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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | ENVIRONMENT | BUGS | FOOTNOTES | SEE ALSO | AUTHOR | COPYRIGHT | COLOPHON |
GCC(1) GNU GCC(1)
gcc - GNU project C and C++ compiler
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...
Only the most useful options are listed here; see below for the
remainder. g++ accepts mostly the same options as gcc.
When you invoke GCC, it normally does preprocessing, compilation,
assembly and linking. The "overall options" allow you to stop this
process at an intermediate stage. For example, the -c option says
not to run the linker. Then the output consists of object files
output by the assembler.
Other options are passed on to one or more stages of processing.
Some options control the preprocessor and others the compiler itself.
Yet other options control the assembler and linker; most of these are
not documented here, since you rarely need to use any of them.
Most of the command-line options that you can use with GCC are useful
for C programs; when an option is only useful with another language
(usually C++), the explanation says so explicitly. If the
description for a particular option does not mention a source
language, you can use that option with all supported languages.
The usual way to run GCC is to run the executable called gcc, or
machine-gcc when cross-compiling, or machine-gcc-version to run a
specific version of GCC. When you compile C++ programs, you should
invoke GCC as g++ instead.
The gcc program accepts options and file names as operands. Many
options have multi-letter names; therefore multiple single-letter
options may not be grouped: -dv is very different from -d -v.
You can mix options and other arguments. For the most part, the
order you use doesn't matter. Order does matter when you use several
options of the same kind; for example, if you specify -L more than
once, the directories are searched in the order specified. Also, the
placement of the -l option is significant.
Many options have long names starting with -f or with -W---for
example, -fmove-loop-invariants, -Wformat and so on. Most of these
have both positive and negative forms; the negative form of -ffoo is
-fno-foo. This manual documents only one of these two forms,
whichever one is not the default.
Option Summary
Here is a summary of all the options, grouped by type. Explanations
are in the following sections.
Overall Options
-c -S -E -o file -x language -v -### --help[=class[,...]]
--target-help --version -pass-exit-codes -pipe -specs=file
-wrapper @file -fplugin=file -fplugin-arg-name=arg
-fdump-ada-spec[-slim] -fada-spec-parent=unit
-fdump-go-spec=file
C Language Options
-ansi -std=standard -fgnu89-inline
-fpermitted-flt-eval-methods=standard -aux-info filename
-fallow-parameterless-variadic-functions -fno-asm -fno-builtin
-fno-builtin-function -fgimple -fhosted -ffreestanding
-fopenacc -fopenmp -fopenmp-simd -fms-extensions
-fplan9-extensions -fsso-struct=endianness
-fallow-single-precision -fcond-mismatch
-flax-vector-conversions -fsigned-bitfields -fsigned-char
-funsigned-bitfields -funsigned-char
C++ Language Options
-fabi-version=n -fno-access-control -faligned-new=n
-fargs-in-order=n -fcheck-new -fconstexpr-depth=n
-fconstexpr-loop-limit=n -ffriend-injection
-fno-elide-constructors -fno-enforce-eh-specs -ffor-scope
-fno-for-scope -fno-gnu-keywords -fno-implicit-templates
-fno-implicit-inline-templates -fno-implement-inlines
-fms-extensions -fnew-inheriting-ctors -fnew-ttp-matching
-fno-nonansi-builtins -fnothrow-opt -fno-operator-names
-fno-optional-diags -fpermissive -fno-pretty-templates -frepo
-fno-rtti -fsized-deallocation -ftemplate-backtrace-limit=n
-ftemplate-depth=n -fno-threadsafe-statics -fuse-cxa-atexit
-fno-weak -nostdinc++ -fvisibility-inlines-hidden
-fvisibility-ms-compat -fext-numeric-literals -Wabi=n -Wabi-tag
-Wconversion-null -Wctor-dtor-privacy -Wdelete-non-virtual-dtor
-Wliteral-suffix -Wmultiple-inheritance -Wnamespaces
-Wnarrowing -Wnoexcept -Wnoexcept-type -Wnon-virtual-dtor
-Wreorder -Wregister -Weffc++ -Wstrict-null-sentinel
-Wtemplates -Wno-non-template-friend -Wold-style-cast
-Woverloaded-virtual -Wno-pmf-conversions -Wsign-promo
-Wvirtual-inheritance
Objective-C and Objective-C++ Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime
-fno-nil-receivers -fobjc-abi-version=n -fobjc-call-cxx-cdtors
-fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc
-fobjc-nilcheck -fobjc-std=objc1 -fno-local-ivars
-fivar-visibility=[public|protected|private|package]
-freplace-objc-classes -fzero-link -gen-decls -Wassign-intercept
-Wno-protocol -Wselector -Wstrict-selector-match
-Wundeclared-selector
Diagnostic Message Formatting Options
-fmessage-length=n -fdiagnostics-show-location=[once|every-line]
-fdiagnostics-color=[auto|never|always]
-fno-diagnostics-show-option -fno-diagnostics-show-caret
-fdiagnostics-parseable-fixits -fdiagnostics-generate-patch
-fno-show-column
Warning Options
-fsyntax-only -fmax-errors=n -Wpedantic -pedantic-errors -w
-Wextra -Wall -Waddress -Waggregate-return -Walloc-zero
-Walloc-size-larger-than=n -Walloca -Walloca-larger-than=n
-Wno-aggressive-loop-optimizations -Warray-bounds
-Warray-bounds=n -Wno-attributes -Wbool-compare
-Wbool-operation -Wno-builtin-declaration-mismatch
-Wno-builtin-macro-redefined -Wc90-c99-compat -Wc99-c11-compat
-Wc++-compat -Wc++11-compat -Wc++14-compat -Wcast-align
-Wcast-qual -Wchar-subscripts -Wchkp -Wclobbered -Wcomment
-Wconditionally-supported -Wconversion -Wcoverage-mismatch
-Wno-cpp -Wdangling-else -Wdate-time -Wdelete-incomplete
-Wno-deprecated -Wno-deprecated-declarations
-Wno-designated-init -Wdisabled-optimization
-Wno-discarded-qualifiers -Wno-discarded-array-qualifiers
-Wno-div-by-zero -Wdouble-promotion -Wduplicated-cond
-Wempty-body -Wenum-compare -Wno-endif-labels
-Wexpansion-to-defined -Werror -Werror=* -Wfatal-errors
-Wfloat-equal -Wformat -Wformat=2 -Wno-format-contains-nul
-Wno-format-extra-args -Wformat-nonliteral -Wformat-overflow=n
-Wformat-security -Wformat-signedness -Wformat-truncation=n
-Wformat-y2k -Wframe-address -Wframe-larger-than=len
-Wno-free-nonheap-object -Wjump-misses-init -Wignored-qualifiers
-Wignored-attributes -Wincompatible-pointer-types -Wimplicit
-Wimplicit-fallthrough -Wimplicit-fallthrough=n
-Wimplicit-function-declaration -Wimplicit-int -Winit-self
-Winline -Wno-int-conversion -Wint-in-bool-context
-Wno-int-to-pointer-cast -Winvalid-memory-model
-Wno-invalid-offsetof -Winvalid-pch -Wlarger-than=len
-Wlogical-op -Wlogical-not-parentheses -Wlong-long -Wmain
-Wmaybe-uninitialized -Wmemset-elt-size
-Wmemset-transposed-args -Wmisleading-indentation
-Wmissing-braces -Wmissing-field-initializers
-Wmissing-include-dirs -Wno-multichar -Wnonnull
-Wnonnull-compare -Wnormalized=[none|id|nfc|nfkc]
-Wnull-dereference -Wodr -Wno-overflow -Wopenmp-simd
-Woverride-init-side-effects -Woverlength-strings -Wpacked
-Wpacked-bitfield-compat -Wpadded -Wparentheses
-Wno-pedantic-ms-format -Wplacement-new -Wplacement-new=n
-Wpointer-arith -Wpointer-compare -Wno-pointer-to-int-cast
-Wno-pragmas -Wredundant-decls -Wrestrict
-Wno-return-local-addr -Wreturn-type -Wsequence-point -Wshadow
-Wno-shadow-ivar -Wshadow=global, -Wshadow=local,
-Wshadow=compatible-local -Wshift-overflow -Wshift-overflow=n
-Wshift-count-negative -Wshift-count-overflow
-Wshift-negative-value -Wsign-compare -Wsign-conversion
-Wfloat-conversion -Wno-scalar-storage-order
-Wsizeof-pointer-memaccess -Wsizeof-array-argument
-Wstack-protector -Wstack-usage=len -Wstrict-aliasing
-Wstrict-aliasing=n -Wstrict-overflow -Wstrict-overflow=n
-Wstringop-overflow=n
-Wsuggest-attribute=[pure|const|noreturn|format]
-Wsuggest-final-types -Wsuggest-final-methods
-Wsuggest-override -Wmissing-format-attribute
-Wsubobject-linkage -Wswitch -Wswitch-bool -Wswitch-default
-Wswitch-enum -Wswitch-unreachable -Wsync-nand -Wsystem-headers
-Wtautological-compare -Wtrampolines -Wtrigraphs -Wtype-limits
-Wundef -Wuninitialized -Wunknown-pragmas
-Wunsafe-loop-optimizations -Wunsuffixed-float-constants
-Wunused -Wunused-function -Wunused-label
-Wunused-local-typedefs -Wunused-macros -Wunused-parameter
-Wno-unused-result -Wunused-value -Wunused-variable
-Wunused-const-variable -Wunused-const-variable=n
-Wunused-but-set-parameter -Wunused-but-set-variable
-Wuseless-cast -Wvariadic-macros -Wvector-operation-performance
-Wvla -Wvla-larger-than=n -Wvolatile-register-var
-Wwrite-strings -Wzero-as-null-pointer-constant -Whsa
C and Objective-C-only Warning Options
-Wbad-function-cast -Wmissing-declarations
-Wmissing-parameter-type -Wmissing-prototypes -Wnested-externs
-Wold-style-declaration -Wold-style-definition
-Wstrict-prototypes -Wtraditional -Wtraditional-conversion
-Wdeclaration-after-statement -Wpointer-sign
Debugging Options
-g -glevel -gcoff -gdwarf -gdwarf-version -ggdb
-grecord-gcc-switches -gno-record-gcc-switches -gstabs -gstabs+
-gstrict-dwarf -gno-strict-dwarf -gcolumn-info -gno-column-info
-gvms -gxcoff -gxcoff+ -gz[=type] -fdebug-prefix-map=old=new
-fdebug-types-section -feliminate-dwarf2-dups
-fno-eliminate-unused-debug-types -femit-struct-debug-baseonly
-femit-struct-debug-reduced -femit-struct-debug-detailed[=spec-
list] -feliminate-unused-debug-symbols -femit-class-debug-always
-fno-merge-debug-strings -fno-dwarf2-cfi-asm -fvar-tracking
-fvar-tracking-assignments
Optimization Options
-faggressive-loop-optimizations -falign-functions[=n]
-falign-jumps[=n] -falign-labels[=n] -falign-loops[=n]
-fassociative-math -fauto-profile -fauto-profile[=path]
-fauto-inc-dec -fbranch-probabilities
-fbranch-target-load-optimize -fbranch-target-load-optimize2
-fbtr-bb-exclusive -fcaller-saves -fcombine-stack-adjustments
-fconserve-stack -fcompare-elim -fcprop-registers
-fcrossjumping -fcse-follow-jumps -fcse-skip-blocks
-fcx-fortran-rules -fcx-limited-range -fdata-sections -fdce
-fdelayed-branch -fdelete-null-pointer-checks -fdevirtualize
-fdevirtualize-speculatively -fdevirtualize-at-ltrans -fdse
-fearly-inlining -fipa-sra -fexpensive-optimizations
-ffat-lto-objects -ffast-math -ffinite-math-only -ffloat-store
-fexcess-precision=style -fforward-propagate -ffp-contract=style
-ffunction-sections -fgcse -fgcse-after-reload -fgcse-las
-fgcse-lm -fgraphite-identity -fgcse-sm -fhoist-adjacent-loads
-fif-conversion -fif-conversion2 -findirect-inlining
-finline-functions -finline-functions-called-once
-finline-limit=n -finline-small-functions -fipa-cp
-fipa-cp-clone -fipa-bit-cp -fipa-vrp -fipa-pta -fipa-profile
-fipa-pure-const -fipa-reference -fipa-icf
-fira-algorithm=algorithm -fira-region=region
-fira-hoist-pressure -fira-loop-pressure
-fno-ira-share-save-slots -fno-ira-share-spill-slots
-fisolate-erroneous-paths-dereference
-fisolate-erroneous-paths-attribute -fivopts
-fkeep-inline-functions -fkeep-static-functions
-fkeep-static-consts -flimit-function-alignment
-flive-range-shrinkage -floop-block -floop-interchange
-floop-strip-mine -floop-unroll-and-jam -floop-nest-optimize
-floop-parallelize-all -flra-remat -flto
-flto-compression-level -flto-partition=alg
-fmerge-all-constants -fmerge-constants -fmodulo-sched
-fmodulo-sched-allow-regmoves -fmove-loop-invariants
-fno-branch-count-reg -fno-defer-pop -fno-fp-int-builtin-inexact
-fno-function-cse -fno-guess-branch-probability -fno-inline
-fno-math-errno -fno-peephole -fno-peephole2
-fno-printf-return-value -fno-sched-interblock -fno-sched-spec
-fno-signed-zeros -fno-toplevel-reorder -fno-trapping-math
-fno-zero-initialized-in-bss -fomit-frame-pointer
-foptimize-sibling-calls -fpartial-inlining -fpeel-loops
-fpredictive-commoning -fprefetch-loop-arrays
-fprofile-correction -fprofile-use -fprofile-use=path
-fprofile-values -fprofile-reorder-functions -freciprocal-math
-free -frename-registers -freorder-blocks
-freorder-blocks-algorithm=algorithm
-freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -freschedule-modulo-scheduled-loops
-frounding-math -fsched2-use-superblocks -fsched-pressure
-fsched-spec-load -fsched-spec-load-dangerous
-fsched-stalled-insns-dep[=n] -fsched-stalled-insns[=n]
-fsched-group-heuristic -fsched-critical-path-heuristic
-fsched-spec-insn-heuristic -fsched-rank-heuristic
-fsched-last-insn-heuristic -fsched-dep-count-heuristic
-fschedule-fusion -fschedule-insns -fschedule-insns2
-fsection-anchors -fselective-scheduling -fselective-scheduling2
-fsel-sched-pipelining -fsel-sched-pipelining-outer-loops
-fsemantic-interposition -fshrink-wrap -fshrink-wrap-separate
-fsignaling-nans -fsingle-precision-constant
-fsplit-ivs-in-unroller -fsplit-loops -fsplit-paths
-fsplit-wide-types -fssa-backprop -fssa-phiopt -fstdarg-opt
-fstore-merging -fstrict-aliasing -fstrict-overflow
-fthread-jumps -ftracer -ftree-bit-ccp -ftree-builtin-call-dce
-ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop
-ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop
-ftree-fre -fcode-hoisting -ftree-loop-if-convert
-ftree-loop-im -ftree-phiprop -ftree-loop-distribution
-ftree-loop-distribute-patterns -ftree-loop-ivcanon
-ftree-loop-linear -ftree-loop-optimize -ftree-loop-vectorize
-ftree-parallelize-loops=n -ftree-pre -ftree-partial-pre
-ftree-pta -ftree-reassoc -ftree-sink -ftree-slsr -ftree-sra
-ftree-switch-conversion -ftree-tail-merge -ftree-ter
-ftree-vectorize -ftree-vrp -funconstrained-commons
-funit-at-a-time -funroll-all-loops -funroll-loops
-funsafe-math-optimizations -funswitch-loops -fipa-ra
-fvariable-expansion-in-unroller -fvect-cost-model -fvpt -fweb
-fwhole-program -fwpa -fuse-linker-plugin --param name=value -O
-O0 -O1 -O2 -O3 -Os -Ofast -Og
Program Instrumentation Options
-p -pg -fprofile-arcs --coverage -ftest-coverage
-fprofile-dir=path -fprofile-generate -fprofile-generate=path
-fsanitize=style -fsanitize-recover -fsanitize-recover=style
-fasan-shadow-offset=number -fsanitize-sections=s1,s2,...
-fsanitize-undefined-trap-on-error -fbounds-check
-fcheck-pointer-bounds -fchkp-check-incomplete-type
-fchkp-first-field-has-own-bounds -fchkp-narrow-bounds
-fchkp-narrow-to-innermost-array -fchkp-optimize
-fchkp-use-fast-string-functions
-fchkp-use-nochk-string-functions -fchkp-use-static-bounds
-fchkp-use-static-const-bounds
-fchkp-treat-zero-dynamic-size-as-infinite -fchkp-check-read
-fchkp-check-read -fchkp-check-write -fchkp-store-bounds
-fchkp-instrument-calls -fchkp-instrument-marked-only
-fchkp-use-wrappers -fchkp-flexible-struct-trailing-arrays
-fstack-protector -fstack-protector-all
-fstack-protector-strong -fstack-protector-explicit
-fstack-check -fstack-limit-register=reg
-fstack-limit-symbol=sym -fno-stack-limit -fsplit-stack
-fvtable-verify=[std|preinit|none] -fvtv-counts -fvtv-debug
-finstrument-functions
-finstrument-functions-exclude-function-list=sym,sym,...
-finstrument-functions-exclude-file-list=file,file,...
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -CC -Dmacro[=defn]
-dD -dI -dM -dN -dU -fdebug-cpp -fdirectives-only
-fdollars-in-identifiers -fexec-charset=charset
-fextended-identifiers -finput-charset=charset
-fno-canonical-system-headers -fpch-deps -fpch-preprocess
-fpreprocessed -ftabstop=width -ftrack-macro-expansion
-fwide-exec-charset=charset -fworking-directory -H -imacros
file -include file -M -MD -MF -MG -MM -MMD -MP -MQ -MT
-no-integrated-cpp -P -pthread -remap -traditional
-traditional-cpp -trigraphs -Umacro -undef -Wp,option
-Xpreprocessor option
Assembler Options
-Wa,option -Xassembler option
Linker Options
object-file-name -fuse-ld=linker -llibrary -nostartfiles
-nodefaultlibs -nostdlib -pie -pthread -rdynamic -s -static
-static-libgcc -static-libstdc++ -static-libasan
-static-libtsan -static-liblsan -static-libubsan -static-libmpx
-static-libmpxwrappers -shared -shared-libgcc -symbolic -T
script -Wl,option -Xlinker option -u symbol -z keyword
Directory Options
-Bprefix -Idir -I- -idirafter dir -imacros file -imultilib dir
-iplugindir=dir -iprefix file -iquote dir -isysroot dir
-isystem dir -iwithprefix dir -iwithprefixbefore dir -Ldir
-no-canonical-prefixes --no-sysroot-suffix -nostdinc
-nostdinc++ --sysroot=dir
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions
-fnon-call-exceptions -fdelete-dead-exceptions -funwind-tables
-fasynchronous-unwind-tables -fno-gnu-unique
-finhibit-size-directive -fno-common -fno-ident
-fpcc-struct-return -fpic -fPIC -fpie -fPIE -fno-plt
-fno-jump-tables -frecord-gcc-switches -freg-struct-return
-fshort-enums -fshort-wchar -fverbose-asm -fpack-struct[=n]
-fleading-underscore -ftls-model=model -fstack-reuse=reuse_level
-ftrampolines -ftrapv -fwrapv
-fvisibility=[default|internal|hidden|protected]
-fstrict-volatile-bitfields -fsync-libcalls
Developer Options
-dletters -dumpspecs -dumpmachine -dumpversion
-dumpfullversion -fchecking -fchecking=n -fdbg-cnt-list
-fdbg-cnt=counter-value-list -fdisable-ipa-pass_name
-fdisable-rtl-pass_name -fdisable-rtl-pass-name=range-list
-fdisable-tree-pass_name -fdisable-tree-pass-name=range-list
-fdump-noaddr -fdump-unnumbered -fdump-unnumbered-links
-fdump-translation-unit[-n] -fdump-class-hierarchy[-n]
-fdump-ipa-all -fdump-ipa-cgraph -fdump-ipa-inline
-fdump-passes -fdump-rtl-pass -fdump-rtl-pass=filename
-fdump-statistics -fdump-final-insns[=file] -fdump-tree-all
-fdump-tree-switch -fdump-tree-switch-options
-fdump-tree-switch-options=filename -fcompare-debug[=opts]
-fcompare-debug-second -fenable-kind-pass
-fenable-kind-pass=range-list -fira-verbose=n -flto-report
-flto-report-wpa -fmem-report-wpa -fmem-report
-fpre-ipa-mem-report -fpost-ipa-mem-report -fopt-info
-fopt-info-options[=file] -fprofile-report -frandom-seed=string
-fsched-verbose=n -fsel-sched-verbose -fsel-sched-dump-cfg
-fsel-sched-pipelining-verbose -fstats -fstack-usage
-ftime-report -ftime-report-details
-fvar-tracking-assignments-toggle -gtoggle
-print-file-name=library -print-libgcc-file-name
-print-multi-directory -print-multi-lib
-print-multi-os-directory -print-prog-name=program
-print-search-dirs -Q -print-sysroot
-print-sysroot-headers-suffix -save-temps -save-temps=cwd
-save-temps=obj -time[=file]
Machine-Dependent Options
AArch64 Options -mabi=name -mbig-endian -mlittle-endian
-mgeneral-regs-only -mcmodel=tiny -mcmodel=small -mcmodel=large
-mstrict-align -momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer -mtls-dialect=desc
-mtls-dialect=traditional -mtls-size=size -mfix-cortex-a53-835769
-mno-fix-cortex-a53-835769 -mfix-cortex-a53-843419
-mno-fix-cortex-a53-843419 -mlow-precision-recip-sqrt
-mno-low-precision-recip-sqrt -mlow-precision-sqrt
-mno-low-precision-sqrt -mlow-precision-div
-mno-low-precision-div -march=name -mcpu=name -mtune=name
Adapteva Epiphany Options -mhalf-reg-file
-mprefer-short-insn-regs -mbranch-cost=num -mcmove -mnops=num
-msoft-cmpsf -msplit-lohi -mpost-inc -mpost-modify
-mstack-offset=num -mround-nearest -mlong-calls -mshort-calls
-msmall16 -mfp-mode=mode -mvect-double -max-vect-align=num
-msplit-vecmove-early -m1reg-reg
ARC Options -mbarrel-shifter -mcpu=cpu -mA6 -mARC600 -mA7
-mARC700 -mdpfp -mdpfp-compact -mdpfp-fast -mno-dpfp-lrsr -mea
-mno-mpy -mmul32x16 -mmul64 -matomic -mnorm -mspfp
-mspfp-compact -mspfp-fast -msimd -msoft-float -mswap -mcrc
-mdsp-packa -mdvbf -mlock -mmac-d16 -mmac-24 -mrtsc -mswape
-mtelephony -mxy -misize -mannotate-align -marclinux
-marclinux_prof -mlong-calls -mmedium-calls -msdata
-mvolatile-cache -mtp-regno=regno -malign-call
-mauto-modify-reg -mbbit-peephole -mno-brcc -mcase-vector-pcrel
-mcompact-casesi -mno-cond-exec -mearly-cbranchsi
-mexpand-adddi -mindexed-loads -mlra -mlra-priority-none
-mlra-priority-compact mlra-priority-noncompact -mno-millicode
-mmixed-code -mq-class -mRcq -mRcw -msize-level=level
-mtune=cpu -mmultcost=num -munalign-prob-threshold=probability
-mmpy-option=multo -mdiv-rem -mcode-density -mll64 -mfpu=fpu
ARM Options -mapcs-frame -mno-apcs-frame -mabi=name
-mapcs-stack-check -mno-apcs-stack-check -mapcs-reentrant
-mno-apcs-reentrant -msched-prolog -mno-sched-prolog
-mlittle-endian -mbig-endian -mfloat-abi=name -mfp16-format=name
-mthumb-interwork -mno-thumb-interwork -mcpu=name -march=name
-mfpu=name -mtune=name -mprint-tune-info
-mstructure-size-boundary=n -mabort-on-noreturn -mlong-calls
-mno-long-calls -msingle-pic-base -mno-single-pic-base
-mpic-register=reg -mnop-fun-dllimport -mpoke-function-name
-mthumb -marm -mtpcs-frame -mtpcs-leaf-frame
-mcaller-super-interworking -mcallee-super-interworking
-mtp=name -mtls-dialect=dialect -mword-relocations
-mfix-cortex-m3-ldrd -munaligned-access -mneon-for-64bits
-mslow-flash-data -masm-syntax-unified -mrestrict-it -mpure-code
-mcmse
AVR Options -mmcu=mcu -mabsdata -maccumulate-args
-mbranch-cost=cost -mcall-prologues -mint8 -mn_flash=size
-mno-interrupts -mrelax -mrmw -mstrict-X -mtiny-stack
-mfract-convert-truncate -nodevicelib -Waddr-space-convert
-Wmisspelled-isr
Blackfin Options -mcpu=cpu[-sirevision] -msim
-momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer
-mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly
-mno-csync-anomaly -mlow-64k -mno-low64k -mstack-check-l1
-mid-shared-library -mno-id-shared-library -mshared-library-id=n
-mleaf-id-shared-library -mno-leaf-id-shared-library -msep-data
-mno-sep-data -mlong-calls -mno-long-calls -mfast-fp
-minline-plt -mmulticore -mcorea -mcoreb -msdram -micplb
C6X Options -mbig-endian -mlittle-endian -march=cpu -msim
-msdata=sdata-type
CRIS Options -mcpu=cpu -march=cpu -mtune=cpu
-mmax-stack-frame=n -melinux-stacksize=n -metrax4 -metrax100
-mpdebug -mcc-init -mno-side-effects -mstack-align
-mdata-align -mconst-align -m32-bit -m16-bit -m8-bit
-mno-prologue-epilogue -mno-gotplt -melf -maout -melinux
-mlinux -sim -sim2 -mmul-bug-workaround
-mno-mul-bug-workaround
CR16 Options -mmac -mcr16cplus -mcr16c -msim -mint32 -mbit-ops
-mdata-model=model
Darwin Options -all_load -allowable_client -arch
-arch_errors_fatal -arch_only -bind_at_load -bundle
-bundle_loader -client_name -compatibility_version
-current_version -dead_strip -dependency-file -dylib_file
-dylinker_install_name -dynamic -dynamiclib
-exported_symbols_list -filelist -flat_namespace
-force_cpusubtype_ALL -force_flat_namespace
-headerpad_max_install_names -iframework -image_base -init
-install_name -keep_private_externs -multi_module
-multiply_defined -multiply_defined_unused -noall_load
-no_dead_strip_inits_and_terms -nofixprebinding -nomultidefs
-noprebind -noseglinkedit -pagezero_size -prebind
-prebind_all_twolevel_modules -private_bundle -read_only_relocs
-sectalign -sectobjectsymbols -whyload -seg1addr -sectcreate
-sectobjectsymbols -sectorder -segaddr -segs_read_only_addr
-segs_read_write_addr -seg_addr_table -seg_addr_table_filename
-seglinkedit -segprot -segs_read_only_addr
-segs_read_write_addr -single_module -static -sub_library
-sub_umbrella -twolevel_namespace -umbrella -undefined
-unexported_symbols_list -weak_reference_mismatches -whatsloaded
-F -gused -gfull -mmacosx-version-min=version -mkernel
-mone-byte-bool
DEC Alpha Options -mno-fp-regs -msoft-float -mieee
-mieee-with-inexact -mieee-conformant -mfp-trap-mode=mode
-mfp-rounding-mode=mode -mtrap-precision=mode -mbuild-constants
-mcpu=cpu-type -mtune=cpu-type -mbwx -mmax -mfix -mcix
-mfloat-vax -mfloat-ieee -mexplicit-relocs -msmall-data
-mlarge-data -msmall-text -mlarge-text -mmemory-latency=time
FR30 Options -msmall-model -mno-lsim
FT32 Options -msim -mlra -mnodiv
FRV Options -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 -mhard-float
-msoft-float -malloc-cc -mfixed-cc -mdword -mno-dword -mdouble
-mno-double -mmedia -mno-media -mmuladd -mno-muladd -mfdpic
-minline-plt -mgprel-ro -multilib-library-pic -mlinked-fp
-mlong-calls -malign-labels -mlibrary-pic -macc-4 -macc-8
-mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move
-moptimize-membar -mno-optimize-membar -mscc -mno-scc
-mcond-exec -mno-cond-exec -mvliw-branch -mno-vliw-branch
-mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec
-mno-nested-cond-exec -mtomcat-stats -mTLS -mtls -mcpu=cpu
GNU/Linux Options -mglibc -muclibc -mmusl -mbionic -mandroid
-tno-android-cc -tno-android-ld
H8/300 Options -mrelax -mh -ms -mn -mexr -mno-exr -mint32
-malign-300
HPPA Options -march=architecture-type -mcaller-copies
-mdisable-fpregs -mdisable-indexing -mfast-indirect-calls -mgas
-mgnu-ld -mhp-ld -mfixed-range=register-range -mjump-in-delay
-mlinker-opt -mlong-calls -mlong-load-store -mno-disable-fpregs
-mno-disable-indexing -mno-fast-indirect-calls -mno-gas
-mno-jump-in-delay -mno-long-load-store -mno-portable-runtime
-mno-soft-float -mno-space-regs -msoft-float -mpa-risc-1-0
-mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime -mschedule=cpu-
type -mspace-regs -msio -mwsio -munix=unix-std -nolibdld
-static -threads
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld
-mno-pic -mvolatile-asm-stop -mregister-names -msdata
-mno-sdata -mconstant-gp -mauto-pic -mfused-madd
-minline-float-divide-min-latency
-minline-float-divide-max-throughput -mno-inline-float-divide
-minline-int-divide-min-latency
-minline-int-divide-max-throughput -mno-inline-int-divide
-minline-sqrt-min-latency -minline-sqrt-max-throughput
-mno-inline-sqrt -mdwarf2-asm -mearly-stop-bits
-mfixed-range=register-range -mtls-size=tls-size -mtune=cpu-type
-milp32 -mlp64 -msched-br-data-spec -msched-ar-data-spec
-msched-control-spec -msched-br-in-data-spec
-msched-ar-in-data-spec -msched-in-control-spec -msched-spec-ldc
-msched-spec-control-ldc -msched-prefer-non-data-spec-insns
-msched-prefer-non-control-spec-insns
-msched-stop-bits-after-every-cycle
-msched-count-spec-in-critical-path
-msel-sched-dont-check-control-spec
-msched-fp-mem-deps-zero-cost -msched-max-memory-insns-hard-limit
-msched-max-memory-insns=max-insns
LM32 Options -mbarrel-shift-enabled -mdivide-enabled
-mmultiply-enabled -msign-extend-enabled -muser-enabled
M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops
-mno-align-loops -missue-rate=number -mbranch-cost=number
-mmodel=code-size-model-type -msdata=sdata-type -mno-flush-func
-mflush-func=name -mno-flush-trap -mflush-trap=number -G num
M32C Options -mcpu=cpu -msim -memregs=number
M680x0 Options -march=arch -mcpu=cpu -mtune=tune -m68000
-m68020 -m68020-40 -m68020-60 -m68030 -m68040 -m68060
-mcpu32 -m5200 -m5206e -m528x -m5307 -m5407 -mcfv4e
-mbitfield -mno-bitfield -mc68000 -mc68020 -mnobitfield -mrtd
-mno-rtd -mdiv -mno-div -mshort -mno-short -mhard-float
-m68881 -msoft-float -mpcrel -malign-int -mstrict-align
-msep-data -mno-sep-data -mshared-library-id=n
-mid-shared-library -mno-id-shared-library -mxgot -mno-xgot
-mlong-jump-table-offsets
MCore Options -mhardlit -mno-hardlit -mdiv -mno-div
-mrelax-immediates -mno-relax-immediates -mwide-bitfields
-mno-wide-bitfields -m4byte-functions -mno-4byte-functions
-mcallgraph-data -mno-callgraph-data -mslow-bytes
-mno-slow-bytes -mno-lsim -mlittle-endian -mbig-endian -m210
-m340 -mstack-increment
MeP Options -mabsdiff -mall-opts -maverage -mbased=n -mbitops
-mc=n -mclip -mconfig=name -mcop -mcop32 -mcop64 -mivc2
-mdc -mdiv -meb -mel -mio-volatile -ml -mleadz -mm
-mminmax -mmult -mno-opts -mrepeat -ms -msatur -msdram
-msim -msimnovec -mtf -mtiny=n
MicroBlaze Options -msoft-float -mhard-float -msmall-divides
-mcpu=cpu -mmemcpy -mxl-soft-mul -mxl-soft-div
-mxl-barrel-shift -mxl-pattern-compare -mxl-stack-check
-mxl-gp-opt -mno-clearbss -mxl-multiply-high -mxl-float-convert
-mxl-float-sqrt -mbig-endian -mlittle-endian -mxl-reorder
-mxl-mode-app-model
MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2
-mips3 -mips4 -mips32 -mips32r2 -mips32r3 -mips32r5
-mips32r6 -mips64 -mips64r2 -mips64r3 -mips64r5 -mips64r6
-mips16 -mno-mips16 -mflip-mips16 -minterlink-compressed
-mno-interlink-compressed -minterlink-mips16
-mno-interlink-mips16 -mabi=abi -mabicalls -mno-abicalls
-mshared -mno-shared -mplt -mno-plt -mxgot -mno-xgot -mgp32
-mgp64 -mfp32 -mfpxx -mfp64 -mhard-float -msoft-float
-mno-float -msingle-float -mdouble-float -modd-spreg
-mno-odd-spreg -mabs=mode -mnan=encoding -mdsp -mno-dsp
-mdspr2 -mno-dspr2 -mmcu -mmno-mcu -meva -mno-eva -mvirt
-mno-virt -mxpa -mno-xpa -mmicromips -mno-micromips -mmsa
-mno-msa -mfpu=fpu-type -msmartmips -mno-smartmips
-mpaired-single -mno-paired-single -mdmx -mno-mdmx -mips3d
-mno-mips3d -mmt -mno-mt -mllsc -mno-llsc -mlong64 -mlong32
-msym32 -mno-sym32 -Gnum -mlocal-sdata -mno-local-sdata
-mextern-sdata -mno-extern-sdata -mgpopt -mno-gopt
-membedded-data -mno-embedded-data -muninit-const-in-rodata
-mno-uninit-const-in-rodata -mcode-readable=setting
-msplit-addresses -mno-split-addresses -mexplicit-relocs
-mno-explicit-relocs -mcheck-zero-division
-mno-check-zero-division -mdivide-traps -mdivide-breaks
-mload-store-pairs -mno-load-store-pairs -mmemcpy -mno-memcpy
-mlong-calls -mno-long-calls -mmad -mno-mad -mimadd
-mno-imadd -mfused-madd -mno-fused-madd -nocpp -mfix-24k
-mno-fix-24k -mfix-r4000 -mno-fix-r4000 -mfix-r4400
-mno-fix-r4400 -mfix-r10000 -mno-fix-r10000 -mfix-rm7000
-mno-fix-rm7000 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130
-mno-fix-vr4130 -mfix-sb1 -mno-fix-sb1 -mflush-func=func
-mno-flush-func -mbranch-cost=num -mbranch-likely
-mno-branch-likely -mcompact-branches=policy -mfp-exceptions
-mno-fp-exceptions -mvr4130-align -mno-vr4130-align -msynci
-mno-synci -mlxc1-sxc1 -mno-lxc1-sxc1 -mmadd4 -mno-madd4
-mrelax-pic-calls -mno-relax-pic-calls -mmcount-ra-address
-mframe-header-opt -mno-frame-header-opt
MMIX Options -mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon
-mabi=gnu -mabi=mmixware -mzero-extend -mknuthdiv
-mtoplevel-symbols -melf -mbranch-predict -mno-branch-predict
-mbase-addresses -mno-base-addresses -msingle-exit
-mno-single-exit
MN10300 Options -mmult-bug -mno-mult-bug -mno-am33 -mam33
-mam33-2 -mam34 -mtune=cpu-type -mreturn-pointer-on-d0 -mno-crt0
-mrelax -mliw -msetlb
Moxie Options -meb -mel -mmul.x -mno-crt0
MSP430 Options -msim -masm-hex -mmcu= -mcpu= -mlarge -msmall
-mrelax -mwarn-mcu -mcode-region= -mdata-region=
-msilicon-errata= -msilicon-errata-warn= -mhwmult= -minrt
NDS32 Options -mbig-endian -mlittle-endian -mreduced-regs
-mfull-regs -mcmov -mno-cmov -mperf-ext -mno-perf-ext -mv3push
-mno-v3push -m16bit -mno-16bit -misr-vector-size=num
-mcache-block-size=num -march=arch -mcmodel=code-model
-mctor-dtor -mrelax
Nios II Options -G num -mgpopt=option -mgpopt -mno-gpopt -mel
-meb -mno-bypass-cache -mbypass-cache -mno-cache-volatile
-mcache-volatile -mno-fast-sw-div -mfast-sw-div -mhw-mul
-mno-hw-mul -mhw-mulx -mno-hw-mulx -mno-hw-div -mhw-div
-mcustom-insn=N -mno-custom-insn -mcustom-fpu-cfg=name -mhal
-msmallc -msys-crt0=name -msys-lib=name -march=arch -mbmx
-mno-bmx -mcdx -mno-cdx
Nvidia PTX Options -m32 -m64 -mmainkernel -moptimize
PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45
-m10 -mbcopy -mbcopy-builtin -mint32 -mno-int16 -mint16
-mno-int32 -mfloat32 -mno-float64 -mfloat64 -mno-float32
-mabshi -mno-abshi -mbranch-expensive -mbranch-cheap -munix-asm
-mdec-asm
picoChip Options -mae=ae_type -mvliw-lookahead=N
-msymbol-as-address -mno-inefficient-warnings
PowerPC Options See RS/6000 and PowerPC Options.
RISC-V Options -mbranch-cost=N-instruction -mmemcpy -mno-memcpy
-mplt -mno-plt -mabi=ABI-string -mfdiv -mno-fdiv -mdiv
-mno-div -march=ISA-string -mtune=processor-string
-msmall-data-limit=N-bytes -msave-restore -mno-save-restore
-mcmodel=code-model -mexplicit-relocs -mno-explicit-relocs
RL78 Options -msim -mmul=none -mmul=g13 -mmul=g14 -mallregs
-mcpu=g10 -mcpu=g13 -mcpu=g14 -mg10 -mg13 -mg14
-m64bit-doubles -m32bit-doubles -msave-mduc-in-interrupts
RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type
-mcmodel=code-model -mpowerpc64 -maltivec -mno-altivec
-mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt
-mno-powerpc-gfxopt -mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb
-mpopcntd -mno-popcntd -mfprnd -mno-fprnd -mcmpb -mno-cmpb
-mmfpgpr -mno-mfpgpr -mhard-dfp -mno-hard-dfp -mfull-toc
-mminimal-toc -mno-fp-in-toc -mno-sum-in-toc -m64 -m32
-mxl-compat -mno-xl-compat -mpe -malign-power -malign-natural
-msoft-float -mhard-float -mmultiple -mno-multiple
-msingle-float -mdouble-float -msimple-fpu -mstring
-mno-string -mupdate -mno-update -mavoid-indexed-addresses
-mno-avoid-indexed-addresses -mfused-madd -mno-fused-madd
-mbit-align -mno-bit-align -mstrict-align -mno-strict-align
-mrelocatable -mno-relocatable -mrelocatable-lib
-mno-relocatable-lib -mtoc -mno-toc -mlittle -mlittle-endian
-mbig -mbig-endian -mdynamic-no-pic -maltivec -mswdiv
-msingle-pic-base -mprioritize-restricted-insns=priority
-msched-costly-dep=dependence_type -minsert-sched-nops=scheme
-mcall-sysv -mcall-netbsd -maix-struct-return
-msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt
-mblock-move-inline-limit=num -misel -mno-isel -misel=yes
-misel=no -mspe -mno-spe -mspe=yes -mspe=no -mpaired
-mgen-cell-microcode -mwarn-cell-microcode -mvrsave -mno-vrsave
-mmulhw -mno-mulhw -mdlmzb -mno-dlmzb -mfloat-gprs=yes
-mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double
-mprototype -mno-prototype -msim -mmvme -mads -myellowknife
-memb -msdata -msdata=opt -mvxworks -G num -mrecip
-mrecip=opt -mno-recip -mrecip-precision -mno-recip-precision
-mveclibabi=type -mfriz -mno-friz
-mpointers-to-nested-functions -mno-pointers-to-nested-functions
-msave-toc-indirect -mno-save-toc-indirect -mpower8-fusion
-mno-mpower8-fusion -mpower8-vector -mno-power8-vector -mcrypto
-mno-crypto -mhtm -mno-htm -mdirect-move -mno-direct-move
-mquad-memory -mno-quad-memory -mquad-memory-atomic
-mno-quad-memory-atomic -mcompat-align-parm
-mno-compat-align-parm -mupper-regs-df -mno-upper-regs-df
-mupper-regs-sf -mno-upper-regs-sf -mupper-regs-di
-mno-upper-regs-di -mupper-regs -mno-upper-regs -mfloat128
-mno-float128 -mfloat128-hardware -mno-float128-hardware
-mgnu-attribute -mno-gnu-attribute -mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset -mlra -mno-lra
RX Options -m64bit-doubles -m32bit-doubles -fpu -nofpu -mcpu=
-mbig-endian-data -mlittle-endian-data -msmall-data -msim
-mno-sim -mas100-syntax -mno-as100-syntax -mrelax
-mmax-constant-size= -mint-register= -mpid -mallow-string-insns
-mno-allow-string-insns -mjsr -mno-warn-multiple-fast-interrupts
-msave-acc-in-interrupts
S/390 and zSeries Options -mtune=cpu-type -march=cpu-type
-mhard-float -msoft-float -mhard-dfp -mno-hard-dfp
-mlong-double-64 -mlong-double-128 -mbackchain -mno-backchain
-mpacked-stack -mno-packed-stack -msmall-exec -mno-small-exec
-mmvcle -mno-mvcle -m64 -m31 -mdebug -mno-debug -mesa
-mzarch -mhtm -mvx -mzvector -mtpf-trace -mno-tpf-trace
-mfused-madd -mno-fused-madd -mwarn-framesize
-mwarn-dynamicstack -mstack-size -mstack-guard
-mhotpatch=halfwords,halfwords
Score Options -meb -mel -mnhwloop -muls -mmac -mscore5
-mscore5u -mscore7 -mscore7d
SH Options -m1 -m2 -m2e -m2a-nofpu -m2a-single-only
-m2a-single -m2a -m3 -m3e -m4-nofpu -m4-single-only
-m4-single -m4 -m4a-nofpu -m4a-single-only -m4a-single -m4a
-m4al -mb -ml -mdalign -mrelax -mbigtable -mfmovd -mrenesas
-mno-renesas -mnomacsave -mieee -mno-ieee -mbitops -misize
-minline-ic_invalidate -mpadstruct -mprefergot -musermode
-multcost=number -mdiv=strategy -mdivsi3_libfunc=name
-mfixed-range=register-range -maccumulate-outgoing-args
-matomic-model=atomic-model -mbranch-cost=num -mzdcbranch
-mno-zdcbranch -mcbranch-force-delay-slot -mfused-madd
-mno-fused-madd -mfsca -mno-fsca -mfsrra -mno-fsrra
-mpretend-cmove -mtas
Solaris 2 Options -mclear-hwcap -mno-clear-hwcap -mimpure-text
-mno-impure-text -pthreads
SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model
-mmemory-model=mem-model -m32 -m64 -mapp-regs -mno-app-regs
-mfaster-structs -mno-faster-structs -mflat -mno-flat -mfpu
-mno-fpu -mhard-float -msoft-float -mhard-quad-float
-msoft-quad-float -mstack-bias -mno-stack-bias
-mstd-struct-return -mno-std-struct-return -munaligned-doubles
-mno-unaligned-doubles -muser-mode -mno-user-mode -mv8plus
-mno-v8plus -mvis -mno-vis -mvis2 -mno-vis2 -mvis3 -mno-vis3
-mcbcond -mno-cbcond -mfmaf -mno-fmaf -mpopc -mno-popc
-msubxc -mno-subxc -mfix-at697f -mfix-ut699 -mlra -mno-lra
SPU Options -mwarn-reloc -merror-reloc -msafe-dma -munsafe-dma
-mbranch-hints -msmall-mem -mlarge-mem -mstdmain
-mfixed-range=register-range -mea32 -mea64
-maddress-space-conversion -mno-address-space-conversion
-mcache-size=cache-size -matomic-updates -mno-atomic-updates
System V Options -Qy -Qn -YP,paths -Ym,dir
TILE-Gx Options -mcpu=CPU -m32 -m64 -mbig-endian
-mlittle-endian -mcmodel=code-model
TILEPro Options -mcpu=cpu -m32
V850 Options -mlong-calls -mno-long-calls -mep -mno-ep
-mprolog-function -mno-prolog-function -mspace -mtda=n -msda=n
-mzda=n -mapp-regs -mno-app-regs -mdisable-callt
-mno-disable-callt -mv850e2v3 -mv850e2 -mv850e1 -mv850es
-mv850e -mv850 -mv850e3v5 -mloop -mrelax -mlong-jumps
-msoft-float -mhard-float -mgcc-abi -mrh850-abi -mbig-switch
VAX Options -mg -mgnu -munix
Visium Options -mdebug -msim -mfpu -mno-fpu -mhard-float
-msoft-float -mcpu=cpu-type -mtune=cpu-type -msv-mode
-muser-mode
VMS Options -mvms-return-codes -mdebug-main=prefix -mmalloc64
-mpointer-size=size
VxWorks Options -mrtp -non-static -Bstatic -Bdynamic
-Xbind-lazy -Xbind-now
x86 Options -mtune=cpu-type -march=cpu-type -mtune-ctrl=feature-
list -mdump-tune-features -mno-default -mfpmath=unit
-masm=dialect -mno-fancy-math-387 -mno-fp-ret-in-387 -m80387
-mhard-float -msoft-float -mno-wide-multiply -mrtd
-malign-double -mpreferred-stack-boundary=num
-mincoming-stack-boundary=num -mcld -mcx16 -msahf -mmovbe
-mcrc32 -mrecip -mrecip=opt -mvzeroupper -mprefer-avx128 -mmmx
-msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx
-mavx2 -mavx512f -mavx512pf -mavx512er -mavx512cd -mavx512vl
-mavx512bw -mavx512dq -mavx512ifma -mavx512vbmi -msha -maes
-mpclmul -mfsgsbase -mrdrnd -mf16c -mfma -mprefetchwt1
-mclflushopt -mxsavec -mxsaves -msse4a -m3dnow -m3dnowa
-mpopcnt -mabm -mbmi -mtbm -mfma4 -mxop -mlzcnt -mbmi2
-mfxsr -mxsave -mxsaveopt -mrtm -mlwp -mmpx -mmwaitx
-mclzero -mpku -mthreads -mms-bitfields -mno-align-stringops
-minline-all-stringops -minline-stringops-dynamically
-mstringop-strategy=alg -mmemcpy-strategy=strategy
-mmemset-strategy=strategy -mpush-args
-maccumulate-outgoing-args -m128bit-long-double
-m96bit-long-double -mlong-double-64 -mlong-double-80
-mlong-double-128 -mregparm=num -msseregparm -mveclibabi=type
-mvect8-ret-in-mem -mpc32 -mpc64 -mpc80 -mstackrealign
-momit-leaf-frame-pointer -mno-red-zone
-mno-tls-direct-seg-refs -mcmodel=code-model -mabi=name
-maddress-mode=mode -m32 -m64 -mx32 -m16 -miamcu
-mlarge-data-threshold=num -msse2avx -mfentry -mrecord-mcount
-mnop-mcount -m8bit-idiv -mavx256-split-unaligned-load
-mavx256-split-unaligned-store -malign-data=type
-mstack-protector-guard=guard -mmitigate-rop -mgeneral-regs-only
x86 Windows Options -mconsole -mcygwin -mno-cygwin -mdll
-mnop-fun-dllimport -mthread -municode -mwin32 -mwindows
-fno-set-stack-executable
Xstormy16 Options -msim
Xtensa Options -mconst16 -mno-const16 -mfused-madd
-mno-fused-madd -mforce-no-pic -mserialize-volatile
-mno-serialize-volatile -mtext-section-literals
-mno-text-section-literals -mauto-litpools -mno-auto-litpools
-mtarget-align -mno-target-align -mlongcalls -mno-longcalls
zSeries Options See S/390 and zSeries Options.
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation
proper, assembly and linking, always in that order. GCC is capable
of preprocessing and compiling several files either into several
assembler input files, or into one assembler input file; then each
assembler input file produces an object file, and linking combines
all the object files (those newly compiled, and those specified as
input) into an executable file.
For any given input file, the file name suffix determines what kind
of compilation is done:
file.c
C source code that must be preprocessed.
file.i
C source code that should not be preprocessed.
file.ii
C++ source code that should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the
libobjc library to make an Objective-C program work.
file.mi
Objective-C source code that should not be preprocessed.
file.mm
file.M
Objective-C++ source code. Note that you must link with the
libobjc library to make an Objective-C++ program work. Note that
.M refers to a literal capital M.
file.mii
Objective-C++ source code that should not be preprocessed.
file.h
C, C++, Objective-C or Objective-C++ header file to be turned
into a precompiled header (default), or C, C++ header file to be
turned into an Ada spec (via the -fdump-ada-spec switch).
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code that must be preprocessed. Note that in .cxx,
the last two letters must both be literally x. Likewise, .C
refers to a literal capital C.
file.mm
file.M
Objective-C++ source code that must be preprocessed.
file.mii
Objective-C++ source code that should not be preprocessed.
file.hh
file.H
file.hp
file.hxx
file.hpp
file.HPP
file.h++
file.tcc
C++ header file to be turned into a precompiled header or Ada
spec.
file.f
file.for
file.ftn
Fixed form Fortran source code that should not be preprocessed.
file.F
file.FOR
file.fpp
file.FPP
file.FTN
Fixed form Fortran source code that must be preprocessed (with
the traditional preprocessor).
file.f90
file.f95
file.f03
file.f08
Free form Fortran source code that should not be preprocessed.
file.F90
file.F95
file.F03
file.F08
Free form Fortran source code that must be preprocessed (with the
traditional preprocessor).
file.go
Go source code.
file.brig
BRIG files (binary representation of HSAIL).
file.ads
Ada source code file that contains a library unit declaration (a
declaration of a package, subprogram, or generic, or a generic
instantiation), or a library unit renaming declaration (a
package, generic, or subprogram renaming declaration). Such
files are also called specs.
file.adb
Ada source code file containing a library unit body (a subprogram
or package body). Such files are also called bodies.
file.s
Assembler code.
file.S
file.sx
Assembler code that must be preprocessed.
other
An object file to be fed straight into linking. Any file name
with no recognized suffix is treated this way.
You can specify the input language explicitly with the -x option:
-x language
Specify explicitly the language for the following input files
(rather than letting the compiler choose a default based on the
file name suffix). This option applies to all following input
files until the next -x option. Possible values for language
are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input f95 f95-cpp-input
go
brig
-x none
Turn off any specification of a language, so that subsequent
files are handled according to their file name suffixes (as they
are if -x has not been used at all).
If you only want some of the stages of compilation, you can use -x
(or filename suffixes) to tell gcc where to start, and one of the
options -c, -S, or -E to say where gcc is to stop. Note that some
combinations (for example, -x cpp-output -E) instruct gcc to do
nothing at all.
-c Compile or assemble the source files, but do not link. The
linking stage simply is not done. The ultimate output is in the
form of an object file for each source file.
By default, the object file name for a source file is made by
replacing the suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly,
are ignored.
-S Stop after the stage of compilation proper; do not assemble. The
output is in the form of an assembler code file for each non-
assembler input file specified.
By default, the assembler file name for a source file is made by
replacing the suffix .c, .i, etc., with .s.
Input files that don't require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler
proper. The output is in the form of preprocessed source code,
which is sent to the standard output.
Input files that don't require preprocessing are ignored.
-o file
Place output in file file. This applies to whatever sort of
output is being produced, whether it be an executable file, an
object file, an assembler file or preprocessed C code.
If -o is not specified, the default is to put an executable file
in a.out, the object file for source.suffix in source.o, its
assembler file in source.s, a precompiled header file in
source.suffix.gch, and all preprocessed C source on standard
output.
-v Print (on standard error output) the commands executed to run the
stages of compilation. Also print the version number of the
compiler driver program and of the preprocessor and the compiler
proper.
-###
Like -v except the commands are not executed and arguments are
quoted unless they contain only alphanumeric characters or
"./-_". This is useful for shell scripts to capture the driver-
generated command lines.
--help
Print (on the standard output) a description of the command-line
options understood by gcc. If the -v option is also specified
then --help is also passed on to the various processes invoked by
gcc, so that they can display the command-line options they
accept. If the -Wextra option has also been specified (prior to
the --help option), then command-line options that have no
documentation associated with them are also displayed.
--target-help
Print (on the standard output) a description of target-specific
command-line options for each tool. For some targets extra
target-specific information may also be printed.
--help={class|[^]qualifier}[,...]
Print (on the standard output) a description of the command-line
options understood by the compiler that fit into all specified
classes and qualifiers. These are the supported classes:
optimizers
Display all of the optimization options supported by the
compiler.
warnings
Display all of the options controlling warning messages
produced by the compiler.
target
Display target-specific options. Unlike the --target-help
option however, target-specific options of the linker and
assembler are not displayed. This is because those tools do
not currently support the extended --help= syntax.
params
Display the values recognized by the --param option.
language
Display the options supported for language, where language is
the name of one of the languages supported in this version of
GCC.
common
Display the options that are common to all languages.
These are the supported qualifiers:
undocumented
Display only those options that are undocumented.
joined
Display options taking an argument that appears after an
equal sign in the same continuous piece of text, such as:
--help=target.
separate
Display options taking an argument that appears as a separate
word following the original option, such as: -o output-file.
Thus for example to display all the undocumented target-specific
switches supported by the compiler, use:
--help=target,undocumented
The sense of a qualifier can be inverted by prefixing it with the
^ character, so for example to display all binary warning options
(i.e., ones that are either on or off and that do not take an
argument) that have a description, use:
--help=warnings,^joined,^undocumented
The argument to --help= should not consist solely of inverted
qualifiers.
Combining several classes is possible, although this usually
restricts the output so much that there is nothing to display.
One case where it does work, however, is when one of the classes
is target. For example, to display all the target-specific
optimization options, use:
--help=target,optimizers
The --help= option can be repeated on the command line. Each
successive use displays its requested class of options, skipping
those that have already been displayed.
If the -Q option appears on the command line before the --help=
option, then the descriptive text displayed by --help= is
changed. Instead of describing the displayed options, an
indication is given as to whether the option is enabled, disabled
or set to a specific value (assuming that the compiler knows this
at the point where the --help= option is used).
Here is a truncated example from the ARM port of gcc:
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]
The output is sensitive to the effects of previous command-line
options, so for example it is possible to find out which
optimizations are enabled at -O2 by using:
-Q -O2 --help=optimizers
Alternatively you can discover which binary optimizations are
enabled by -O3 by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
--version
Display the version number and copyrights of the invoked GCC.
-pass-exit-codes
Normally the gcc program exits with the code of 1 if any phase of
the compiler returns a non-success return code. If you specify
-pass-exit-codes, the gcc program instead returns with the
numerically highest error produced by any phase returning an
error indication. The C, C++, and Fortran front ends return 4 if
an internal compiler error is encountered.
-pipe
Use pipes rather than temporary files for communication between
the various stages of compilation. This fails to work on some
systems where the assembler is unable to read from a pipe; but
the GNU assembler has no trouble.
-specs=file
Process file after the compiler reads in the standard specs file,
in order to override the defaults which the gcc driver program
uses when determining what switches to pass to cc1, cc1plus, as,
ld, etc. More than one -specs=file can be specified on the
command line, and they are processed in order, from left to
right.
-wrapper
Invoke all subcommands under a wrapper program. The name of the
wrapper program and its parameters are passed as a comma
separated list.
gcc -c t.c -wrapper gdb,--args
This invokes all subprograms of gcc under gdb --args, thus the
invocation of cc1 is gdb --args cc1 ....
-fplugin=name.so
Load the plugin code in file name.so, assumed to be a shared
object to be dlopen'd by the compiler. The base name of the
shared object file is used to identify the plugin for the
purposes of argument parsing (See -fplugin-arg-name-key=value
below). Each plugin should define the callback functions
specified in the Plugins API.
-fplugin-arg-name-key=value
Define an argument called key with a value of value for the
plugin called name.
-fdump-ada-spec[-slim]
For C and C++ source and include files, generate corresponding
Ada specs.
-fada-spec-parent=unit
In conjunction with -fdump-ada-spec[-slim] above, generate Ada
specs as child units of parent unit.
-fdump-go-spec=file
For input files in any language, generate corresponding Go
declarations in file. This generates Go "const", "type", "var",
and "func" declarations which may be a useful way to start
writing a Go interface to code written in some other language.
@file
Read command-line options from file. The options read are
inserted in place of the original @file option. If file does not
exist, or cannot be read, then the option will be treated
literally, and not removed.
Options in file are separated by whitespace. A whitespace
character may be included in an option by surrounding the entire
option in either single or double quotes. Any character
(including a backslash) may be included by prefixing the
character to be included with a backslash. The file may itself
contain additional @file options; any such options will be
processed recursively.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc,
.cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp,
.H, or (for shared template code) .tcc; and preprocessed C++ files
use the suffix .ii. GCC recognizes files with these names and
compiles them as C++ programs even if you call the compiler the same
way as for compiling C programs (usually with the name gcc).
However, the use of gcc does not add the C++ library. g++ is a
program that calls GCC and automatically specifies linking against
the C++ library. It treats .c, .h and .i files as C++ source files
instead of C source files unless -x is used. This program is also
useful when precompiling a C header file with a .h extension for use
in C++ compilations. On many systems, g++ is also installed with the
name c++.
When you compile C++ programs, you may specify many of the same
command-line options that you use for compiling programs in any
language; or command-line options meaningful for C and related
languages; or options that are meaningful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived
from C, such as C++, Objective-C and Objective-C++) that the compiler
accepts:
-ansi
In C mode, this is equivalent to -std=c90. In C++ mode, it is
equivalent to -std=c++98.
This turns off certain features of GCC that are incompatible with
ISO C90 (when compiling C code), or of standard C++ (when
compiling C++ code), such as the "asm" and "typeof" keywords, and
predefined macros such as "unix" and "vax" that identify the type
of system you are using. It also enables the undesirable and
rarely used ISO trigraph feature. For the C compiler, it
disables recognition of C++ style // comments as well as the
"inline" keyword.
The alternate keywords "__asm__", "__extension__", "__inline__"
and "__typeof__" continue to work despite -ansi. You would not
want to use them in an ISO C program, of course, but it is useful
to put them in header files that might be included in
compilations done with -ansi. Alternate predefined macros such
as "__unix__" and "__vax__" are also available, with or without
-ansi.
The -ansi option does not cause non-ISO programs to be rejected
gratuitously. For that, -Wpedantic is required in addition to
-ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi option
is used. Some header files may notice this macro and refrain
from declaring certain functions or defining certain macros that
the ISO standard doesn't call for; this is to avoid interfering
with any programs that might use these names for other things.
Functions that are normally built in but do not have semantics
defined by ISO C (such as "alloca" and "ffs") are not built-in
functions when -ansi is used.
-std=
Determine the language standard. This option is currently only
supported when compiling C or C++.
The compiler can accept several base standards, such as c90 or
c++98, and GNU dialects of those standards, such as gnu90 or
gnu++98. When a base standard is specified, the compiler accepts
all programs following that standard plus those using GNU
extensions that do not contradict it. For example, -std=c90
turns off certain features of GCC that are incompatible with ISO
C90, such as the "asm" and "typeof" keywords, but not other GNU
extensions that do not have a meaning in ISO C90, such as
omitting the middle term of a "?:" expression. On the other hand,
when a GNU dialect of a standard is specified, all features
supported by the compiler are enabled, even when those features
change the meaning of the base standard. As a result, some
strict-conforming programs may be rejected. The particular
standard is used by -Wpedantic to identify which features are GNU
extensions given that version of the standard. For example
-std=gnu90 -Wpedantic warns about C++ style // comments, while
-std=gnu99 -Wpedantic does not.
A value for this option must be provided; possible values are
c90
c89
iso9899:1990
Support all ISO C90 programs (certain GNU extensions that
conflict with ISO C90 are disabled). Same as -ansi for C
code.
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. This standard is substantially completely
supported, modulo bugs and floating-point issues (mainly but
not entirely relating to optional C99 features from Annexes F
and G). See <http://gcc.gnu.org/c99status.html > for more
information. The names c9x and iso9899:199x are deprecated.
c11
c1x
iso9899:2011
ISO C11, the 2011 revision of the ISO C standard. This
standard is substantially completely supported, modulo bugs,
floating-point issues (mainly but not entirely relating to
optional C11 features from Annexes F and G) and the optional
Annexes K (Bounds-checking interfaces) and L (Analyzability).
The name c1x is deprecated.
gnu90
gnu89
GNU dialect of ISO C90 (including some C99 features).
gnu99
gnu9x
GNU dialect of ISO C99. The name gnu9x is deprecated.
gnu11
gnu1x
GNU dialect of ISO C11. This is the default for C code. The
name gnu1x is deprecated.
c++98
c++03
The 1998 ISO C++ standard plus the 2003 technical corrigendum
and some additional defect reports. Same as -ansi for C++
code.
gnu++98
gnu++03
GNU dialect of -std=c++98.
c++11
c++0x
The 2011 ISO C++ standard plus amendments. The name c++0x is
deprecated.
gnu++11
gnu++0x
GNU dialect of -std=c++11. The name gnu++0x is deprecated.
c++14
c++1y
The 2014 ISO C++ standard plus amendments. The name c++1y is
deprecated.
gnu++14
gnu++1y
GNU dialect of -std=c++14. This is the default for C++ code.
The name gnu++1y is deprecated.
c++1z
The next revision of the ISO C++ standard, tentatively
planned for 2017. Support is highly experimental, and will
almost certainly change in incompatible ways in future
releases.
gnu++1z
GNU dialect of -std=c++1z. Support is highly experimental,
and will almost certainly change in incompatible ways in
future releases.
-fgnu89-inline
The option -fgnu89-inline tells GCC to use the traditional GNU
semantics for "inline" functions when in C99 mode.
Using this option is roughly equivalent to adding the
"gnu_inline" function attribute to all inline functions.
The option -fno-gnu89-inline explicitly tells GCC to use the C99
semantics for "inline" when in C99 or gnu99 mode (i.e., it
specifies the default behavior). This option is not supported in
-std=c90 or -std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and
"__GNUC_STDC_INLINE__" may be used to check which semantics are
in effect for "inline" functions.
-fpermitted-flt-eval-methods=style
ISO/IEC TS 18661-3 defines new permissible values for
"FLT_EVAL_METHOD" that indicate that operations and constants
with a semantic type that is an interchange or extended format
should be evaluated to the precision and range of that type.
These new values are a superset of those permitted under C99/C11,
which does not specify the meaning of other positive values of
"FLT_EVAL_METHOD". As such, code conforming to C11 may not have
been written expecting the possibility of the new values.
-fpermitted-flt-eval-methods specifies whether the compiler
should allow only the values of "FLT_EVAL_METHOD" specified in
C99/C11, or the extended set of values specified in ISO/IEC TS
18661-3.
style is either "c11" or "ts-18661-3" as appropriate.
The default when in a standards compliant mode (-std=c11 or
similar) is -fpermitted-flt-eval-methods=c11. The default when
in a GNU dialect (-std=gnu11 or similar) is
-fpermitted-flt-eval-methods=ts-18661-3.
-aux-info filename
Output to the given filename prototyped declarations for all
functions declared and/or defined in a translation unit,
including those in header files. This option is silently ignored
in any language other than C.
Besides declarations, the file indicates, in comments, the origin
of each declaration (source file and line), whether the
declaration was implicit, prototyped or unprototyped (I, N for
new or O for old, respectively, in the first character after the
line number and the colon), and whether it came from a
declaration or a definition (C or F, respectively, in the
following character). In the case of function definitions, a
K&R-style list of arguments followed by their declarations is
also provided, inside comments, after the declaration.
-fallow-parameterless-variadic-functions
Accept variadic functions without named parameters.
Although it is possible to define such a function, this is not
very useful as it is not possible to read the arguments. This is
only supported for C as this construct is allowed by C++.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so
that code can use these words as identifiers. You can use the
keywords "__asm__", "__inline__" and "__typeof__" instead. -ansi
implies -fno-asm.
In C++, this switch only affects the "typeof" keyword, since
"asm" and "inline" are standard keywords. You may want to use
the -fno-gnu-keywords flag instead, which has the same effect.
In C99 mode (-std=c99 or -std=gnu99), this switch only affects
the "asm" and "typeof" keywords, since "inline" is a standard
keyword in ISO C99.
-fno-builtin
-fno-builtin-function
Don't recognize built-in functions that do not begin with
__builtin_ as prefix.
GCC normally generates special code to handle certain built-in
functions more efficiently; for instance, calls to "alloca" may
become single instructions which adjust the stack directly, and
calls to "memcpy" may become inline copy loops. The resulting
code is often both smaller and faster, but since the function
calls no longer appear as such, you cannot set a breakpoint on
those calls, nor can you change the behavior of the functions by
linking with a different library. In addition, when a function
is recognized as a built-in function, GCC may use information
about that function to warn about problems with calls to that
function, or to generate more efficient code, even if the
resulting code still contains calls to that function. For
example, warnings are given with -Wformat for bad calls to
"printf" when "printf" is built in and "strlen" is known not to
modify global memory.
With the -fno-builtin-function option only the built-in function
function is disabled. function must not begin with __builtin_.
If a function is named that is not built-in in this version of
GCC, this option is ignored. There is no corresponding
-fbuiltin-function option; if you wish to enable built-in
functions selectively when using -fno-builtin or -ffreestanding,
you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fgimple
Enable parsing of function definitions marked with "__GIMPLE".
This is an experimental feature that allows unit testing of
GIMPLE passes.
-fhosted
Assert that compilation targets a hosted environment. This
implies -fbuiltin. A hosted environment is one in which the
entire standard library is available, and in which "main" has a
return type of "int". Examples are nearly everything except a
kernel. This is equivalent to -fno-freestanding.
-ffreestanding
Assert that compilation targets a freestanding environment. This
implies -fno-builtin. A freestanding environment is one in which
the standard library may not exist, and program startup may not
necessarily be at "main". The most obvious example is an OS
kernel. This is equivalent to -fno-hosted.
-fopenacc
Enable handling of OpenACC directives "#pragma acc" in C/C++ and
"!$acc" in Fortran. When -fopenacc is specified, the compiler
generates accelerated code according to the OpenACC Application
Programming Interface v2.0 <http://www.openacc.org/ >. This
option implies -pthread, and thus is only supported on targets
that have support for -pthread.
-fopenacc-dim=geom
Specify default compute dimensions for parallel offload regions
that do not explicitly specify. The geom value is a triple of
':'-separated sizes, in order 'gang', 'worker' and, 'vector'. A
size can be omitted, to use a target-specific default value.
-fopenmp
Enable handling of OpenMP directives "#pragma omp" in C/C++ and
"!$omp" in Fortran. When -fopenmp is specified, the compiler
generates parallel code according to the OpenMP Application
Program Interface v4.5 <http://www.openmp.org/ >. This option
implies -pthread, and thus is only supported on targets that have
support for -pthread. -fopenmp implies -fopenmp-simd.
-fopenmp-simd
Enable handling of OpenMP's SIMD directives with "#pragma omp" in
C/C++ and "!$omp" in Fortran. Other OpenMP directives are
ignored.
-fcilkplus
Enable the usage of Cilk Plus language extension features for
C/C++. When the option -fcilkplus is specified, enable the usage
of the Cilk Plus Language extension features for C/C++. The
present implementation follows ABI version 1.2. This is an
experimental feature that is only partially complete, and whose
interface may change in future versions of GCC as the official
specification changes. Currently, all features but "_Cilk_for"
have been implemented.
-fgnu-tm
When the option -fgnu-tm is specified, the compiler generates
code for the Linux variant of Intel's current Transactional
Memory ABI specification document (Revision 1.1, May 6 2009).
This is an experimental feature whose interface may change in
future versions of GCC, as the official specification changes.
Please note that not all architectures are supported for this
feature.
For more information on GCC's support for transactional memory,
Note that the transactional memory feature is not supported with
non-call exceptions (-fnon-call-exceptions).
-fms-extensions
Accept some non-standard constructs used in Microsoft header
files.
In C++ code, this allows member names in structures to be similar
to previous types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
};
Some cases of unnamed fields in structures and unions are only
accepted with this option.
Note that this option is off for all targets but x86 targets
using ms-abi.
-fplan9-extensions
Accept some non-standard constructs used in Plan 9 code.
This enables -fms-extensions, permits passing pointers to
structures with anonymous fields to functions that expect
pointers to elements of the type of the field, and permits
referring to anonymous fields declared using a typedef. This
is only supported for C, not C++.
-fcond-mismatch
Allow conditional expressions with mismatched types in the second
and third arguments. The value of such an expression is void.
This option is not supported for C++.
-flax-vector-conversions
Allow implicit conversions between vectors with differing numbers
of elements and/or incompatible element types. This option
should not be used for new code.
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should be. It
is either like "unsigned char" by default or like "signed char"
by default.
Ideally, a portable program should always use "signed char" or
"unsigned char" when it depends on the signedness of an object.
But many programs have been written to use plain "char" and
expect it to be signed, or expect it to be unsigned, depending on
the machines they were written for. This option, and its
inverse, let you make such a program work with the opposite
default.
The type "char" is always a distinct type from each of "signed
char" or "unsigned char", even though its behavior is always just
like one of those two.
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is the
negative form of -funsigned-char. Likewise, the option
-fno-signed-char is equivalent to -funsigned-char.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned,
when the declaration does not use either "signed" or "unsigned".
By default, such a bit-field is signed, because this is
consistent: the basic integer types such as "int" are signed
types.
-fsso-struct=endianness
Set the default scalar storage order of structures and unions to
the specified endianness. The accepted values are big-endian,
little-endian and native for the native endianness of the target
(the default). This option is not supported for C++.
Warning: the -fsso-struct switch causes GCC to generate code that
is not binary compatible with code generated without it if the
specified endianness is not the native endianness of the target.
Options Controlling C++ Dialect
This section describes the command-line options that are only
meaningful for C++ programs. You can also use most of the GNU
compiler options regardless of what language your program is in. For
example, you might compile a file firstClass.C like this:
g++ -g -fstrict-enums -O -c firstClass.C
In this example, only -fstrict-enums is an option meant only for C++
programs; you can use the other options with any language supported
by GCC.
Some options for compiling C programs, such as -std, are also
relevant for C++ programs.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. The default is version 0.
Version 0 refers to the version conforming most closely to the
C++ ABI specification. Therefore, the ABI obtained using version
0 will change in different versions of G++ as ABI bugs are fixed.
Version 1 is the version of the C++ ABI that first appeared in
G++ 3.2.
Version 2 is the version of the C++ ABI that first appeared in
G++ 3.4, and was the default through G++ 4.9.
Version 3 corrects an error in mangling a constant address as a
template argument.
Version 4, which first appeared in G++ 4.5, implements a standard
mangling for vector types.
Version 5, which first appeared in G++ 4.6, corrects the mangling
of attribute const/volatile on function pointer types, decltype
of a plain decl, and use of a function parameter in the
declaration of another parameter.
Version 6, which first appeared in G++ 4.7, corrects the
promotion behavior of C++11 scoped enums and the mangling of
template argument packs, const/static_cast, prefix ++ and --, and
a class scope function used as a template argument.
Version 7, which first appeared in G++ 4.8, that treats nullptr_t
as a builtin type and corrects the mangling of lambdas in default
argument scope.
Version 8, which first appeared in G++ 4.9, corrects the
substitution behavior of function types with function-cv-
qualifiers.
Version 9, which first appeared in G++ 5.2, corrects the
alignment of "nullptr_t".
Version 10, which first appeared in G++ 6.1, adds mangling of
attributes that affect type identity, such as ia32 calling
convention attributes (e.g. stdcall).
Version 11, which first appeared in G++ 7, corrects the mangling
of sizeof... expressions and operator names. For multiple
entities with the same name within a function, that are declared
in different scopes, the mangling now changes starting with the
twelfth occurrence. It also implies -fnew-inheriting-ctors.
See also -Wabi.
-fabi-compat-version=n
On targets that support strong aliases, G++ works around mangling
changes by creating an alias with the correct mangled name when
defining a symbol with an incorrect mangled name. This switch
specifies which ABI version to use for the alias.
With -fabi-version=0 (the default), this defaults to 8 (GCC 5
compatibility). If another ABI version is explicitly selected,
this defaults to 0. For compatibility with GCC versions 3.2
through 4.9, use -fabi-compat-version=2.
If this option is not provided but -Wabi=n is, that version is
used for compatibility aliases. If this option is provided along
with -Wabi (without the version), the version from this option is
used for the warning.
-fno-access-control
Turn off all access checking. This switch is mainly useful for
working around bugs in the access control code.
-faligned-new
Enable support for C++17 "new" of types that require more
alignment than "void* ::operator new(std::size_t)" provides. A
numeric argument such as "-faligned-new=32" can be used to
specify how much alignment (in bytes) is provided by that
function, but few users will need to override the default of
"alignof(std::max_align_t)".
-fcheck-new
Check that the pointer returned by "operator new" is non-null
before attempting to modify the storage allocated. This check is
normally unnecessary because the C++ standard specifies that
"operator new" only returns 0 if it is declared "throw()", in
which case the compiler always checks the return value even
without this option. In all other cases, when "operator new" has
a non-empty exception specification, memory exhaustion is
signalled by throwing "std::bad_alloc". See also new (nothrow).
-fconcepts
Enable support for the C++ Extensions for Concepts Technical
Specification, ISO 19217 (2015), which allows code like
template <class T> concept bool Addable = requires (T t) { t + t; };
template <Addable T> T add (T a, T b) { return a + b; }
-fconstexpr-depth=n
Set the maximum nested evaluation depth for C++11 constexpr
functions to n. A limit is needed to detect endless recursion
during constant expression evaluation. The minimum specified by
the standard is 512.
-fconstexpr-loop-limit=n
Set the maximum number of iterations for a loop in C++14
constexpr functions to n. A limit is needed to detect infinite
loops during constant expression evaluation. The default is
262144 (1<<18).
-fdeduce-init-list
Enable deduction of a template type parameter as
"std::initializer_list" from a brace-enclosed initializer list,
i.e.
template <class T> auto forward(T t) -> decltype (realfn (t))
{
return realfn (t);
}
void f()
{
forward({1,2}); // call forward<std::initializer_list<int>>
}
This deduction was implemented as a possible extension to the
originally proposed semantics for the C++11 standard, but was not
part of the final standard, so it is disabled by default. This
option is deprecated, and may be removed in a future version of
G++.
-ffriend-injection
Inject friend functions into the enclosing namespace, so that
they are visible outside the scope of the class in which they are
declared. Friend functions were documented to work this way in
the old Annotated C++ Reference Manual. However, in ISO C++ a
friend function that is not declared in an enclosing scope can
only be found using argument dependent lookup. GCC defaults to
the standard behavior.
This option is for compatibility, and may be removed in a future
release of G++.
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a
temporary that is only used to initialize another object of the
same type. Specifying this option disables that optimization,
and forces G++ to call the copy constructor in all cases. This
option also causes G++ to call trivial member functions which
otherwise would be expanded inline.
In C++17, the compiler is required to omit these temporaries, but
this option still affects trivial member functions.
-fno-enforce-eh-specs
Don't generate code to check for violation of exception
specifications at run time. This option violates the C++
standard, but may be useful for reducing code size in production
builds, much like defining "NDEBUG". This does not give user
code permission to throw exceptions in violation of the exception
specifications; the compiler still optimizes based on the
specifications, so throwing an unexpected exception results in
undefined behavior at run time.
-fextern-tls-init
-fno-extern-tls-init
The C++11 and OpenMP standards allow "thread_local" and
"threadprivate" variables to have dynamic (runtime)
initialization. To support this, any use of such a variable goes
through a wrapper function that performs any necessary
initialization. When the use and definition of the variable are
in the same translation unit, this overhead can be optimized
away, but when the use is in a different translation unit there
is significant overhead even if the variable doesn't actually
need dynamic initialization. If the programmer can be sure that
no use of the variable in a non-defining TU needs to trigger
dynamic initialization (either because the variable is statically
initialized, or a use of the variable in the defining TU will be
executed before any uses in another TU), they can avoid this
overhead with the -fno-extern-tls-init option.
On targets that support symbol aliases, the default is
-fextern-tls-init. On targets that do not support symbol
aliases, the default is -fno-extern-tls-init.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a
for-init-statement is limited to the "for" loop itself, as
specified by the C++ standard. If -fno-for-scope is specified,
the scope of variables declared in a for-init-statement extends
to the end of the enclosing scope, as was the case in old
versions of G++, and other (traditional) implementations of C++.
If neither flag is given, the default is to follow the standard,
but to allow and give a warning for old-style code that would
otherwise be invalid, or have different behavior.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this
word as an identifier. You can use the keyword "__typeof__"
instead. This option is implied by the strict ISO C++ dialects:
-ansi, -std=c++98, -std=c++11, etc.
-fno-implicit-templates
Never emit code for non-inline templates that are instantiated
implicitly (i.e. by use); only emit code for explicit
instantiations.
-fno-implicit-inline-templates
Don't emit code for implicit instantiations of inline templates,
either. The default is to handle inlines differently so that
compiles with and without optimization need the same set of
explicit instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions
controlled by "#pragma implementation". This causes linker
errors if these functions are not inlined everywhere they are
called.
-fms-extensions
Disable Wpedantic warnings about constructs used in MFC, such as
implicit int and getting a pointer to member function via non-
standard syntax.
-fnew-inheriting-ctors
Enable the P0136 adjustment to the semantics of C++11 constructor
inheritance. This is part of C++17 but also considered to be a
Defect Report against C++11 and C++14. This flag is enabled by
default unless -fabi-version=10 or lower is specified.
-fnew-ttp-matching
Enable the P0522 resolution to Core issue 150, template template
parameters and default arguments: this allows a template with
default template arguments as an argument for a template template
parameter with fewer template parameters. This flag is enabled
by default for -std=c++1z.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated
by ANSI/ISO C. These include "ffs", "alloca", "_exit", "index",
"bzero", "conjf", and other related functions.
-fnothrow-opt
Treat a "throw()" exception specification as if it were a
"noexcept" specification to reduce or eliminate the text size
overhead relative to a function with no exception specification.
If the function has local variables of types with non-trivial
destructors, the exception specification actually makes the
function smaller because the EH cleanups for those variables can
be optimized away. The semantic effect is that an exception
thrown out of a function with such an exception specification
results in a call to "terminate" rather than "unexpected".
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor",
"compl", "not", "or" and "xor" as synonyms as keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not
need to issue. Currently, the only such diagnostic issued by G++
is the one for a name having multiple meanings within a class.
-fpermissive
Downgrade some diagnostics about nonconformant code from errors
to warnings. Thus, using -fpermissive allows some nonconforming
code to compile.
-fno-pretty-templates
When an error message refers to a specialization of a function
template, the compiler normally prints the signature of the
template followed by the template arguments and any typedefs or
typenames in the signature (e.g. "void f(T) [with T = int]"
rather than "void f(int)") so that it's clear which template is
involved. When an error message refers to a specialization of a
class template, the compiler omits any template arguments that
match the default template arguments for that template. If
either of these behaviors make it harder to understand the error
message rather than easier, you can use -fno-pretty-templates to
disable them.
-frepo
Enable automatic template instantiation at link time. This
option also implies -fno-implicit-templates.
-fno-rtti
Disable generation of information about every class with virtual
functions for use by the C++ run-time type identification
features ("dynamic_cast" and "typeid"). If you don't use those
parts of the language, you can save some space by using this
flag. Note that exception handling uses the same information,
but G++ generates it as needed. The "dynamic_cast" operator can
still be used for casts that do not require run-time type
information, i.e. casts to "void *" or to unambiguous base
classes.
-fsized-deallocation
Enable the built-in global declarations
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
as introduced in C++14. This is useful for user-defined
replacement deallocation functions that, for example, use the
size of the object to make deallocation faster. Enabled by
default under -std=c++14 and above. The flag
-Wsized-deallocation warns about places that might want to add a
definition.
-fstrict-enums
Allow the compiler to optimize using the assumption that a value
of enumerated type can only be one of the values of the
enumeration (as defined in the C++ standard; basically, a value
that can be represented in the minimum number of bits needed to
represent all the enumerators). This assumption may not be valid
if the program uses a cast to convert an arbitrary integer value
to the enumerated type.
-fstrong-eval-order
Evaluate member access, array subscripting, and shift expressions
in left-to-right order, and evaluate assignment in right-to-left
order, as adopted for C++17. Enabled by default with -std=c++1z.
-fstrong-eval-order=some enables just the ordering of member
access and shift expressions, and is the default without
-std=c++1z.
-ftemplate-backtrace-limit=n
Set the maximum number of template instantiation notes for a
single warning or error to n. The default value is 10.
-ftemplate-depth=n
Set the maximum instantiation depth for template classes to n. A
limit on the template instantiation depth is needed to detect
endless recursions during template class instantiation. ANSI/ISO
C++ conforming programs must not rely on a maximum depth greater
than 17 (changed to 1024 in C++11). The default value is 900, as
the compiler can run out of stack space before hitting 1024 in
some situations.
-fno-threadsafe-statics
Do not emit the extra code to use the routines specified in the
C++ ABI for thread-safe initialization of local statics. You can
use this option to reduce code size slightly in code that doesn't
need to be thread-safe.
-fuse-cxa-atexit
Register destructors for objects with static storage duration
with the "__cxa_atexit" function rather than the "atexit"
function. This option is required for fully standards-compliant
handling of static destructors, but only works if your C library
supports "__cxa_atexit".
-fno-use-cxa-get-exception-ptr
Don't use the "__cxa_get_exception_ptr" runtime routine. This
causes "std::uncaught_exception" to be incorrect, but is
necessary if the runtime routine is not available.
-fvisibility-inlines-hidden
This switch declares that the user does not attempt to compare
pointers to inline functions or methods where the addresses of
the two functions are taken in different shared objects.
The effect of this is that GCC may, effectively, mark inline
methods with "__attribute__ ((visibility ("hidden")))" so that
they do not appear in the export table of a DSO and do not
require a PLT indirection when used within the DSO. Enabling
this option can have a dramatic effect on load and link times of
a DSO as it massively reduces the size of the dynamic export
table when the library makes heavy use of templates.
The behavior of this switch is not quite the same as marking the
methods as hidden directly, because it does not affect static
variables local to the function or cause the compiler to deduce
that the function is defined in only one shared object.
You may mark a method as having a visibility explicitly to negate
the effect of the switch for that method. For example, if you do
want to compare pointers to a particular inline method, you might
mark it as having default visibility. Marking the enclosing
class with explicit visibility has no effect.
Explicitly instantiated inline methods are unaffected by this
option as their linkage might otherwise cross a shared library
boundary.
-fvisibility-ms-compat
This flag attempts to use visibility settings to make GCC's C++
linkage model compatible with that of Microsoft Visual Studio.
The flag makes these changes to GCC's linkage model:
1. It sets the default visibility to "hidden", like
-fvisibility=hidden.
2. Types, but not their members, are not hidden by default.
3. The One Definition Rule is relaxed for types without explicit
visibility specifications that are defined in more than one
shared object: those declarations are permitted if they are
permitted when this option is not used.
In new code it is better to use -fvisibility=hidden and export
those classes that are intended to be externally visible.
Unfortunately it is possible for code to rely, perhaps
accidentally, on the Visual Studio behavior.
Among the consequences of these changes are that static data
members of the same type with the same name but defined in
different shared objects are different, so changing one does not
change the other; and that pointers to function members defined
in different shared objects may not compare equal. When this
flag is given, it is a violation of the ODR to define types with
the same name differently.
-fno-weak
Do not use weak symbol support, even if it is provided by the
linker. By default, G++ uses weak symbols if they are available.
This option exists only for testing, and should not be used by
end-users; it results in inferior code and has no benefits. This
option may be removed in a future release of G++.
-nostdinc++
Do not search for header files in the standard directories
specific to C++, but do still search the other standard
directories. (This option is used when building the C++
library.)
In addition, these optimization, warning, and code generation options
have meanings only for C++ programs:
-Wabi (C, Objective-C, C++ and Objective-C++ only)
Warn when G++ it generates code that is probably not compatible
with the vendor-neutral C++ ABI. Since G++ now defaults to
updating the ABI with each major release, normally -Wabi will
warn only if there is a check added later in a release series for
an ABI issue discovered since the initial release. -Wabi will
warn about more things if an older ABI version is selected (with
-fabi-version=n).
-Wabi can also be used with an explicit version number to warn
about compatibility with a particular -fabi-version level, e.g.
-Wabi=2 to warn about changes relative to -fabi-version=2.
If an explicit version number is provided and
-fabi-compat-version is not specified, the version number from
this option is used for compatibility aliases. If no explicit
version number is provided with this option, but
-fabi-compat-version is specified, that version number is used
for ABI warnings.
Although an effort has been made to warn about all such cases,
there are probably some cases that are not warned about, even
though G++ is generating incompatible code. There may also be
cases where warnings are emitted even though the code that is
generated is compatible.
You should rewrite your code to avoid these warnings if you are
concerned about the fact that code generated by G++ may not be
binary compatible with code generated by other compilers.
Known incompatibilities in -fabi-version=2 (which was the default
from GCC 3.4 to 4.9) include:
* A template with a non-type template parameter of reference
type was mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}
This was fixed in -fabi-version=3.
* SIMD vector types declared using "__attribute
((vector_size))" were mangled in a non-standard way that does
not allow for overloading of functions taking vectors of
different sizes.
The mangling was changed in -fabi-version=4.
* "__attribute ((const))" and "noreturn" were mangled as type
qualifiers, and "decltype" of a plain declaration was folded
away.
These mangling issues were fixed in -fabi-version=5.
* Scoped enumerators passed as arguments to a variadic function
are promoted like unscoped enumerators, causing "va_arg" to
complain. On most targets this does not actually affect the
parameter passing ABI, as there is no way to pass an argument
smaller than "int".
Also, the ABI changed the mangling of template argument
packs, "const_cast", "static_cast", prefix
increment/decrement, and a class scope function used as a
template argument.
These issues were corrected in -fabi-version=6.
* Lambdas in default argument scope were mangled incorrectly,
and the ABI changed the mangling of "nullptr_t".
These issues were corrected in -fabi-version=7.
* When mangling a function type with function-cv-qualifiers,
the un-qualified function type was incorrectly treated as a
substitution candidate.
This was fixed in -fabi-version=8, the default for GCC 5.1.
* "decltype(nullptr)" incorrectly had an alignment of 1,
leading to unaligned accesses. Note that this did not affect
the ABI of a function with a "nullptr_t" parameter, as
parameters have a minimum alignment.
This was fixed in -fabi-version=9, the default for GCC 5.2.
* Target-specific attributes that affect the identity of a
type, such as ia32 calling conventions on a function type
(stdcall, regparm, etc.), did not affect the mangled name,
leading to name collisions when function pointers were used
as template arguments.
This was fixed in -fabi-version=10, the default for GCC 6.1.
It also warns about psABI-related changes. The known psABI
changes at this point include:
* For SysV/x86-64, unions with "long double" members are passed
in memory as specified in psABI. For example:
union U {
long double ld;
int i;
};
"union U" is always passed in memory.
-Wabi-tag (C++ and Objective-C++ only)
Warn when a type with an ABI tag is used in a context that does
not have that ABI tag. See C++ Attributes for more information
about ABI tags.
-Wctor-dtor-privacy (C++ and Objective-C++ only)
Warn when a class seems unusable because all the constructors or
destructors in that class are private, and it has neither friends
nor public static member functions. Also warn if there are no
non-private methods, and there's at least one private member
function that isn't a constructor or destructor.
-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
Warn when "delete" is used to destroy an instance of a class that
has virtual functions and non-virtual destructor. It is unsafe to
delete an instance of a derived class through a pointer to a base
class if the base class does not have a virtual destructor. This
warning is enabled by -Wall.
-Wliteral-suffix (C++ and Objective-C++ only)
Warn when a string or character literal is followed by a ud-
suffix which does not begin with an underscore. As a conforming
extension, GCC treats such suffixes as separate preprocessing
tokens in order to maintain backwards compatibility with code
that uses formatting macros from "<inttypes.h>". For example:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
printf("My int64: %" PRId64"\n", i64);
}
In this case, "PRId64" is treated as a separate preprocessing
token.
Additionally, warn when a user-defined literal operator is
declared with a literal suffix identifier that doesn't begin with
an underscore. Literal suffix identifiers that don't begin with
an underscore are reserved for future standardization.
This warning is enabled by default.
-Wlto-type-mismatch
During the link-time optimization warn about type mismatches in
global declarations from different compilation units. Requires
-flto to be enabled. Enabled by default.
-Wno-narrowing (C++ and Objective-C++ only)
For C++11 and later standards, narrowing conversions are
diagnosed by default, as required by the standard. A narrowing
conversion from a constant produces an error, and a narrowing
conversion from a non-constant produces a warning, but
-Wno-narrowing suppresses the diagnostic. Note that this does
not affect the meaning of well-formed code; narrowing conversions
are still considered ill-formed in SFINAE contexts.
With -Wnarrowing in C++98, warn when a narrowing conversion
prohibited by C++11 occurs within { }, e.g.
int i = { 2.2 }; // error: narrowing from double to int
This flag is included in -Wall and -Wc++11-compat.
-Wnoexcept (C++ and Objective-C++ only)
Warn when a noexcept-expression evaluates to false because of a
call to a function that does not have a non-throwing exception
specification (i.e. "throw()" or "noexcept") but is known by the
compiler to never throw an exception.
-Wnoexcept (C++ and Objective-C++ only)
Warn if the C++1z feature making "noexcept" part of a function
type changes the mangled name of a symbol relative to C++14.
Enabled by -Wabi and -Wc++1z-compat.
template <class T> void f(T t) { t(); };
void g() noexcept;
void h() { f(g); } // in C++14 calls f<void(*)()>, in C++1z calls f<void(*)()noexcept>
-Wnon-virtual-dtor (C++ and Objective-C++ only)
Warn when a class has virtual functions and an accessible non-
virtual destructor itself or in an accessible polymorphic base
class, in which case it is possible but unsafe to delete an
instance of a derived class through a pointer to the class itself
or base class. This warning is automatically enabled if -Weffc++
is specified.
-Wregister (C++ and Objective-C++ only)
Warn on uses of the "register" storage class specifier, except
when it is part of the GNU Explicit Register Variables extension.
The use of the "register" keyword as storage class specifier has
been deprecated in C++11 and removed in C++17. Enabled by
default with -std=c++1z.
-Wreorder (C++ and Objective-C++ only)
Warn when the order of member initializers given in the code does
not match the order in which they must be executed. For
instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler rearranges the member initializers for "i" and "j"
to match the declaration order of the members, emitting a warning
to that effect. This warning is enabled by -Wall.
-fext-numeric-literals (C++ and Objective-C++ only)
Accept imaginary, fixed-point, or machine-defined literal number
suffixes as GNU extensions. When this option is turned off these
suffixes are treated as C++11 user-defined literal numeric
suffixes. This is on by default for all pre-C++11 dialects and
all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11,
-std=gnu++14. This option is off by default for ISO C++11
onwards (-std=c++11, ...).
The following -W... options are not affected by -Wall.
-Weffc++ (C++ and Objective-C++ only)
Warn about violations of the following style guidelines from
Scott Meyers' Effective C++ series of books:
* Define a copy constructor and an assignment operator for
classes with dynamically-allocated memory.
* Prefer initialization to assignment in constructors.
* Have "operator=" return a reference to *this.
* Don't try to return a reference when you must return an
object.
* Distinguish between prefix and postfix forms of increment and
decrement operators.
* Never overload "&&", "||", or ",".
This option also enables -Wnon-virtual-dtor, which is also one of
the effective C++ recommendations. However, the check is
extended to warn about the lack of virtual destructor in
accessible non-polymorphic bases classes too.
When selecting this option, be aware that the standard library
headers do not obey all of these guidelines; use grep -v to
filter out those warnings.
-Wstrict-null-sentinel (C++ and Objective-C++ only)
Warn about the use of an uncasted "NULL" as sentinel. When
compiling only with GCC this is a valid sentinel, as "NULL" is
defined to "__null". Although it is a null pointer constant
rather than a null pointer, it is guaranteed to be of the same
size as a pointer. But this use is not portable across different
compilers.
-Wno-non-template-friend (C++ and Objective-C++ only)
Disable warnings when non-template friend functions are declared
within a template. In very old versions of GCC that predate
implementation of the ISO standard, declarations such as friend
int foo(int), where the name of the friend is an unqualified-id,
could be interpreted as a particular specialization of a template
function; the warning exists to diagnose compatibility problems,
and is enabled by default.
-Wold-style-cast (C++ and Objective-C++ only)
Warn if an old-style (C-style) cast to a non-void type is used
within a C++ program. The new-style casts ("dynamic_cast",
"static_cast", "reinterpret_cast", and "const_cast") are less
vulnerable to unintended effects and much easier to search for.
-Woverloaded-virtual (C++ and Objective-C++ only)
Warn when a function declaration hides virtual functions from a
base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();
fails to compile.
-Wno-pmf-conversions (C++ and Objective-C++ only)
Disable the diagnostic for converting a bound pointer to member
function to a plain pointer.
-Wsign-promo (C++ and Objective-C++ only)
Warn when overload resolution chooses a promotion from unsigned
or enumerated type to a signed type, over a conversion to an
unsigned type of the same size. Previous versions of G++ tried
to preserve unsignedness, but the standard mandates the current
behavior.
-Wtemplates (C++ and Objective-C++ only)
Warn when a primary template declaration is encountered. Some
coding rules disallow templates, and this may be used to enforce
that rule. The warning is inactive inside a system header file,
such as the STL, so one can still use the STL. One may also
instantiate or specialize templates.
-Wmultiple-inheritance (C++ and Objective-C++ only)
Warn when a class is defined with multiple direct base classes.
Some coding rules disallow multiple inheritance, and this may be
used to enforce that rule. The warning is inactive inside a
system header file, such as the STL, so one can still use the
STL. One may also define classes that indirectly use multiple
inheritance.
-Wvirtual-inheritance
Warn when a class is defined with a virtual direct base class.
Some coding rules disallow multiple inheritance, and this may be
used to enforce that rule. The warning is inactive inside a
system header file, such as the STL, so one can still use the
STL. One may also define classes that indirectly use virtual
inheritance.
-Wnamespaces
Warn when a namespace definition is opened. Some coding rules
disallow namespaces, and this may be used to enforce that rule.
The warning is inactive inside a system header file, such as the
STL, so one can still use the STL. One may also use using
directives and qualified names.
-Wno-terminate (C++ and Objective-C++ only)
Disable the warning about a throw-expression that will
immediately result in a call to "terminate".
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and
Objective-C++ languages themselves.
This section describes the command-line options that are only
meaningful for Objective-C and Objective-C++ programs. You can also
use most of the language-independent GNU compiler options. For
example, you might compile a file some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-
C and Objective-C++ programs; you can use the other options with any
language supported by GCC.
Note that since Objective-C is an extension of the C language,
Objective-C compilations may also use options specific to the C
front-end (e.g., -Wtraditional). Similarly, Objective-C++
compilations may use C++-specific options (e.g., -Wabi).
Here is a list of options that are only for compiling Objective-C and
Objective-C++ programs:
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate for each
literal string specified with the syntax "@"..."". The default
class name is "NXConstantString" if the GNU runtime is being
used, and "NSConstantString" if the NeXT runtime is being used
(see below). The -fconstant-cfstrings option, if also present,
overrides the -fconstant-string-class setting and cause "@"...""
literals to be laid out as constant CoreFoundation strings.
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C
runtime. This is the default for most types of systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the
default for NeXT-based systems, including Darwin and Mac OS X.
The macro "__NEXT_RUNTIME__" is predefined if (and only if) this
option is used.
-fno-nil-receivers
Assume that all Objective-C message dispatches ("[receiver
message:arg]") in this translation unit ensure that the receiver
is not "nil". This allows for more efficient entry points in the
runtime to be used. This option is only available in conjunction
with the NeXT runtime and ABI version 0 or 1.
-fobjc-abi-version=n
Use version n of the Objective-C ABI for the selected runtime.
This option is currently supported only for the NeXT runtime. In
that case, Version 0 is the traditional (32-bit) ABI without
support for properties and other Objective-C 2.0 additions.
Version 1 is the traditional (32-bit) ABI with support for
properties and other Objective-C 2.0 additions. Version 2 is the
modern (64-bit) ABI. If nothing is specified, the default is
Version 0 on 32-bit target machines, and Version 2 on 64-bit
target machines.
-fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance
variables is a C++ object with a non-trivial default constructor.
If so, synthesize a special "- (id) .cxx_construct" instance
method which runs non-trivial default constructors on any such
instance variables, in order, and then return "self". Similarly,
check if any instance variable is a C++ object with a non-trivial
destructor, and if so, synthesize a special "- (void)
.cxx_destruct" method which runs all such default destructors, in
reverse order.
The "- (id) .cxx_construct" and "- (void) .cxx_destruct" methods
thusly generated only operate on instance variables declared in
the current Objective-C class, and not those inherited from
superclasses. It is the responsibility of the Objective-C
runtime to invoke all such methods in an object's inheritance
hierarchy. The "- (id) .cxx_construct" methods are invoked by
the runtime immediately after a new object instance is allocated;
the "- (void) .cxx_destruct" methods are invoked immediately
before the runtime deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and
later has support for invoking the "- (id) .cxx_construct" and "-
(void) .cxx_destruct" methods.
-fobjc-direct-dispatch
Allow fast jumps to the message dispatcher. On Darwin this is
accomplished via the comm page.
-fobjc-exceptions
Enable syntactic support for structured exception handling in
Objective-C, similar to what is offered by C++. This option is
required to use the Objective-C keywords @try, @throw, @catch,
@finally and @synchronized. This option is available with both
the GNU runtime and the NeXT runtime (but not available in
conjunction with the NeXT runtime on Mac OS X 10.2 and earlier).
-fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C++
programs. This option is only available with the NeXT runtime;
the GNU runtime has a different garbage collection implementation
that does not require special compiler flags.
-fobjc-nilcheck
For the NeXT runtime with version 2 of the ABI, check for a nil
receiver in method invocations before doing the actual method
call. This is the default and can be disabled using
-fno-objc-nilcheck. Class methods and super calls are never
checked for nil in this way no matter what this flag is set to.
Currently this flag does nothing when the GNU runtime, or an
older version of the NeXT runtime ABI, is used.
-fobjc-std=objc1
Conform to the language syntax of Objective-C 1.0, the language
recognized by GCC 4.0. This only affects the Objective-C
additions to the C/C++ language; it does not affect conformance
to C/C++ standards, which is controlled by the separate C/C++
dialect option flags. When this option is used with the
Objective-C or Objective-C++ compiler, any Objective-C syntax
that is not recognized by GCC 4.0 is rejected. This is useful if
you need to make sure that your Objective-C code can be compiled
with older versions of GCC.
-freplace-objc-classes
Emit a special marker instructing ld(1) not to statically link in
the resulting object file, and allow dyld(1) to load it in at run
time instead. This is used in conjunction with the Fix-and-
Continue debugging mode, where the object file in question may be
recompiled and dynamically reloaded in the course of program
execution, without the need to restart the program itself.
Currently, Fix-and-Continue functionality is only available in
conjunction with the NeXT runtime on Mac OS X 10.3 and later.
-fzero-link
When compiling for the NeXT runtime, the compiler ordinarily
replaces calls to "objc_getClass("...")" (when the name of the
class is known at compile time) with static class references that
get initialized at load time, which improves run-time
performance. Specifying the -fzero-link flag suppresses this
behavior and causes calls to "objc_getClass("...")" to be
retained. This is useful in Zero-Link debugging mode, since it
allows for individual class implementations to be modified during
program execution. The GNU runtime currently always retains
calls to "objc_get_class("...")" regardless of command-line
options.
-fno-local-ivars
By default instance variables in Objective-C can be accessed as
if they were local variables from within the methods of the class
they're declared in. This can lead to shadowing between instance
variables and other variables declared either locally inside a
class method or globally with the same name. Specifying the
-fno-local-ivars flag disables this behavior thus avoiding
variable shadowing issues.
-fivar-visibility=[public|protected|private|package]
Set the default instance variable visibility to the specified
option so that instance variables declared outside the scope of
any access modifier directives default to the specified
visibility.
-gen-decls
Dump interface declarations for all classes seen in the source
file to a file named sourcename.decl.
-Wassign-intercept (Objective-C and Objective-C++ only)
Warn whenever an Objective-C assignment is being intercepted by
the garbage collector.
-Wno-protocol (Objective-C and Objective-C++ only)
If a class is declared to implement a protocol, a warning is
issued for every method in the protocol that is not implemented
by the class. The default behavior is to issue a warning for
every method not explicitly implemented in the class, even if a
method implementation is inherited from the superclass. If you
use the -Wno-protocol option, then methods inherited from the
superclass are considered to be implemented, and no warning is
issued for them.
-Wselector (Objective-C and Objective-C++ only)
Warn if multiple methods of different types for the same selector
are found during compilation. The check is performed on the list
of methods in the final stage of compilation. Additionally, a
check is performed for each selector appearing in a
"@selector(...)" expression, and a corresponding method for that
selector has been found during compilation. Because these checks
scan the method table only at the end of compilation, these
warnings are not produced if the final stage of compilation is
not reached, for example because an error is found during
compilation, or because the -fsyntax-only option is being used.
-Wstrict-selector-match (Objective-C and Objective-C++ only)
Warn if multiple methods with differing argument and/or return
types are found for a given selector when attempting to send a
message using this selector to a receiver of type "id" or
"Class". When this flag is off (which is the default behavior),
the compiler omits such warnings if any differences found are
confined to types that share the same size and alignment.
-Wundeclared-selector (Objective-C and Objective-C++ only)
Warn if a "@selector(...)" expression referring to an undeclared
selector is found. A selector is considered undeclared if no
method with that name has been declared before the
"@selector(...)" expression, either explicitly in an @interface
or @protocol declaration, or implicitly in an @implementation
section. This option always performs its checks as soon as a
"@selector(...)" expression is found, while -Wselector only
performs its checks in the final stage of compilation. This also
enforces the coding style convention that methods and selectors
must be declared before being used.
-print-objc-runtime-info
Generate C header describing the largest structure that is passed
by value, if any.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective
of the output device's aspect (e.g. its width, ...). You can use the
options described below to control the formatting algorithm for
diagnostic messages, e.g. how many characters per line, how often
source location information should be reported. Note that some
language front ends may not honor these options.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n
characters. If n is zero, then no line-wrapping is done; each
error message appears on a single line. This is the default for
all front ends.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit source location information once; that
is, in case the message is too long to fit on a single physical
line and has to be wrapped, the source location won't be emitted
(as prefix) again, over and over, in subsequent continuation
lines. This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit the same source location information
(as prefix) for physical lines that result from the process of
breaking a message which is too long to fit on a single line.
-fdiagnostics-color[=WHEN]
-fno-diagnostics-color
Use color in diagnostics. WHEN is never, always, or auto. The
default depends on how the compiler has been configured, it can
be any of the above WHEN options or also never if GCC_COLORS
environment variable isn't present in the environment, and auto
otherwise. auto means to use color only when the standard error
is a terminal. The forms -fdiagnostics-color and
-fno-diagnostics-color are aliases for -fdiagnostics-color=always
and -fdiagnostics-color=never, respectively.
The colors are defined by the environment variable GCC_COLORS.
Its value is a colon-separated list of capabilities and Select
Graphic Rendition (SGR) substrings. SGR commands are interpreted
by the terminal or terminal emulator. (See the section in the
documentation of your text terminal for permitted values and
their meanings as character attributes.) These substring values
are integers in decimal representation and can be concatenated
with semicolons. Common values to concatenate include 1 for
bold, 4 for underline, 5 for blink, 7 for inverse, 39 for default
foreground color, 30 to 37 for foreground colors, 90 to 97 for
16-color mode foreground colors, 38;5;0 to 38;5;255 for 88-color
and 256-color modes foreground colors, 49 for default background
color, 40 to 47 for background colors, 100 to 107 for 16-color
mode background colors, and 48;5;0 to 48;5;255 for 88-color and
256-color modes background colors.
The default GCC_COLORS is
error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
quote=01:fixit-insert=32:fixit-delete=31:\
diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32
where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold
cyan, 32 is green, 34 is blue, 01 is bold, and 31 is red.
Setting GCC_COLORS to the empty string disables colors.
Supported capabilities are as follows.
"error="
SGR substring for error: markers.
"warning="
SGR substring for warning: markers.
"note="
SGR substring for note: markers.
"range1="
SGR substring for first additional range.
"range2="
SGR substring for second additional range.
"locus="
SGR substring for location information, file:line or
file:line:column etc.
"quote="
SGR substring for information printed within quotes.
"fixit-insert="
SGR substring for fix-it hints suggesting text to be inserted
or replaced.
"fixit-delete="
SGR substring for fix-it hints suggesting text to be deleted.
"diff-filename="
SGR substring for filename headers within generated patches.
"diff-hunk="
SGR substring for the starts of hunks within generated
patches.
"diff-delete="
SGR substring for deleted lines within generated patches.
"diff-insert="
SGR substring for inserted lines within generated patches.
-fno-diagnostics-show-option
By default, each diagnostic emitted includes text indicating the
command-line option that directly controls the diagnostic (if
such an option is known to the diagnostic machinery). Specifying
the -fno-diagnostics-show-option flag suppresses that behavior.
-fno-diagnostics-show-caret
By default, each diagnostic emitted includes the original source
line and a caret ^ indicating the column. This option suppresses
this information. The source line is truncated to n characters,
if the -fmessage-length=n option is given. When the output is
done to the terminal, the width is limited to the width given by
the COLUMNS environment variable or, if not set, to the terminal
width.
-fdiagnostics-parseable-fixits
Emit fix-it hints in a machine-parseable format, suitable for
consumption by IDEs. For each fix-it, a line will be printed
after the relevant diagnostic, starting with the string "fix-
it:". For example:
fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"
The location is expressed as a half-open range, expressed as a
count of bytes, starting at byte 1 for the initial column. In
the above example, bytes 3 through 20 of line 45 of "test.c" are
to be replaced with the given string:
00000000011111111112222222222
12345678901234567890123456789
gtk_widget_showall (dlg);
^^^^^^^^^^^^^^^^^^
gtk_widget_show_all
The filename and replacement string escape backslash as "\\", tab
as "\t", newline as "\n", double quotes as "\"", non-printable
characters as octal (e.g. vertical tab as "\013").
An empty replacement string indicates that the given range is to
be removed. An empty range (e.g. "45:3-45:3") indicates that the
string is to be inserted at the given position.
-fdiagnostics-generate-patch
Print fix-it hints to stderr in unified diff format, after any
diagnostics are printed. For example:
--- test.c
+++ test.c
@ -42,5 +42,5 @
void show_cb(GtkDialog *dlg)
{
- gtk_widget_showall(dlg);
+ gtk_widget_show_all(dlg);
}
The diff may or may not be colorized, following the same rules as
for diagnostics (see -fdiagnostics-color).
-fno-show-column
Do not print column numbers in diagnostics. This may be
necessary if diagnostics are being scanned by a program that does
not understand the column numbers, such as dejagnu.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions that are
not inherently erroneous but that are risky or suggest there may have
been an error.
The following language-independent options do not enable specific
warnings but control the kinds of diagnostics produced by GCC.
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond
that.
-fmax-errors=n
Limits the maximum number of error messages to n, at which point
GCC bails out rather than attempting to continue processing the
source code. If n is 0 (the default), there is no limit on the
number of error messages produced. If -Wfatal-errors is also
specified, then -Wfatal-errors takes precedence over this option.
-w Inhibit all warning messages.
-Werror
Make all warnings into errors.
-Werror=
Make the specified warning into an error. The specifier for a
warning is appended; for example -Werror=switch turns the
warnings controlled by -Wswitch into errors. This switch takes a
negative form, to be used to negate -Werror for specific
warnings; for example -Wno-error=switch makes -Wswitch warnings
not be errors, even when -Werror is in effect.
The warning message for each controllable warning includes the
option that controls the warning. That option can then be used
with -Werror= and -Wno-error= as described above. (Printing of
the option in the warning message can be disabled using the
-fno-diagnostics-show-option flag.)
Note that specifying -Werror=foo automatically implies -Wfoo.
However, -Wno-error=foo does not imply anything.
-Wfatal-errors
This option causes the compiler to abort compilation on the first
error occurred rather than trying to keep going and printing
further error messages.
You can request many specific warnings with options beginning with
-W, for example -Wimplicit to request warnings on implicit
declarations. Each of these specific warning options also has a
negative form beginning -Wno- to turn off warnings; for example,
-Wno-implicit. This manual lists only one of the two forms,
whichever is not the default. For further language-specific options
also refer to C++ Dialect Options and Objective-C and Objective-C++
Dialect Options.
Some options, such as -Wall and -Wextra, turn on other options, such
as -Wunused, which may turn on further options, such as
-Wunused-value. The combined effect of positive and negative forms is
that more specific options have priority over less specific ones,
independently of their position in the command-line. For options of
the same specificity, the last one takes effect. Options enabled or
disabled via pragmas take effect as if they appeared at the end of
the command-line.
When an unrecognized warning option is requested (e.g.,
-Wunknown-warning), GCC emits a diagnostic stating that the option is
not recognized. However, if the -Wno- form is used, the behavior is
slightly different: no diagnostic is produced for
-Wno-unknown-warning unless other diagnostics are being produced.
This allows the use of new -Wno- options with old compilers, but if
something goes wrong, the compiler warns that an unrecognized option
is present.
-Wpedantic
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++;
reject all programs that use forbidden extensions, and some other
programs that do not follow ISO C and ISO C++. For ISO C,
follows the version of the ISO C standard specified by any -std
option used.
Valid ISO C and ISO C++ programs should compile properly with or
without this option (though a rare few require -ansi or a -std
option specifying the required version of ISO C). However,
without this option, certain GNU extensions and traditional C and
C++ features are supported as well. With this option, they are
rejected.
-Wpedantic does not cause warning messages for use of the
alternate keywords whose names begin and end with __. Pedantic
warnings are also disabled in the expression that follows
"__extension__". However, only system header files should use
these escape routes; application programs should avoid them.
Some users try to use -Wpedantic to check programs for strict ISO
C conformance. They soon find that it does not do quite what
they want: it finds some non-ISO practices, but not all---only
those for which ISO C requires a diagnostic, and some others for
which diagnostics have been added.
A feature to report any failure to conform to ISO C might be
useful in some instances, but would require considerable
additional work and would be quite different from -Wpedantic. We
don't have plans to support such a feature in the near future.
Where the standard specified with -std represents a GNU extended
dialect of C, such as gnu90 or gnu99, there is a corresponding
base standard, the version of ISO C on which the GNU extended
dialect is based. Warnings from -Wpedantic are given where they
are required by the base standard. (It does not make sense for
such warnings to be given only for features not in the specified
GNU C dialect, since by definition the GNU dialects of C include
all features the compiler supports with the given option, and
there would be nothing to warn about.)
-pedantic-errors
Give an error whenever the base standard (see -Wpedantic)
requires a diagnostic, in some cases where there is undefined
behavior at compile-time and in some other cases that do not
prevent compilation of programs that are valid according to the
standard. This is not equivalent to -Werror=pedantic, since there
are errors enabled by this option and not enabled by the latter
and vice versa.
-Wall
This enables all the warnings about constructions that some users
consider questionable, and that are easy to avoid (or modify to
prevent the warning), even in conjunction with macros. This also
enables some language-specific warnings described in C++ Dialect
Options and Objective-C and Objective-C++ Dialect Options.
-Wall turns on the following warning flags:
-Waddress -Warray-bounds=1 (only with -O2) -Wbool-compare
-Wbool-operation -Wc++11-compat -Wc++14-compat -Wchar-subscripts
-Wcomment -Wduplicate-decl-specifier (C and Objective-C only)
-Wenum-compare (in C/ObjC; this is on by default in C++) -Wformat
-Wint-in-bool-context -Wimplicit (C and Objective-C only)
-Wimplicit-int (C and Objective-C only)
-Wimplicit-function-declaration (C and Objective-C only)
-Winit-self (only for C++) -Wlogical-not-parentheses -Wmain (only
for C/ObjC and unless -ffreestanding) -Wmaybe-uninitialized
-Wmemset-elt-size -Wmemset-transposed-args
-Wmisleading-indentation (only for C/C++) -Wmissing-braces (only
for C/ObjC) -Wnarrowing (only for C++) -Wnonnull
-Wnonnull-compare -Wopenmp-simd -Wparentheses -Wpointer-sign
-Wreorder -Wreturn-type -Wsequence-point -Wsign-compare (only in
C++) -Wsizeof-pointer-memaccess -Wstrict-aliasing
-Wstrict-overflow=1 -Wswitch -Wtautological-compare -Wtrigraphs
-Wuninitialized -Wunknown-pragmas -Wunused-function
-Wunused-label -Wunused-value -Wunused-variable
-Wvolatile-register-var
Note that some warning flags are not implied by -Wall. Some of
them warn about constructions that users generally do not
consider questionable, but which occasionally you might wish to
check for; others warn about constructions that are necessary or
hard to avoid in some cases, and there is no simple way to modify
the code to suppress the warning. Some of them are enabled by
-Wextra but many of them must be enabled individually.
-Wextra
This enables some extra warning flags that are not enabled by
-Wall. (This option used to be called -W. The older name is
still supported, but the newer name is more descriptive.)
-Wclobbered -Wempty-body -Wignored-qualifiers
-Wimplicit-fallthrough=3 -Wmissing-field-initializers
-Wmissing-parameter-type (C only) -Wold-style-declaration (C
only) -Woverride-init -Wsign-compare (C only) -Wtype-limits
-Wuninitialized -Wshift-negative-value (in C++03 and in C99 and
newer) -Wunused-parameter (only with -Wunused or -Wall)
-Wunused-but-set-parameter (only with -Wunused or -Wall)
The option -Wextra also prints warning messages for the following
cases:
* A pointer is compared against integer zero with "<", "<=",
">", or ">=".
* (C++ only) An enumerator and a non-enumerator both appear in
a conditional expression.
* (C++ only) Ambiguous virtual bases.
* (C++ only) Subscripting an array that has been declared
"register".
* (C++ only) Taking the address of a variable that has been
declared "register".
* (C++ only) A base class is not initialized in the copy
constructor of a derived class.
-Wchar-subscripts
Warn if an array subscript has type "char". This is a common
cause of error, as programmers often forget that this type is
signed on some machines. This warning is enabled by -Wall.
-Wchkp
Warn about an invalid memory access that is found by Pointer
Bounds Checker (-fcheck-pointer-bounds).
-Wno-coverage-mismatch
Warn if feedback profiles do not match when using the
-fprofile-use option. If a source file is changed between
compiling with -fprofile-gen and with -fprofile-use, the files
with the profile feedback can fail to match the source file and
GCC cannot use the profile feedback information. By default,
this warning is enabled and is treated as an error.
-Wno-coverage-mismatch can be used to disable the warning or
-Wno-error=coverage-mismatch can be used to disable the error.
Disabling the error for this warning can result in poorly
optimized code and is useful only in the case of very minor
changes such as bug fixes to an existing code-base. Completely
disabling the warning is not recommended.
-Wno-cpp
(C, Objective-C, C++, Objective-C++ and Fortran only)
Suppress warning messages emitted by "#warning" directives.
-Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
Give a warning when a value of type "float" is implicitly
promoted to "double". CPUs with a 32-bit "single-precision"
floating-point unit implement "float" in hardware, but emulate
"double" in software. On such a machine, doing computations
using "double" values is much more expensive because of the
overhead required for software emulation.
It is easy to accidentally do computations with "double" because
floating-point literals are implicitly of type "double". For
example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}
the compiler performs the entire computation with "double"
because the floating-point literal is a "double".
-Wduplicate-decl-specifier (C and Objective-C only)
Warn if a declaration has duplicate "const", "volatile",
"restrict" or "_Atomic" specifier. This warning is enabled by
-Wall.
-Wformat
-Wformat=n
Check calls to "printf" and "scanf", etc., to make sure that the
arguments supplied have types appropriate to the format string
specified, and that the conversions specified in the format
string make sense. This includes standard functions, and others
specified by format attributes, in the "printf", "scanf",
"strftime" and "strfmon" (an X/Open extension, not in the C
standard) families (or other target-specific families). Which
functions are checked without format attributes having been
specified depends on the standard version selected, and such
checks of functions without the attribute specified are disabled
by -ffreestanding or -fno-builtin.
The formats are checked against the format features supported by
GNU libc version 2.2. These include all ISO C90 and C99
features, as well as features from the Single Unix Specification
and some BSD and GNU extensions. Other library implementations
may not support all these features; GCC does not support warning
about features that go beyond a particular library's limitations.
However, if -Wpedantic is used with -Wformat, warnings are given
about format features not in the selected standard version (but
not for "strfmon" formats, since those are not in any version of
the C standard).
-Wformat=1
-Wformat
Option -Wformat is equivalent to -Wformat=1, and -Wno-format
is equivalent to -Wformat=0. Since -Wformat also checks for
null format arguments for several functions, -Wformat also
implies -Wnonnull. Some aspects of this level of format
checking can be disabled by the options:
-Wno-format-contains-nul, -Wno-format-extra-args, and
-Wno-format-zero-length. -Wformat is enabled by -Wall.
-Wno-format-contains-nul
If -Wformat is specified, do not warn about format strings
that contain NUL bytes.
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess arguments
to a "printf" or "scanf" format function. The C standard
specifies that such arguments are ignored.
Where the unused arguments lie between used arguments that
are specified with $ operand number specifications, normally
warnings are still given, since the implementation could not
know what type to pass to "va_arg" to skip the unused
arguments. However, in the case of "scanf" formats, this
option suppresses the warning if the unused arguments are all
pointers, since the Single Unix Specification says that such
unused arguments are allowed.
-Wformat-overflow
-Wformat-overflow=level
Warn about calls to formatted input/output functions such as
"sprintf" and "vsprintf" that might overflow the destination
buffer. When the exact number of bytes written by a format
directive cannot be determined at compile-time it is
estimated based on heuristics that depend on the level
argument and on optimization. While enabling optimization
will in most cases improve the accuracy of the warning, it
may also result in false positives.
-Wformat-overflow
-Wformat-overflow=1
Level 1 of -Wformat-overflow enabled by -Wformat employs
a conservative approach that warns only about calls that
most likely overflow the buffer. At this level, numeric
arguments to format directives with unknown values are
assumed to have the value of one, and strings of unknown
length to be empty. Numeric arguments that are known to
be bounded to a subrange of their type, or string
arguments whose output is bounded either by their
directive's precision or by a finite set of string
literals, are assumed to take on the value within the
range that results in the most bytes on output. For
example, the call to "sprintf" below is diagnosed because
even with both a and b equal to zero, the terminating NUL
character ('\0') appended by the function to the
destination buffer will be written past its end.
Increasing the size of the buffer by a single byte is
sufficient to avoid the warning, though it may not be
sufficient to avoid the overflow.
void f (int a, int b)
{
char buf [12];
sprintf (buf, "a = %i, b = %i\n", a, b);
}
-Wformat-overflow=2
Level 2 warns also about calls that might overflow the
destination buffer given an argument of sufficient length
or magnitude. At level 2, unknown numeric arguments are
assumed to have the minimum representable value for
signed types with a precision greater than 1, and the
maximum representable value otherwise. Unknown string
arguments whose length cannot be assumed to be bounded
either by the directive's precision, or by a finite set
of string literals they may evaluate to, or the character
array they may point to, are assumed to be 1 character
long.
At level 2, the call in the example above is again
diagnosed, but this time because with a equal to a 32-bit
"INT_MIN" the first %i directive will write some of its
digits beyond the end of the destination buffer. To make
the call safe regardless of the values of the two
variables, the size of the destination buffer must be
increased to at least 34 bytes. GCC includes the minimum
size of the buffer in an informational note following the
warning.
An alternative to increasing the size of the destination
buffer is to constrain the range of formatted values.
The maximum length of string arguments can be bounded by
specifying the precision in the format directive. When
numeric arguments of format directives can be assumed to
be bounded by less than the precision of their type,
choosing an appropriate length modifier to the format
specifier will reduce the required buffer size. For
example, if a and b in the example above can be assumed
to be within the precision of the "short int" type then
using either the %hi format directive or casting the
argument to "short" reduces the maximum required size of
the buffer to 24 bytes.
void f (int a, int b)
{
char buf [23];
sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-length
formats. The C standard specifies that zero-length formats
are allowed.
-Wformat=2
Enable -Wformat plus additional format checks. Currently
equivalent to -Wformat -Wformat-nonliteral -Wformat-security
-Wformat-y2k.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is
not a string literal and so cannot be checked, unless the
format function takes its format arguments as a "va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of format
functions that represent possible security problems. At
present, this warns about calls to "printf" and "scanf"
functions where the format string is not a string literal and
there are no format arguments, as in "printf (foo);". This
may be a security hole if the format string came from
untrusted input and contains %n. (This is currently a subset
of what -Wformat-nonliteral warns about, but in future
warnings may be added to -Wformat-security that are not
included in -Wformat-nonliteral.)
-Wformat-signedness
If -Wformat is specified, also warn if the format string
requires an unsigned argument and the argument is signed and
vice versa.
-Wformat-truncation
-Wformat-truncation=level
Warn about calls to formatted input/output functions such as
"snprintf" and "vsnprintf" that might result in output
truncation. When the exact number of bytes written by a
format directive cannot be determined at compile-time it is
estimated based on heuristics that depend on the level
argument and on optimization. While enabling optimization
will in most cases improve the accuracy of the warning, it
may also result in false positives. Except as noted
otherwise, the option uses the same logic -Wformat-overflow.
-Wformat-truncation
-Wformat-truncation=1
Level 1 of -Wformat-truncation enabled by -Wformat
employs a conservative approach that warns only about
calls to bounded functions whose return value is unused
and that will most likely result in output truncation.
-Wformat-truncation=2
Level 2 warns also about calls to bounded functions whose
return value is used and that might result in truncation
given an argument of sufficient length or magnitude.
-Wformat-y2k
If -Wformat is specified, also warn about "strftime" formats
that may yield only a two-digit year.
-Wnonnull
Warn about passing a null pointer for arguments marked as
requiring a non-null value by the "nonnull" function attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled
with the -Wno-nonnull option.
-Wnonnull-compare
Warn when comparing an argument marked with the "nonnull"
function attribute against null inside the function.
-Wnonnull-compare is included in -Wall. It can be disabled with
the -Wno-nonnull-compare option.
-Wnull-dereference
Warn if the compiler detects paths that trigger erroneous or
undefined behavior due to dereferencing a null pointer. This
option is only active when -fdelete-null-pointer-checks is
active, which is enabled by optimizations in most targets. The
precision of the warnings depends on the optimization options
used.
-Winit-self (C, C++, Objective-C and Objective-C++ only)
Warn about uninitialized variables that are initialized with
themselves. Note this option can only be used with the
-Wuninitialized option.
For example, GCC warns about "i" being uninitialized in the
following snippet only when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}
This warning is enabled by -Wall in C++.
-Wimplicit-int (C and Objective-C only)
Warn when a declaration does not specify a type. This warning is
enabled by -Wall.
-Wimplicit-function-declaration (C and Objective-C only)
Give a warning whenever a function is used before being declared.
In C99 mode (-std=c99 or -std=gnu99), this warning is enabled by
default and it is made into an error by -pedantic-errors. This
warning is also enabled by -Wall.
-Wimplicit (C and Objective-C only)
Same as -Wimplicit-int and -Wimplicit-function-declaration. This
warning is enabled by -Wall.
-Wimplicit-fallthrough
-Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3
and -Wno-implicit-fallthrough is the same as
-Wimplicit-fallthrough=0.
-Wimplicit-fallthrough=n
Warn when a switch case falls through. For example:
switch (cond)
{
case 1:
a = 1;
break;
case 2:
a = 2;
case 3:
a = 3;
break;
}
This warning does not warn when the last statement of a case
cannot fall through, e.g. when there is a return statement or a
call to function declared with the noreturn attribute.
-Wimplicit-fallthrough= also takes into account control flow
statements, such as ifs, and only warns when appropriate. E.g.
switch (cond)
{
case 1:
if (i > 3) {
bar (5);
break;
} else if (i < 1) {
bar (0);
} else
return;
default:
...
}
Since there are occasions where a switch case fall through is
desirable, GCC provides an attribute, "__attribute__
((fallthrough))", that is to be used along with a null statement
to suppress this warning that would normally occur:
switch (cond)
{
case 1:
bar (0);
__attribute__ ((fallthrough));
default:
...
}
C++17 provides a standard way to suppress the
-Wimplicit-fallthrough warning using "[[fallthrough]];" instead
of the GNU attribute. In C++11 or C++14 users can use
"[[gnu::fallthrough]];", which is a GNU extension. Instead of
the these attributes, it is also possible to add a fallthrough
comment to silence the warning. The whole body of the C or C++
style comment should match the given regular expressions listed
below. The option argument n specifies what kind of comments are
accepted:
*<-Wimplicit-fallthrough=0 disables the warning altogether.>
*<-Wimplicit-fallthrough=1 matches ".*" regular>
expression, any comment is used as fallthrough comment.
*<-Wimplicit-fallthrough=2 case insensitively matches>
".*falls?[ \t-]*thr(ough|u).*" regular expression.
*<-Wimplicit-fallthrough=3 case sensitively matches one of the>
following regular expressions:
*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
*<"[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S |
|-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?">
*<"[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s |
|-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?">
*<"[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s |
|-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?">
*<-Wimplicit-fallthrough=4 case sensitively matches one of the>
following regular expressions:
*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
*<"[ \t]*FALLTHR(OUGH|U)[ \t]*">
*<-Wimplicit-fallthrough=5 doesn't recognize any comments as>
fallthrough comments, only attributes disable the warning.
The comment needs to be followed after optional whitespace and
other comments by "case" or "default" keywords or by a user label
that precedes some "case" or "default" label.
switch (cond)
{
case 1:
bar (0);
/* FALLTHRU */
default:
...
}
The -Wimplicit-fallthrough=3 warning is enabled by -Wextra.
-Wignored-qualifiers (C and C++ only)
Warn if the return type of a function has a type qualifier such
as "const". For ISO C such a type qualifier has no effect, since
the value returned by a function is not an lvalue. For C++, the
warning is only emitted for scalar types or "void". ISO C
prohibits qualified "void" return types on function definitions,
so such return types always receive a warning even without this
option.
This warning is also enabled by -Wextra.
-Wignored-attributes (C and C++ only)
Warn when an attribute is ignored. This is different from the
-Wattributes option in that it warns whenever the compiler
decides to drop an attribute, not that the attribute is either
unknown, used in a wrong place, etc. This warning is enabled by
default.
-Wmain
Warn if the type of "main" is suspicious. "main" should be a
function with external linkage, returning int, taking either zero
arguments, two, or three arguments of appropriate types. This
warning is enabled by default in C++ and is enabled by either
-Wall or -Wpedantic.
-Wmisleading-indentation (C and C++ only)
Warn when the indentation of the code does not reflect the block
structure. Specifically, a warning is issued for "if", "else",
"while", and "for" clauses with a guarded statement that does not
use braces, followed by an unguarded statement with the same
indentation.
In the following example, the call to "bar" is misleadingly
indented as if it were guarded by the "if" conditional.
if (some_condition ())
foo ();
bar (); /* Gotcha: this is not guarded by the "if". */
In the case of mixed tabs and spaces, the warning uses the
-ftabstop= option to determine if the statements line up
(defaulting to 8).
The warning is not issued for code involving multiline
preprocessor logic such as the following example.
if (flagA)
foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
if (flagB)
#endif
foo (1);
The warning is not issued after a "#line" directive, since this
typically indicates autogenerated code, and no assumptions can be
made about the layout of the file that the directive references.
This warning is enabled by -Wall in C and C++.
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed.
In the following example, the initializer for "a" is not fully
bracketed, but that for "b" is fully bracketed. This warning is
enabled by -Wall in C.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
Warn if a user-supplied include directory does not exist.
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when
there is an assignment in a context where a truth value is
expected, or when operators are nested whose precedence people
often get confused about.
Also warn if a comparison like "x<=y<=z" appears; this is
equivalent to "(x<=y ? 1 : 0) <= z", which is a different
interpretation from that of ordinary mathematical notation.
Also warn for dangerous uses of the GNU extension to "?:" with
omitted middle operand. When the condition in the "?": operator
is a boolean expression, the omitted value is always 1. Often
programmers expect it to be a value computed inside the
conditional expression instead.
This warning is enabled by -Wall.
-Wsequence-point
Warn about code that may have undefined semantics because of
violations of sequence point rules in the C and C++ standards.
The C and C++ standards define the order in which expressions in
a C/C++ program are evaluated in terms of sequence points, which
represent a partial ordering between the execution of parts of
the program: those executed before the sequence point, and those
executed after it. These occur after the evaluation of a full
expression (one which is not part of a larger expression), after
the evaluation of the first operand of a "&&", "||", "? :" or ","
(comma) operator, before a function is called (but after the
evaluation of its arguments and the expression denoting the
called function), and in certain other places. Other than as
expressed by the sequence point rules, the order of evaluation of
subexpressions of an expression is not specified. All these
rules describe only a partial order rather than a total order,
since, for example, if two functions are called within one
expression with no sequence point between them, the order in
which the functions are called is not specified. However, the
standards committee have ruled that function calls do not
overlap.
It is not specified when between sequence points modifications to
the values of objects take effect. Programs whose behavior
depends on this have undefined behavior; the C and C++ standards
specify that "Between the previous and next sequence point an
object shall have its stored value modified at most once by the
evaluation of an expression. Furthermore, the prior value shall
be read only to determine the value to be stored.". If a program
breaks these rules, the results on any particular implementation
are entirely unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n] =
b[n++]" and "a[i++] = i;". Some more complicated cases are not
diagnosed by this option, and it may give an occasional false
positive result, but in general it has been found fairly
effective at detecting this sort of problem in programs.
The C++17 standard will define the order of evaluation of
operands in more cases: in particular it requires that the right-
hand side of an assignment be evaluated before the left-hand
side, so the above examples are no longer undefined. But this
warning will still warn about them, to help people avoid writing
code that is undefined in C and earlier revisions of C++.
The standard is worded confusingly, therefore there is some
debate over the precise meaning of the sequence point rules in
subtle cases. Links to discussions of the problem, including
proposed formal definitions, may be found on the GCC readings
page, at <http://gcc.gnu.org/readings.html >.
This warning is enabled by -Wall for C and C++.
-Wno-return-local-addr
Do not warn about returning a pointer (or in C++, a reference) to
a variable that goes out of scope after the function returns.
-Wreturn-type
Warn whenever a function is defined with a return type that
defaults to "int". Also warn about any "return" statement with
no return value in a function whose return type is not "void"
(falling off the end of the function body is considered returning
without a value).
For C only, warn about a "return" statement with an expression in
a function whose return type is "void", unless the expression
type is also "void". As a GNU extension, the latter case is
accepted without a warning unless -Wpedantic is used.
For C++, a function without return type always produces a
diagnostic message, even when -Wno-return-type is specified. The
only exceptions are "main" and functions defined in system
headers.
This warning is enabled by -Wall.
-Wshift-count-negative
Warn if shift count is negative. This warning is enabled by
default.
-Wshift-count-overflow
Warn if shift count >= width of type. This warning is enabled by
default.
-Wshift-negative-value
Warn if left shifting a negative value. This warning is enabled
by -Wextra in C99 and C++11 modes (and newer).
-Wshift-overflow
-Wshift-overflow=n
Warn about left shift overflows. This warning is enabled by
default in C99 and C++11 modes (and newer).
-Wshift-overflow=1
This is the warning level of -Wshift-overflow and is enabled
by default in C99 and C++11 modes (and newer). This warning
level does not warn about left-shifting 1 into the sign bit.
(However, in C, such an overflow is still rejected in
contexts where an integer constant expression is required.)
-Wshift-overflow=2
This warning level also warns about left-shifting 1 into the
sign bit, unless C++14 mode is active.
-Wswitch
Warn whenever a "switch" statement has an index of enumerated
type and lacks a "case" for one or more of the named codes of
that enumeration. (The presence of a "default" label prevents
this warning.) "case" labels outside the enumeration range also
provoke warnings when this option is used (even if there is a
"default" label). This warning is enabled by -Wall.
-Wswitch-default
Warn whenever a "switch" statement does not have a "default"
case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumerated
type and lacks a "case" for one or more of the named codes of
that enumeration. "case" labels outside the enumeration range
also provoke warnings when this option is used. The only
difference between -Wswitch and this option is that this option
gives a warning about an omitted enumeration code even if there
is a "default" label.
-Wswitch-bool
Warn whenever a "switch" statement has an index of boolean type
and the case values are outside the range of a boolean type. It
is possible to suppress this warning by casting the controlling
expression to a type other than "bool". For example:
switch ((int) (a == 4))
{
...
}
This warning is enabled by default for C and C++ programs.
-Wswitch-unreachable
Warn whenever a "switch" statement contains statements between
the controlling expression and the first case label, which will
never be executed. For example:
switch (cond)
{
i = 15;
...
case 5:
...
}
-Wswitch-unreachable does not warn if the statement between the
controlling expression and the first case label is just a
declaration:
switch (cond)
{
int i;
...
case 5:
i = 5;
...
}
This warning is enabled by default for C and C++ programs.
-Wsync-nand (C and C++ only)
Warn when "__sync_fetch_and_nand" and "__sync_nand_and_fetch"
built-in functions are used. These functions changed semantics
in GCC 4.4.
-Wunused-but-set-parameter
Warn whenever a function parameter is assigned to, but otherwise
unused (aside from its declaration).
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused together with -Wextra.
-Wunused-but-set-variable
Warn whenever a local variable is assigned to, but otherwise
unused (aside from its declaration). This warning is enabled by
-Wall.
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused, which is enabled by
-Wall.
-Wunused-function
Warn whenever a static function is declared but not defined or a
non-inline static function is unused. This warning is enabled by
-Wall.
-Wunused-label
Warn whenever a label is declared but not used. This warning is
enabled by -Wall.
To suppress this warning use the "unused" attribute.
-Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only)
Warn when a typedef locally defined in a function is not used.
This warning is enabled by -Wall.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its
declaration.
To suppress this warning use the "unused" attribute.
-Wno-unused-result
Do not warn if a caller of a function marked with attribute
"warn_unused_result" does not use its return value. The default
is -Wunused-result.
-Wunused-variable
Warn whenever a local or static variable is unused aside from its
declaration. This option implies -Wunused-const-variable=1 for C,
but not for C++. This warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
-Wunused-const-variable
-Wunused-const-variable=n
Warn whenever a constant static variable is unused aside from its
declaration. -Wunused-const-variable=1 is enabled by
-Wunused-variable for C, but not for C++. In C this declares
variable storage, but in C++ this is not an error since const
variables take the place of "#define"s.
To suppress this warning use the "unused" attribute.
-Wunused-const-variable=1
This is the warning level that is enabled by
-Wunused-variable for C. It warns only about unused static
const variables defined in the main compilation unit, but not
about static const variables declared in any header included.
-Wunused-const-variable=2
This warning level also warns for unused constant static
variables in headers (excluding system headers). This is the
warning level of -Wunused-const-variable and must be
explicitly requested since in C++ this isn't an error and in
C it might be harder to clean up all headers included.
-Wunused-value
Warn whenever a statement computes a result that is explicitly
not used. To suppress this warning cast the unused expression to
"void". This includes an expression-statement or the left-hand
side of a comma expression that contains no side effects. For
example, an expression such as "x[i,j]" causes a warning, while
"x[(void)i,j]" does not.
This warning is enabled by -Wall.
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you
must either specify -Wextra -Wunused (note that -Wall implies
-Wunused), or separately specify -Wunused-parameter.
-Wuninitialized
Warn if an automatic variable is used without first being
initialized or if a variable may be clobbered by a "setjmp" call.
In C++, warn if a non-static reference or non-static "const"
member appears in a class without constructors.
If you want to warn about code that uses the uninitialized value
of the variable in its own initializer, use the -Winit-self
option.
These warnings occur for individual uninitialized or clobbered
elements of structure, union or array variables as well as for
variables that are uninitialized or clobbered as a whole. They
do not occur for variables or elements declared "volatile".
Because these warnings depend on optimization, the exact
variables or elements for which there are warnings depends on the
precise optimization options and version of GCC used.
Note that there may be no warning about a variable that is used
only to compute a value that itself is never used, because such
computations may be deleted by data flow analysis before the
warnings are printed.
-Winvalid-memory-model
Warn for invocations of __atomic Builtins, __sync Builtins, and
the C11 atomic generic functions with a memory consistency
argument that is either invalid for the operation or outside the
range of values of the "memory_order" enumeration. For example,
since the "__atomic_store" and "__atomic_store_n" built-ins are
only defined for the relaxed, release, and sequentially
consistent memory orders the following code is diagnosed:
void store (int *i)
{
__atomic_store_n (i, 0, memory_order_consume);
}
-Winvalid-memory-model is enabled by default.
-Wmaybe-uninitialized
For an automatic variable, if there exists a path from the
function entry to a use of the variable that is initialized, but
there exist some other paths for which the variable is not
initialized, the compiler emits a warning if it cannot prove the
uninitialized paths are not executed at run time. These warnings
are made optional because GCC is not smart enough to see all the
reasons why the code might be correct in spite of appearing to
have an error. Here is one example of how this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is always
initialized, but GCC doesn't know this. To suppress the warning,
you need to provide a default case with assert(0) or similar
code.
This option also warns when a non-volatile automatic variable
might be changed by a call to "longjmp". These warnings as well
are possible only in optimizing compilation.
The compiler sees only the calls to "setjmp". It cannot know
where "longjmp" will be called; in fact, a signal handler could
call it at any point in the code. As a result, you may get a
warning even when there is in fact no problem because "longjmp"
cannot in fact be called at the place that would cause a problem.
Some spurious warnings can be avoided if you declare all the
functions you use that never return as "noreturn".
This warning is enabled by -Wall or -Wextra.
-Wunknown-pragmas
Warn when a "#pragma" directive is encountered that is not
understood by GCC. If this command-line option is used, warnings
are even issued for unknown pragmas in system header files. This
is not the case if the warnings are only enabled by the -Wall
command-line option.
-Wno-pragmas
Do not warn about misuses of pragmas, such as incorrect
parameters, invalid syntax, or conflicts between pragmas. See
also -Wunknown-pragmas.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It
warns about code that might break the strict aliasing rules that
the compiler is using for optimization. The warning does not
catch all cases, but does attempt to catch the more common
pitfalls. It is included in -Wall. It is equivalent to
-Wstrict-aliasing=3
-Wstrict-aliasing=n
This option is only active when -fstrict-aliasing is active. It
warns about code that might break the strict aliasing rules that
the compiler is using for optimization. Higher levels correspond
to higher accuracy (fewer false positives). Higher levels also
correspond to more effort, similar to the way -O works.
-Wstrict-aliasing is equivalent to -Wstrict-aliasing=3.
Level 1: Most aggressive, quick, least accurate. Possibly useful
when higher levels do not warn but -fstrict-aliasing still breaks
the code, as it has very few false negatives. However, it has
many false positives. Warns for all pointer conversions between
possibly incompatible types, even if never dereferenced. Runs in
the front end only.
Level 2: Aggressive, quick, not too precise. May still have many
false positives (not as many as level 1 though), and few false
negatives (but possibly more than level 1). Unlike level 1, it
only warns when an address is taken. Warns about incomplete
types. Runs in the front end only.
Level 3 (default for -Wstrict-aliasing): Should have very few
false positives and few false negatives. Slightly slower than
levels 1 or 2 when optimization is enabled. Takes care of the
common pun+dereference pattern in the front end:
"*(int*)&some_float". If optimization is enabled, it also runs
in the back end, where it deals with multiple statement cases
using flow-sensitive points-to information. Only warns when the
converted pointer is dereferenced. Does not warn about
incomplete types.
-Wstrict-overflow
-Wstrict-overflow=n
This option is only active when -fstrict-overflow is active. It
warns about cases where the compiler optimizes based on the
assumption that signed overflow does not occur. Note that it
does not warn about all cases where the code might overflow: it
only warns about cases where the compiler implements some
optimization. Thus this warning depends on the optimization
level.
An optimization that assumes that signed overflow does not occur
is perfectly safe if the values of the variables involved are
such that overflow never does, in fact, occur. Therefore this
warning can easily give a false positive: a warning about code
that is not actually a problem. To help focus on important
issues, several warning levels are defined. No warnings are
issued for the use of undefined signed overflow when estimating
how many iterations a loop requires, in particular when
determining whether a loop will be executed at all.
-Wstrict-overflow=1
Warn about cases that are both questionable and easy to
avoid. For example, with -fstrict-overflow, the compiler
simplifies "x + 1 > x" to 1. This level of -Wstrict-overflow
is enabled by -Wall; higher levels are not, and must be
explicitly requested.
-Wstrict-overflow=2
Also warn about other cases where a comparison is simplified
to a constant. For example: "abs (x) >= 0". This can only
be simplified when -fstrict-overflow is in effect, because
"abs (INT_MIN)" overflows to "INT_MIN", which is less than
zero. -Wstrict-overflow (with no level) is the same as
-Wstrict-overflow=2.
-Wstrict-overflow=3
Also warn about other cases where a comparison is simplified.
For example: "x + 1 > 1" is simplified to "x > 0".
-Wstrict-overflow=4
Also warn about other simplifications not covered by the
above cases. For example: "(x * 10) / 5" is simplified to "x
* 2".
-Wstrict-overflow=5
Also warn about cases where the compiler reduces the
magnitude of a constant involved in a comparison. For
example: "x + 2 > y" is simplified to "x + 1 >= y". This is
reported only at the highest warning level because this
simplification applies to many comparisons, so this warning
level gives a very large number of false positives.
-Wstringop-overflow
-Wstringop-overflow=type
Warn for calls to string manipulation functions such as "memcpy"
and "strcpy" that are determined to overflow the destination
buffer. The optional argument is one greater than the type of
Object Size Checking to perform to determine the size of the
destination. The argument is meaningful only for functions that
operate on character arrays but not for raw memory functions like
"memcpy" which always make use of Object Size type-0. The option
also warns for calls that specify a size in excess of the largest
possible object or at most "SIZE_MAX / 2" bytes. The option
produces the best results with optimization enabled but can
detect a small subset of simple buffer overflows even without
optimization in calls to the GCC built-in functions like
"__builtin_memcpy" that correspond to the standard functions. In
any case, the option warns about just a subset of buffer
overflows detected by the corresponding overflow checking built-
ins. For example, the option will issue a warning for the
"strcpy" call below because it copies at least 5 characters (the
string "blue" including the terminating NUL) into the buffer of
size 4.
enum Color { blue, purple, yellow };
const char* f (enum Color clr)
{
static char buf [4];
const char *str;
switch (clr)
{
case blue: str = "blue"; break;
case purple: str = "purple"; break;
case yellow: str = "yellow"; break;
}
return strcpy (buf, str); // warning here
}
Option -Wstringop-overflow=2 is enabled by default.
-Wstringop-overflow
-Wstringop-overflow=1
The -Wstringop-overflow=1 option uses type-zero Object Size
Checking to determine the sizes of destination objects. This
is the default setting of the option. At this setting the
option will not warn for writes past the end of subobjects of
larger objects accessed by pointers unless the size of the
largest surrounding object is known. When the destination
may be one of several objects it is assumed to be the largest
one of them. On Linux systems, when optimization is enabled
at this setting the option warns for the same code as when
the "_FORTIFY_SOURCE" macro is defined to a non-zero value.
-Wstringop-overflow=2
The -Wstringop-overflow=2 option uses type-one Object Size
Checking to determine the sizes of destination objects. At
this setting the option will warn about overflows when
writing to members of the largest complete objects whose
exact size is known. It will, however, not warn for
excessive writes to the same members of unknown objects
referenced by pointers since they may point to arrays
containing unknown numbers of elements.
-Wstringop-overflow=3
The -Wstringop-overflow=3 option uses type-two Object Size
Checking to determine the sizes of destination objects. At
this setting the option warns about overflowing the smallest
object or data member. This is the most restrictive setting
of the option that may result in warnings for safe code.
-Wstringop-overflow=4
The -Wstringop-overflow=4 option uses type-three Object Size
Checking to determine the sizes of destination objects. At
this setting the option will warn about overflowing any data
members, and when the destination is one of several objects
it uses the size of the largest of them to decide whether to
issue a warning. Similarly to -Wstringop-overflow=3 this
setting of the option may result in warnings for benign code.
-Wsuggest-attribute=[pure|const|noreturn|format]
Warn for cases where adding an attribute may be beneficial. The
attributes currently supported are listed below.
-Wsuggest-attribute=pure
-Wsuggest-attribute=const
-Wsuggest-attribute=noreturn
Warn about functions that might be candidates for attributes
"pure", "const" or "noreturn". The compiler only warns for
functions visible in other compilation units or (in the case
of "pure" and "const") if it cannot prove that the function
returns normally. A function returns normally if it doesn't
contain an infinite loop or return abnormally by throwing,
calling "abort" or trapping. This analysis requires option
-fipa-pure-const, which is enabled by default at -O and
higher. Higher optimization levels improve the accuracy of
the analysis.
-Wsuggest-attribute=format
-Wmissing-format-attribute
Warn about function pointers that might be candidates for
"format" attributes. Note these are only possible
candidates, not absolute ones. GCC guesses that function
pointers with "format" attributes that are used in
assignment, initialization, parameter passing or return
statements should have a corresponding "format" attribute in
the resulting type. I.e. the left-hand side of the
assignment or initialization, the type of the parameter
variable, or the return type of the containing function
respectively should also have a "format" attribute to avoid
the warning.
GCC also warns about function definitions that might be
candidates for "format" attributes. Again, these are only
possible candidates. GCC guesses that "format" attributes
might be appropriate for any function that calls a function
like "vprintf" or "vscanf", but this might not always be the
case, and some functions for which "format" attributes are
appropriate may not be detected.
-Wsuggest-final-types
Warn about types with virtual methods where code quality would be
improved if the type were declared with the C++11 "final"
specifier, or, if possible, declared in an anonymous namespace.
This allows GCC to more aggressively devirtualize the polymorphic
calls. This warning is more effective with link time
optimization, where the information about the class hierarchy
graph is more complete.
-Wsuggest-final-methods
Warn about virtual methods where code quality would be improved
if the method were declared with the C++11 "final" specifier, or,
if possible, its type were declared in an anonymous namespace or
with the "final" specifier. This warning is more effective with
link-time optimization, where the information about the class
hierarchy graph is more complete. It is recommended to first
consider suggestions of -Wsuggest-final-types and then rebuild
with new annotations.
-Wsuggest-override
Warn about overriding virtual functions that are not marked with
the override keyword.
-Walloc-zero
Warn about calls to allocation functions decorated with attribute
"alloc_size" that specify zero bytes, including those to the
built-in forms of the functions "aligned_alloc", "alloca",
"calloc", "malloc", and "realloc". Because the behavior of these
functions when called with a zero size differs among
implementations (and in the case of "realloc" has been
deprecated) relying on it may result in subtle portability bugs
and should be avoided.
-Walloc-size-larger-than=n
Warn about calls to functions decorated with attribute
"alloc_size" that attempt to allocate objects larger than the
specified number of bytes, or where the result of the size
computation in an integer type with infinite precision would
exceed "SIZE_MAX / 2". The option argument n may end in one of
the standard suffixes designating a multiple of bytes such as
"kB" and "KiB" for kilobyte and kibibyte, respectively, "MB" and
"MiB" for megabyte and mebibyte, and so on.
-Walloca
This option warns on all uses of "alloca" in the source.
-Walloca-larger-than=n
This option warns on calls to "alloca" that are not bounded by a
controlling predicate limiting its argument of integer type to at
most n bytes, or calls to "alloca" where the bound is unknown.
Arguments of non-integer types are considered unbounded even if
they appear to be constrained to the expected range.
For example, a bounded case of "alloca" could be:
void func (size_t n)
{
void *p;
if (n <= 1000)
p = alloca (n);
else
p = malloc (n);
f (p);
}
In the above example, passing "-Walloca-larger-than=1000" would
not issue a warning because the call to "alloca" is known to be
at most 1000 bytes. However, if "-Walloca-larger-than=500" were
passed, the compiler would emit a warning.
Unbounded uses, on the other hand, are uses of "alloca" with no
controlling predicate constraining its integer argument. For
example:
void func ()
{
void *p = alloca (n);
f (p);
}
If "-Walloca-larger-than=500" were passed, the above would
trigger a warning, but this time because of the lack of bounds
checking.
Note, that even seemingly correct code involving signed integers
could cause a warning:
void func (signed int n)
{
if (n < 500)
{
p = alloca (n);
f (p);
}
}
In the above example, n could be negative, causing a larger than
expected argument to be implicitly cast into the "alloca" call.
This option also warns when "alloca" is used in a loop.
This warning is not enabled by -Wall, and is only active when
-ftree-vrp is active (default for -O2 and above).
See also -Wvla-larger-than=n.
-Warray-bounds
-Warray-bounds=n
This option is only active when -ftree-vrp is active (default for
-O2 and above). It warns about subscripts to arrays that are
always out of bounds. This warning is enabled by -Wall.
-Warray-bounds=1
This is the warning level of -Warray-bounds and is enabled by
-Wall; higher levels are not, and must be explicitly
requested.
-Warray-bounds=2
This warning level also warns about out of bounds access for
arrays at the end of a struct and for arrays accessed through
pointers. This warning level may give a larger number of
false positives and is deactivated by default.
-Wbool-compare
Warn about boolean expression compared with an integer value
different from "true"/"false". For instance, the following
comparison is always false:
int n = 5;
...
if ((n > 1) == 2) { ... }
This warning is enabled by -Wall.
-Wbool-operation
Warn about suspicious operations on expressions of a boolean
type. For instance, bitwise negation of a boolean is very likely
a bug in the program. For C, this warning also warns about
incrementing or decrementing a boolean, which rarely makes sense.
(In C++, decrementing a boolean is always invalid. Incrementing
a boolean is invalid in C++1z, and deprecated otherwise.)
This warning is enabled by -Wall.
-Wduplicated-branches
Warn when an if-else has identical branches. This warning
detects cases like
if (p != NULL)
return 0;
else
return 0;
It doesn't warn when both branches contain just a null statement.
This warning also warn for conditional operators:
int i = x ? *p : *p;
-Wduplicated-cond
Warn about duplicated conditions in an if-else-if chain. For
instance, warn for the following code:
if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }
-Wframe-address
Warn when the __builtin_frame_address or __builtin_return_address
is called with an argument greater than 0. Such calls may return
indeterminate values or crash the program. The warning is
included in -Wall.
-Wno-discarded-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on pointers are being discarded.
Typically, the compiler warns if a "const char *" variable is
passed to a function that takes a "char *" parameter. This
option can be used to suppress such a warning.
-Wno-discarded-array-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on arrays which are pointer
targets are being discarded. Typically, the compiler warns if a
"const int (*)[]" variable is passed to a function that takes a
"int (*)[]" parameter. This option can be used to suppress such
a warning.
-Wno-incompatible-pointer-types (C and Objective-C only)
Do not warn when there is a conversion between pointers that have
incompatible types. This warning is for cases not covered by
-Wno-pointer-sign, which warns for pointer argument passing or
assignment with different signedness.
-Wno-int-conversion (C and Objective-C only)
Do not warn about incompatible integer to pointer and pointer to
integer conversions. This warning is about implicit conversions;
for explicit conversions the warnings -Wno-int-to-pointer-cast
and -Wno-pointer-to-int-cast may be used.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero.
Floating-point division by zero is not warned about, as it can be
a legitimate way of obtaining infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header
files. Warnings from system headers are normally suppressed, on
the assumption that they usually do not indicate real problems
and would only make the compiler output harder to read. Using
this command-line option tells GCC to emit warnings from system
headers as if they occurred in user code. However, note that
using -Wall in conjunction with this option does not warn about
unknown pragmas in system headers---for that, -Wunknown-pragmas
must also be used.
-Wtautological-compare
Warn if a self-comparison always evaluates to true or false.
This warning detects various mistakes such as:
int i = 1;
...
if (i > i) { ... }
This warning is enabled by -Wall.
-Wtrampolines
Warn about trampolines generated for pointers to nested
functions. A trampoline is a small piece of data or code that is
created at run time on the stack when the address of a nested
function is taken, and is used to call the nested function
indirectly. For some targets, it is made up of data only and
thus requires no special treatment. But, for most targets, it is
made up of code and thus requires the stack to be made executable
in order for the program to work properly.
-Wfloat-equal
Warn if floating-point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the
programmer) to consider floating-point values as approximations
to infinitely precise real numbers. If you are doing this, then
you need to compute (by analyzing the code, or in some other way)
the maximum or likely maximum error that the computation
introduces, and allow for it when performing comparisons (and
when producing output, but that's a different problem). In
particular, instead of testing for equality, you should check to
see whether the two values have ranges that overlap; and this is
done with the relational operators, so equality comparisons are
probably mistaken.
-Wtraditional (C and Objective-C only)
Warn about certain constructs that behave differently in
traditional and ISO C. Also warn about ISO C constructs that
have no traditional C equivalent, and/or problematic constructs
that should be avoided.
* Macro parameters that appear within string literals in the
macro body. In traditional C macro replacement takes place
within string literals, but in ISO C it does not.
* In traditional C, some preprocessor directives did not exist.
Traditional preprocessors only considered a line to be a
directive if the # appeared in column 1 on the line.
Therefore -Wtraditional warns about directives that
traditional C understands but ignores because the # does not
appear as the first character on the line. It also suggests
you hide directives like "#pragma" not understood by
traditional C by indenting them. Some traditional
implementations do not recognize "#elif", so this option
suggests avoiding it altogether.
* A function-like macro that appears without arguments.
* The unary plus operator.
* The U integer constant suffix, or the F or L floating-point
constant suffixes. (Traditional C does support the L suffix
on integer constants.) Note, these suffixes appear in macros
defined in the system headers of most modern systems, e.g.
the _MIN/_MAX macros in "<limits.h>". Use of these macros in
user code might normally lead to spurious warnings, however
GCC's integrated preprocessor has enough context to avoid
warning in these cases.
* A function declared external in one block and then used after
the end of the block.
* A "switch" statement has an operand of type "long".
* A non-"static" function declaration follows a "static" one.
This construct is not accepted by some traditional C
compilers.
* The ISO type of an integer constant has a different width or
signedness from its traditional type. This warning is only
issued if the base of the constant is ten. I.e. hexadecimal
or octal values, which typically represent bit patterns, are
not warned about.
* Usage of ISO string concatenation is detected.
* Initialization of automatic aggregates.
* Identifier conflicts with labels. Traditional C lacks a
separate namespace for labels.
* Initialization of unions. If the initializer is zero, the
warning is omitted. This is done under the assumption that
the zero initializer in user code appears conditioned on e.g.
"__STDC__" to avoid missing initializer warnings and relies
on default initialization to zero in the traditional C case.
* Conversions by prototypes between fixed/floating-point values
and vice versa. The absence of these prototypes when
compiling with traditional C causes serious problems. This
is a subset of the possible conversion warnings; for the full
set use -Wtraditional-conversion.
* Use of ISO C style function definitions. This warning
intentionally is not issued for prototype declarations or
variadic functions because these ISO C features appear in
your code when using libiberty's traditional C compatibility
macros, "PARAMS" and "VPARAMS". This warning is also
bypassed for nested functions because that feature is already
a GCC extension and thus not relevant to traditional C
compatibility.
-Wtraditional-conversion (C and Objective-C only)
Warn if a prototype causes a type conversion that is different
from what would happen to the same argument in the absence of a
prototype. This includes conversions of fixed point to floating
and vice versa, and conversions changing the width or signedness
of a fixed-point argument except when the same as the default
promotion.
-Wdeclaration-after-statement (C and Objective-C only)
Warn when a declaration is found after a statement in a block.
This construct, known from C++, was introduced with ISO C99 and
is by default allowed in GCC. It is not supported by ISO C90.
-Wshadow
Warn whenever a local variable or type declaration shadows
another variable, parameter, type, class member (in C++), or
instance variable (in Objective-C) or whenever a built-in
function is shadowed. Note that in C++, the compiler warns if a
local variable shadows an explicit typedef, but not if it shadows
a struct/class/enum. Same as -Wshadow=global.
-Wno-shadow-ivar (Objective-C only)
Do not warn whenever a local variable shadows an instance
variable in an Objective-C method.
-Wshadow=global
The default for -Wshadow. Warns for any (global) shadowing.
-Wshadow=local
Warn when a local variable shadows another local variable or
parameter. This warning is enabled by -Wshadow=global.
-Wshadow=compatible-local
Warn when a local variable shadows another local variable or
parameter whose type is compatible with that of the shadowing
variable. In C++, type compatibility here means the type of the
shadowing variable can be converted to that of the shadowed
variable. The creation of this flag (in addition to
-Wshadow=local) is based on the idea that when a local variable
shadows another one of incompatible type, it is most likely
intentional, not a bug or typo, as shown in the following
example:
for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
for (int i = 0; i < N; ++i)
{
...
}
...
}
Since the two variable "i" in the example above have incompatible
types, enabling only -Wshadow=compatible-local will not emit a
warning. Because their types are incompatible, if a programmer
accidentally uses one in place of the other, type checking will
catch that and emit an error or warning. So not warning (about
shadowing) in this case will not lead to undetected bugs. Use of
this flag instead of -Wshadow=local can possibly reduce the
number of warnings triggered by intentional shadowing.
This warning is enabled by -Wshadow=local.
-Wlarger-than=len
Warn whenever an object of larger than len bytes is defined.
-Wframe-larger-than=len
Warn if the size of a function frame is larger than len bytes.
The computation done to determine the stack frame size is
approximate and not conservative. The actual requirements may be
somewhat greater than len even if you do not get a warning. In
addition, any space allocated via "alloca", variable-length
arrays, or related constructs is not included by the compiler
when determining whether or not to issue a warning.
-Wno-free-nonheap-object
Do not warn when attempting to free an object that was not
allocated on the heap.
-Wstack-usage=len
Warn if the stack usage of a function might be larger than len
bytes. The computation done to determine the stack usage is
conservative. Any space allocated via "alloca", variable-length
arrays, or related constructs is included by the compiler when
determining whether or not to issue a warning.
The message is in keeping with the output of -fstack-usage.
* If the stack usage is fully static but exceeds the specified
amount, it's:
warning: stack usage is 1120 bytes
* If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes
* If the stack usage is (partly) dynamic and not bounded, it's:
warning: stack usage might be unbounded
-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler cannot
assume anything on the bounds of the loop indices. With
-funsafe-loop-optimizations warn if the compiler makes such
assumptions.
-Wno-pedantic-ms-format (MinGW targets only)
When used in combination with -Wformat and -pedantic without GNU
extensions, this option disables the warnings about non-ISO
"printf" / "scanf" format width specifiers "I32", "I64", and "I"
used on Windows targets, which depend on the MS runtime.
-Waligned-new
Warn about a new-expression of a type that requires greater
alignment than the "alignof(std::max_align_t)" but uses an
allocation function without an explicit alignment parameter. This
option is enabled by -Wall.
Normally this only warns about global allocation functions, but
-Waligned-new=all also warns about class member allocation
functions.
-Wplacement-new
-Wplacement-new=n
Warn about placement new expressions with undefined behavior,
such as constructing an object in a buffer that is smaller than
the type of the object. For example, the placement new
expression below is diagnosed because it attempts to construct an
array of 64 integers in a buffer only 64 bytes large.
char buf [64];
new (buf) int[64];
This warning is enabled by default.
-Wplacement-new=1
This is the default warning level of -Wplacement-new. At
this level the warning is not issued for some strictly
undefined constructs that GCC allows as extensions for
compatibility with legacy code. For example, the following
"new" expression is not diagnosed at this level even though
it has undefined behavior according to the C++ standard
because it writes past the end of the one-element array.
struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
new (s->a)int [32]();
-Wplacement-new=2
At this level, in addition to diagnosing all the same
constructs as at level 1, a diagnostic is also issued for
placement new expressions that construct an object in the
last member of structure whose type is an array of a single
element and whose size is less than the size of the object
being constructed. While the previous example would be
diagnosed, the following construct makes use of the flexible
member array extension to avoid the warning at level 2.
struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
new (s->a)int [32]();
-Wpointer-arith
Warn about anything that depends on the "size of" a function type
or of "void". GNU C assigns these types a size of 1, for
convenience in calculations with "void *" pointers and pointers
to functions. In C++, warn also when an arithmetic operation
involves "NULL". This warning is also enabled by -Wpedantic.
-Wpointer-compare
Warn if a pointer is compared with a zero character constant.
This usually means that the pointer was meant to be dereferenced.
For example:
const char *p = foo ();
if (p == '\0')
return 42;
Note that the code above is invalid in C++11.
This warning is enabled by default.
-Wtype-limits
Warn if a comparison is always true or always false due to the
limited range of the data type, but do not warn for constant
expressions. For example, warn if an unsigned variable is
compared against zero with "<" or ">=". This warning is also
enabled by -Wextra.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /*
comment, or whenever a backslash-newline appears in a // comment.
This warning is enabled by -Wall.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the
meaning of the program. Trigraphs within comments are not warned
about, except those that would form escaped newlines.
This option is implied by -Wall. If -Wall is not given, this
option is still enabled unless trigraphs are enabled. To get
trigraph conversion without warnings, but get the other -Wall
warnings, use -trigraphs -Wall -Wno-trigraphs.
-Wundef
Warn if an undefined identifier is evaluated in an "#if"
directive. Such identifiers are replaced with zero.
-Wexpansion-to-defined
Warn whenever defined is encountered in the expansion of a macro
(including the case where the macro is expanded by an #if
directive). Such usage is not portable. This warning is also
enabled by -Wpedantic and -Wextra.
-Wunused-macros
Warn about macros defined in the main file that are unused. A
macro is used if it is expanded or tested for existence at least
once. The preprocessor also warns if the macro has not been used
at the time it is redefined or undefined.
Built-in macros, macros defined on the command line, and macros
defined in include files are not warned about.
Note: If a macro is actually used, but only used in skipped
conditional blocks, then the preprocessor reports it as unused.
To avoid the warning in such a case, you might improve the scope
of the macro's definition by, for example, moving it into the
first skipped block. Alternatively, you could provide a dummy
use with something like:
#if defined the_macro_causing_the_warning
#endif
-Wno-endif-labels
Do not warn whenever an "#else" or an "#endif" are followed by
text. This sometimes happens in older programs with code of the
form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments. This warning
is on by default.
-Wbad-function-cast (C and Objective-C only)
Warn when a function call is cast to a non-matching type. For
example, warn if a call to a function returning an integer type
is cast to a pointer type.
-Wc90-c99-compat (C and Objective-C only)
Warn about features not present in ISO C90, but present in ISO
C99. For instance, warn about use of variable length arrays,
"long long" type, "bool" type, compound literals, designated
initializers, and so on. This option is independent of the
standards mode. Warnings are disabled in the expression that
follows "__extension__".
-Wc99-c11-compat (C and Objective-C only)
Warn about features not present in ISO C99, but present in ISO
C11. For instance, warn about use of anonymous structures and
unions, "_Atomic" type qualifier, "_Thread_local" storage-class
specifier, "_Alignas" specifier, "Alignof" operator, "_Generic"
keyword, and so on. This option is independent of the standards
mode. Warnings are disabled in the expression that follows
"__extension__".
-Wc++-compat (C and Objective-C only)
Warn about ISO C constructs that are outside of the common subset
of ISO C and ISO C++, e.g. request for implicit conversion from
"void *" to a pointer to non-"void" type.
-Wc++11-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++
1998 and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are
keywords in ISO C++ 2011. This warning turns on -Wnarrowing and
is enabled by -Wall.
-Wc++14-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++
2011 and ISO C++ 2014. This warning is enabled by -Wall.
-Wc++1z-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++
2014 and the forthoming ISO C++ 2017(?). This warning is enabled
by -Wall.
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier
from the target type. For example, warn if a "const char *" is
cast to an ordinary "char *".
Also warn when making a cast that introduces a type qualifier in
an unsafe way. For example, casting "char **" to "const char **"
is unsafe, as in this example:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';
-Wcast-align
Warn whenever a pointer is cast such that the required alignment
of the target is increased. For example, warn if a "char *" is
cast to an "int *" on machines where integers can only be
accessed at two- or four-byte boundaries.
-Wwrite-strings
When compiling C, give string constants the type "const
char[length]" so that copying the address of one into a
non-"const" "char *" pointer produces a warning. These warnings
help you find at compile time code that can try to write into a
string constant, but only if you have been very careful about
using "const" in declarations and prototypes. Otherwise, it is
just a nuisance. This is why we did not make -Wall request these
warnings.
When compiling C++, warn about the deprecated conversion from
string literals to "char *". This warning is enabled by default
for C++ programs.
-Wclobbered
Warn for variables that might be changed by "longjmp" or "vfork".
This warning is also enabled by -Wextra.
-Wconditionally-supported (C++ and Objective-C++ only)
Warn for conditionally-supported (C++11 [intro.defs]) constructs.
-Wconversion
Warn for implicit conversions that may alter a value. This
includes conversions between real and integer, like "abs (x)"
when "x" is "double"; conversions between signed and unsigned,
like "unsigned ui = -1"; and conversions to smaller types, like
"sqrtf (M_PI)". Do not warn for explicit casts like "abs ((int)
x)" and "ui = (unsigned) -1", or if the value is not changed by
the conversion like in "abs (2.0)". Warnings about conversions
between signed and unsigned integers can be disabled by using
-Wno-sign-conversion.
For C++, also warn for confusing overload resolution for user-
defined conversions; and conversions that never use a type
conversion operator: conversions to "void", the same type, a base
class or a reference to them. Warnings about conversions between
signed and unsigned integers are disabled by default in C++
unless -Wsign-conversion is explicitly enabled.
-Wno-conversion-null (C++ and Objective-C++ only)
Do not warn for conversions between "NULL" and non-pointer types.
-Wconversion-null is enabled by default.
-Wzero-as-null-pointer-constant (C++ and Objective-C++ only)
Warn when a literal 0 is used as null pointer constant. This can
be useful to facilitate the conversion to "nullptr" in C++11.
-Wsubobject-linkage (C++ and Objective-C++ only)
Warn if a class type has a base or a field whose type uses the
anonymous namespace or depends on a type with no linkage. If a
type A depends on a type B with no or internal linkage, defining
it in multiple translation units would be an ODR violation
because the meaning of B is different in each translation unit.
If A only appears in a single translation unit, the best way to
silence the warning is to give it internal linkage by putting it
in an anonymous namespace as well. The compiler doesn't give
this warning for types defined in the main .C file, as those are
unlikely to have multiple definitions. -Wsubobject-linkage is
enabled by default.
-Wdangling-else
Warn about constructions where there may be confusion to which
"if" statement an "else" branch belongs. Here is an example of
such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C/C++, every "else" branch belongs to the innermost possible
"if" statement, which in this example is "if (b)". This is often
not what the programmer expected, as illustrated in the above
example by indentation the programmer chose. When there is the
potential for this confusion, GCC issues a warning when this flag
is specified. To eliminate the warning, add explicit braces
around the innermost "if" statement so there is no way the "else"
can belong to the enclosing "if". The resulting code looks like
this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
This warning is enabled by -Wparentheses.
-Wdate-time
Warn when macros "__TIME__", "__DATE__" or "__TIMESTAMP__" are
encountered as they might prevent bit-wise-identical reproducible
compilations.
-Wdelete-incomplete (C++ and Objective-C++ only)
Warn when deleting a pointer to incomplete type, which may cause
undefined behavior at runtime. This warning is enabled by
default.
-Wuseless-cast (C++ and Objective-C++ only)
Warn when an expression is casted to its own type.
-Wempty-body
Warn if an empty body occurs in an "if", "else" or "do while"
statement. This warning is also enabled by -Wextra.
-Wenum-compare
Warn about a comparison between values of different enumerated
types. In C++ enumerated type mismatches in conditional
expressions are also diagnosed and the warning is enabled by
default. In C this warning is enabled by -Wall.
-Wjump-misses-init (C, Objective-C only)
Warn if a "goto" statement or a "switch" statement jumps forward
across the initialization of a variable, or jumps backward to a
label after the variable has been initialized. This only warns
about variables that are initialized when they are declared.
This warning is only supported for C and Objective-C; in C++ this
sort of branch is an error in any case.
-Wjump-misses-init is included in -Wc++-compat. It can be
disabled with the -Wno-jump-misses-init option.
-Wsign-compare
Warn when a comparison between signed and unsigned values could
produce an incorrect result when the signed value is converted to
unsigned. In C++, this warning is also enabled by -Wall. In C,
it is also enabled by -Wextra.
-Wsign-conversion
Warn for implicit conversions that may change the sign of an
integer value, like assigning a signed integer expression to an
unsigned integer variable. An explicit cast silences the warning.
In C, this option is enabled also by -Wconversion.
-Wfloat-conversion
Warn for implicit conversions that reduce the precision of a real
value. This includes conversions from real to integer, and from
higher precision real to lower precision real values. This
option is also enabled by -Wconversion.
-Wno-scalar-storage-order
Do not warn on suspicious constructs involving reverse scalar
storage order.
-Wsized-deallocation (C++ and Objective-C++ only)
Warn about a definition of an unsized deallocation function
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;
without a definition of the corresponding sized deallocation
function
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
or vice versa. Enabled by -Wextra along with
-fsized-deallocation.
-Wsizeof-pointer-memaccess
Warn for suspicious length parameters to certain string and
memory built-in functions if the argument uses "sizeof". This
warning warns e.g. about "memset (ptr, 0, sizeof (ptr));" if
"ptr" is not an array, but a pointer, and suggests a possible
fix, or about "memcpy (&foo, ptr, sizeof (&foo));". This warning
is enabled by -Wall.
-Wsizeof-array-argument
Warn when the "sizeof" operator is applied to a parameter that is
declared as an array in a function definition. This warning is
enabled by default for C and C++ programs.
-Wmemset-elt-size
Warn for suspicious calls to the "memset" built-in function, if
the first argument references an array, and the third argument is
a number equal to the number of elements, but not equal to the
size of the array in memory. This indicates that the user has
omitted a multiplication by the element size. This warning is
enabled by -Wall.
-Wmemset-transposed-args
Warn for suspicious calls to the "memset" built-in function, if
the second argument is not zero and the third argument is zero.
This warns e.g.@ about "memset (buf, sizeof buf, 0)" where most
probably "memset (buf, 0, sizeof buf)" was meant instead. The
diagnostics is only emitted if the third argument is literal
zero. If it is some expression that is folded to zero, a cast of
zero to some type, etc., it is far less likely that the user has
mistakenly exchanged the arguments and no warning is emitted.
This warning is enabled by -Wall.
-Waddress
Warn about suspicious uses of memory addresses. These include
using the address of a function in a conditional expression, such
as "void func(void); if (func)", and comparisons against the
memory address of a string literal, such as "if (x == "abc")".
Such uses typically indicate a programmer error: the address of a
function always evaluates to true, so their use in a conditional
usually indicate that the programmer forgot the parentheses in a
function call; and comparisons against string literals result in
unspecified behavior and are not portable in C, so they usually
indicate that the programmer intended to use "strcmp". This
warning is enabled by -Wall.
-Wlogical-op
Warn about suspicious uses of logical operators in expressions.
This includes using logical operators in contexts where a bit-
wise operator is likely to be expected. Also warns when the
operands of a logical operator are the same:
extern int a;
if (a < 0 && a < 0) { ... }
-Wlogical-not-parentheses
Warn about logical not used on the left hand side operand of a
comparison. This option does not warn if the right operand is
considered to be a boolean expression. Its purpose is to detect
suspicious code like the following:
int a;
...
if (!a > 1) { ... }
It is possible to suppress the warning by wrapping the LHS into
parentheses:
if ((!a) > 1) { ... }
This warning is enabled by -Wall.
-Waggregate-return
Warn if any functions that return structures or unions are
defined or called. (In languages where you can return an array,
this also elicits a warning.)
-Wno-aggressive-loop-optimizations
Warn if in a loop with constant number of iterations the compiler
detects undefined behavior in some statement during one or more
of the iterations.
-Wno-attributes
Do not warn if an unexpected "__attribute__" is used, such as
unrecognized attributes, function attributes applied to
variables, etc. This does not stop errors for incorrect use of
supported attributes.
-Wno-builtin-declaration-mismatch
Warn if a built-in function is declared with the wrong signature.
This warning is enabled by default.
-Wno-builtin-macro-redefined
Do not warn if certain built-in macros are redefined. This
suppresses warnings for redefinition of "__TIMESTAMP__",
"__TIME__", "__DATE__", "__FILE__", and "__BASE_FILE__".
-Wstrict-prototypes (C and Objective-C only)
Warn if a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted
without a warning if preceded by a declaration that specifies the
argument types.)
-Wold-style-declaration (C and Objective-C only)
Warn for obsolescent usages, according to the C Standard, in a
declaration. For example, warn if storage-class specifiers like
"static" are not the first things in a declaration. This warning
is also enabled by -Wextra.
-Wold-style-definition (C and Objective-C only)
Warn if an old-style function definition is used. A warning is
given even if there is a previous prototype.
-Wmissing-parameter-type (C and Objective-C only)
A function parameter is declared without a type specifier in
K&R-style functions:
void foo(bar) { }
This warning is also enabled by -Wextra.
-Wmissing-prototypes (C and Objective-C only)
Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition
itself provides a prototype. Use this option to detect global
functions that do not have a matching prototype declaration in a
header file. This option is not valid for C++ because all
function declarations provide prototypes and a non-matching
declaration declares an overload rather than conflict with an
earlier declaration. Use -Wmissing-declarations to detect
missing declarations in C++.
-Wmissing-declarations
Warn if a global function is defined without a previous
declaration. Do so even if the definition itself provides a
prototype. Use this option to detect global functions that are
not declared in header files. In C, no warnings are issued for
functions with previous non-prototype declarations; use
-Wmissing-prototypes to detect missing prototypes. In C++, no
warnings are issued for function templates, or for inline
functions, or for functions in anonymous namespaces.
-Wmissing-field-initializers
Warn if a structure's initializer has some fields missing. For
example, the following code causes such a warning, because "x.h"
is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
This option does not warn about designated initializers, so the
following modification does not trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
In C++ this option does not warn either about the empty { }
initializer, for example:
struct s { int f, g, h; };
s x = { };
This warning is included in -Wextra. To get other -Wextra
warnings without this one, use -Wextra
-Wno-missing-field-initializers.
-Wno-multichar
Do not warn if a multicharacter constant ('FOOF') is used.
Usually they indicate a typo in the user's code, as they have
implementation-defined values, and should not be used in portable
code.
-Wnormalized=[none|id|nfc|nfkc]
In ISO C and ISO C++, two identifiers are different if they are
different sequences of characters. However, sometimes when
characters outside the basic ASCII character set are used, you
can have two different character sequences that look the same.
To avoid confusion, the ISO 10646 standard sets out some
normalization rules which when applied ensure that two sequences
that look the same are turned into the same sequence. GCC can
warn you if you are using identifiers that have not been
normalized; this option controls that warning.
There are four levels of warning supported by GCC. The default
is -Wnormalized=nfc, which warns about any identifier that is not
in the ISO 10646 "C" normalized form, NFC. NFC is the
recommended form for most uses. It is equivalent to
-Wnormalized.
Unfortunately, there are some characters allowed in identifiers
by ISO C and ISO C++ that, when turned into NFC, are not allowed
in identifiers. That is, there's no way to use these symbols in
portable ISO C or C++ and have all your identifiers in NFC.
-Wnormalized=id suppresses the warning for these characters. It
is hoped that future versions of the standards involved will
correct this, which is why this option is not the default.
You can switch the warning off for all characters by writing
-Wnormalized=none or -Wno-normalized. You should only do this if
you are using some other normalization scheme (like "D"), because
otherwise you can easily create bugs that are literally
impossible to see.
Some characters in ISO 10646 have distinct meanings but look
identical in some fonts or display methodologies, especially once
formatting has been applied. For instance "\u207F", "SUPERSCRIPT
LATIN SMALL LETTER N", displays just like a regular "n" that has
been placed in a superscript. ISO 10646 defines the NFKC
normalization scheme to convert all these into a standard form as
well, and GCC warns if your code is not in NFKC if you use
-Wnormalized=nfkc. This warning is comparable to warning about
every identifier that contains the letter O because it might be
confused with the digit 0, and so is not the default, but may be
useful as a local coding convention if the programming
environment cannot be fixed to display these characters
distinctly.
-Wno-deprecated
Do not warn about usage of deprecated features.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked
as deprecated by using the "deprecated" attribute.
-Wno-overflow
Do not warn about compile-time overflow in constant expressions.
-Wno-odr
Warn about One Definition Rule violations during link-time
optimization. Requires -flto-odr-type-merging to be enabled.
Enabled by default.
-Wopenmp-simd
Warn if the vectorizer cost model overrides the OpenMP or the
Cilk Plus simd directive set by user. The
-fsimd-cost-model=unlimited option can be used to relax the cost
model.
-Woverride-init (C and Objective-C only)
Warn if an initialized field without side effects is overridden
when using designated initializers.
This warning is included in -Wextra. To get other -Wextra
warnings without this one, use -Wextra -Wno-override-init.
-Woverride-init-side-effects (C and Objective-C only)
Warn if an initialized field with side effects is overridden when
using designated initializers. This warning is enabled by
default.
-Wpacked
Warn if a structure is given the packed attribute, but the packed
attribute has no effect on the layout or size of the structure.
Such structures may be mis-aligned for little benefit. For
instance, in this code, the variable "f.x" in "struct bar" is
misaligned even though "struct bar" does not itself have the
packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wpacked-bitfield-compat
The 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute
on bit-fields of type "char". This has been fixed in GCC 4.4 but
the change can lead to differences in the structure layout. GCC
informs you when the offset of such a field has changed in GCC
4.4. For example there is no longer a 4-bit padding between
field "a" and "b" in this structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));
This warning is enabled by default. Use
-Wno-packed-bitfield-compat to disable this warning.
-Wpadded
Warn if padding is included in a structure, either to align an
element of the structure or to align the whole structure.
Sometimes when this happens it is possible to rearrange the
fields of the structure to reduce the padding and so make the
structure smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope,
even in cases where multiple declaration is valid and changes
nothing.
-Wrestrict
Warn when an argument passed to a restrict-qualified parameter
aliases with another argument.
-Wnested-externs (C and Objective-C only)
Warn if an "extern" declaration is encountered within a function.
-Wno-inherited-variadic-ctor
Suppress warnings about use of C++11 inheriting constructors when
the base class inherited from has a C variadic constructor; the
warning is on by default because the ellipsis is not inherited.
-Winline
Warn if a function that is declared as inline cannot be inlined.
Even with this option, the compiler does not warn about failures
to inline functions declared in system headers.
The compiler uses a variety of heuristics to determine whether or
not to inline a function. For example, the compiler takes into
account the size of the function being inlined and the amount of
inlining that has already been done in the current function.
Therefore, seemingly insignificant changes in the source program
can cause the warnings produced by -Winline to appear or
disappear.
-Wno-invalid-offsetof (C++ and Objective-C++ only)
Suppress warnings from applying the "offsetof" macro to a non-POD
type. According to the 2014 ISO C++ standard, applying
"offsetof" to a non-standard-layout type is undefined. In
existing C++ implementations, however, "offsetof" typically gives
meaningful results. This flag is for users who are aware that
they are writing nonportable code and who have deliberately
chosen to ignore the warning about it.
The restrictions on "offsetof" may be relaxed in a future version
of the C++ standard.
-Wint-in-bool-context
Warn for suspicious use of integer values where boolean values
are expected, such as conditional expressions (?:) using non-
boolean integer constants in boolean context, like "if (a <= b ?
2 : 3)". Or left shifting of signed integers in boolean context,
like "for (a = 0; 1 << a; a++);". Likewise for all kinds of
multiplications regardless of the data type. This warning is
enabled by -Wall.
-Wno-int-to-pointer-cast
Suppress warnings from casts to pointer type of an integer of a
different size. In C++, casting to a pointer type of smaller size
is an error. Wint-to-pointer-cast is enabled by default.
-Wno-pointer-to-int-cast (C and Objective-C only)
Suppress warnings from casts from a pointer to an integer type of
a different size.
-Winvalid-pch
Warn if a precompiled header is found in the search path but
cannot be used.
-Wlong-long
Warn if "long long" type is used. This is enabled by either
-Wpedantic or -Wtraditional in ISO C90 and C++98 modes. To
inhibit the warning messages, use -Wno-long-long.
-Wvariadic-macros
Warn if variadic macros are used in ISO C90 mode, or if the GNU
alternate syntax is used in ISO C99 mode. This is enabled by
either -Wpedantic or -Wtraditional. To inhibit the warning
messages, use -Wno-variadic-macros.
-Wvarargs
Warn upon questionable usage of the macros used to handle
variable arguments like "va_start". This is default. To inhibit
the warning messages, use -Wno-varargs.
-Wvector-operation-performance
Warn if vector operation is not implemented via SIMD capabilities
of the architecture. Mainly useful for the performance tuning.
Vector operation can be implemented "piecewise", which means that
the scalar operation is performed on every vector element; "in
parallel", which means that the vector operation is implemented
using scalars of wider type, which normally is more performance
efficient; and "as a single scalar", which means that vector fits
into a scalar type.
-Wno-virtual-move-assign
Suppress warnings about inheriting from a virtual base with a
non-trivial C++11 move assignment operator. This is dangerous
because if the virtual base is reachable along more than one
path, it is moved multiple times, which can mean both objects end
up in the moved-from state. If the move assignment operator is
written to avoid moving from a moved-from object, this warning
can be disabled.
-Wvla
Warn if a variable-length array is used in the code. -Wno-vla
prevents the -Wpedantic warning of the variable-length array.
-Wvla-larger-than=n
If this option is used, the compiler will warn on uses of
variable-length arrays where the size is either unbounded, or
bounded by an argument that can be larger than n bytes. This is
similar to how -Walloca-larger-than=n works, but with variable-
length arrays.
Note that GCC may optimize small variable-length arrays of a
known value into plain arrays, so this warning may not get
triggered for such arrays.
This warning is not enabled by -Wall, and is only active when
-ftree-vrp is active (default for -O2 and above).
See also -Walloca-larger-than=n.
-Wvolatile-register-var
Warn if a register variable is declared volatile. The volatile
modifier does not inhibit all optimizations that may eliminate
reads and/or writes to register variables. This warning is
enabled by -Wall.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning
does not generally indicate that there is anything wrong with
your code; it merely indicates that GCC's optimizers are unable
to handle the code effectively. Often, the problem is that your
code is too big or too complex; GCC refuses to optimize programs
when the optimization itself is likely to take inordinate amounts
of time.
-Wpointer-sign (C and Objective-C only)
Warn for pointer argument passing or assignment with different
signedness. This option is only supported for C and Objective-C.
It is implied by -Wall and by -Wpedantic, which can be disabled
with -Wno-pointer-sign.
-Wstack-protector
This option is only active when -fstack-protector is active. It
warns about functions that are not protected against stack
smashing.
-Woverlength-strings
Warn about string constants that are longer than the "minimum
maximum" length specified in the C standard. Modern compilers
generally allow string constants that are much longer than the
standard's minimum limit, but very portable programs should avoid
using longer strings.
The limit applies after string constant concatenation, and does
not count the trailing NUL. In C90, the limit was 509
characters; in C99, it was raised to 4095. C++98 does not
specify a normative minimum maximum, so we do not diagnose
overlength strings in C++.
This option is implied by -Wpedantic, and can be disabled with
-Wno-overlength-strings.
-Wunsuffixed-float-constants (C and Objective-C only)
Issue a warning for any floating constant that does not have a
suffix. When used together with -Wsystem-headers it warns about
such constants in system header files. This can be useful when
preparing code to use with the "FLOAT_CONST_DECIMAL64" pragma
from the decimal floating-point extension to C99.
-Wno-designated-init (C and Objective-C only)
Suppress warnings when a positional initializer is used to
initialize a structure that has been marked with the
"designated_init" attribute.
-Whsa
Issue a warning when HSAIL cannot be emitted for the compiled
function or OpenMP construct.
Options for Debugging Your Program
To tell GCC to emit extra information for use by a debugger, in
almost all cases you need only to add -g to your other options.
GCC allows you to use -g with -O. The shortcuts taken by optimized
code may occasionally be surprising: some variables you declared may
not exist at all; flow of control may briefly move where you did not
expect it; some statements may not be executed because they compute
constant results or their values are already at hand; some statements
may execute in different places because they have been moved out of
loops. Nevertheless it is possible to debug optimized output. This
makes it reasonable to use the optimizer for programs that might have
bugs.
If you are not using some other optimization option, consider using
-Og with -g. With no -O option at all, some compiler passes that
collect information useful for debugging do not run at all, so that
-Og may result in a better debugging experience.
-g Produce debugging information in the operating system's native
format (stabs, COFF, XCOFF, or DWARF). GDB can work with this
debugging information.
On most systems that use stabs format, -g enables use of extra
debugging information that only GDB can use; this extra
information makes debugging work better in GDB but probably makes
other debuggers crash or refuse to read the program. If you want
to control for certain whether to generate the extra information,
use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).
-ggdb
Produce debugging information for use by GDB. This means to use
the most expressive format available (DWARF, stabs, or the native
format if neither of those are supported), including GDB
extensions if at all possible.
-gdwarf
-gdwarf-version
Produce debugging information in DWARF format (if that is
supported). The value of version may be either 2, 3, 4 or 5; the
default version for most targets is 4. DWARF Version 5 is only
experimental.
Note that with DWARF Version 2, some ports require and always use
some non-conflicting DWARF 3 extensions in the unwind tables.
Version 4 may require GDB 7.0 and -fvar-tracking-assignments for
maximum benefit.
GCC no longer supports DWARF Version 1, which is substantially
different than Version 2 and later. For historical reasons, some
other DWARF-related options (including -feliminate-dwarf2-dups
and -fno-dwarf2-cfi-asm) retain a reference to DWARF Version 2 in
their names, but apply to all currently-supported versions of
DWARF.
-gstabs
Produce debugging information in stabs format (if that is
supported), without GDB extensions. This is the format used by
DBX on most BSD systems. On MIPS, Alpha and System V Release 4
systems this option produces stabs debugging output that is not
understood by DBX or SDB. On System V Release 4 systems this
option requires the GNU assembler.
-gstabs+
Produce debugging information in stabs format (if that is
supported), using GNU extensions understood only by the GNU
debugger (GDB). The use of these extensions is likely to make
other debuggers crash or refuse to read the program.
-gcoff
Produce debugging information in COFF format (if that is
supported). This is the format used by SDB on most System V
systems prior to System V Release 4.
-gxcoff
Produce debugging information in XCOFF format (if that is
supported). This is the format used by the DBX debugger on IBM
RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if that is
supported), using GNU extensions understood only by the GNU
debugger (GDB). The use of these extensions is likely to make
other debuggers crash or refuse to read the program, and may
cause assemblers other than the GNU assembler (GAS) to fail with
an error.
-gvms
Produce debugging information in Alpha/VMS debug format (if that
is supported). This is the format used by DEBUG on Alpha/VMS
systems.
-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gvmslevel
Request debugging information and also use level to specify how
much information. The default level is 2.
Level 0 produces no debug information at all. Thus, -g0 negates
-g.
Level 1 produces minimal information, enough for making
backtraces in parts of the program that you don't plan to debug.
This includes descriptions of functions and external variables,
and line number tables, but no information about local variables.
Level 3 includes extra information, such as all the macro
definitions present in the program. Some debuggers support macro
expansion when you use -g3.
-gdwarf does not accept a concatenated debug level, to avoid
confusion with -gdwarf-level. Instead use an additional -glevel
option to change the debug level for DWARF.
-feliminate-unused-debug-symbols
Produce debugging information in stabs format (if that is
supported), for only symbols that are actually used.
-femit-class-debug-always
Instead of emitting debugging information for a C++ class in only
one object file, emit it in all object files using the class.
This option should be used only with debuggers that are unable to
handle the way GCC normally emits debugging information for
classes because using this option increases the size of debugging
information by as much as a factor of two.
-fno-merge-debug-strings
Direct the linker to not merge together strings in the debugging
information that are identical in different object files.
Merging is not supported by all assemblers or linkers. Merging
decreases the size of the debug information in the output file at
the cost of increasing link processing time. Merging is enabled
by default.
-fdebug-prefix-map=old=new
When compiling files in directory old, record debugging
information describing them as in new instead.
-fvar-tracking
Run variable tracking pass. It computes where variables are
stored at each position in code. Better debugging information is
then generated (if the debugging information format supports this
information).
It is enabled by default when compiling with optimization (-Os,
-O, -O2, ...), debugging information (-g) and the debug info
format supports it.
-fvar-tracking-assignments
Annotate assignments to user variables early in the compilation
and attempt to carry the annotations over throughout the
compilation all the way to the end, in an attempt to improve
debug information while optimizing. Use of -gdwarf-4 is
recommended along with it.
It can be enabled even if var-tracking is disabled, in which case
annotations are created and maintained, but discarded at the end.
By default, this flag is enabled together with -fvar-tracking,
except when selective scheduling is enabled.
-gsplit-dwarf
Separate as much DWARF debugging information as possible into a
separate output file with the extension .dwo. This option allows
the build system to avoid linking files with debug information.
To be useful, this option requires a debugger capable of reading
.dwo files.
-gpubnames
Generate DWARF ".debug_pubnames" and ".debug_pubtypes" sections.
-ggnu-pubnames
Generate ".debug_pubnames" and ".debug_pubtypes" sections in a
format suitable for conversion into a GDB index. This option is
only useful with a linker that can produce GDB index version 7.
-fdebug-types-section
When using DWARF Version 4 or higher, type DIEs can be put into
their own ".debug_types" section instead of making them part of
the ".debug_info" section. It is more efficient to put them in a
separate comdat sections since the linker can then remove
duplicates. But not all DWARF consumers support ".debug_types"
sections yet and on some objects ".debug_types" produces larger
instead of smaller debugging information.
-grecord-gcc-switches
-gno-record-gcc-switches
This switch causes the command-line options used to invoke the
compiler that may affect code generation to be appended to the
DW_AT_producer attribute in DWARF debugging information. The
options are concatenated with spaces separating them from each
other and from the compiler version. It is enabled by default.
See also -frecord-gcc-switches for another way of storing
compiler options into the object file.
-gstrict-dwarf
Disallow using extensions of later DWARF standard version than
selected with -gdwarf-version. On most targets using non-
conflicting DWARF extensions from later standard versions is
allowed.
-gno-strict-dwarf
Allow using extensions of later DWARF standard version than
selected with -gdwarf-version.
-gcolumn-info
-gno-column-info
Emit location column information into DWARF debugging
information, rather than just file and line. This option is
disabled by default.
-gz[=type]
Produce compressed debug sections in DWARF format, if that is
supported. If type is not given, the default type depends on the
capabilities of the assembler and linker used. type may be one
of none (don't compress debug sections), zlib (use zlib
compression in ELF gABI format), or zlib-gnu (use zlib
compression in traditional GNU format). If the linker doesn't
support writing compressed debug sections, the option is
rejected. Otherwise, if the assembler does not support them, -gz
is silently ignored when producing object files.
-feliminate-dwarf2-dups
Compress DWARF debugging information by eliminating duplicated
information about each symbol. This option only makes sense when
generating DWARF debugging information.
-femit-struct-debug-baseonly
Emit debug information for struct-like types only when the base
name of the compilation source file matches the base name of file
in which the struct is defined.
This option substantially reduces the size of debugging
information, but at significant potential loss in type
information to the debugger. See -femit-struct-debug-reduced for
a less aggressive option. See -femit-struct-debug-detailed for
more detailed control.
This option works only with DWARF debug output.
-femit-struct-debug-reduced
Emit debug information for struct-like types only when the base
name of the compilation source file matches the base name of file
in which the type is defined, unless the struct is a template or
defined in a system header.
This option significantly reduces the size of debugging
information, with some potential loss in type information to the
debugger. See -femit-struct-debug-baseonly for a more aggressive
option. See -femit-struct-debug-detailed for more detailed
control.
This option works only with DWARF debug output.
-femit-struct-debug-detailed[=spec-list]
Specify the struct-like types for which the compiler generates
debug information. The intent is to reduce duplicate struct
debug information between different object files within the same
program.
This option is a detailed version of -femit-struct-debug-reduced
and -femit-struct-debug-baseonly, which serves for most needs.
A specification has the
syntax[dir:|ind:][ord:|gen:](any|sys|base|none)
The optional first word limits the specification to structs that
are used directly (dir:) or used indirectly (ind:). A struct
type is used directly when it is the type of a variable, member.
Indirect uses arise through pointers to structs. That is, when
use of an incomplete struct is valid, the use is indirect. An
example is struct one direct; struct two * indirect;.
The optional second word limits the specification to ordinary
structs (ord:) or generic structs (gen:). Generic structs are a
bit complicated to explain. For C++, these are non-explicit
specializations of template classes, or non-template classes
within the above. Other programming languages have generics, but
-femit-struct-debug-detailed does not yet implement them.
The third word specifies the source files for those structs for
which the compiler should emit debug information. The values
none and any have the normal meaning. The value base means that
the base of name of the file in which the type declaration
appears must match the base of the name of the main compilation
file. In practice, this means that when compiling foo.c, debug
information is generated for types declared in that file and
foo.h, but not other header files. The value sys means those
types satisfying base or declared in system or compiler headers.
You may need to experiment to determine the best settings for
your application.
The default is -femit-struct-debug-detailed=all.
This option works only with DWARF debug output.
-fno-dwarf2-cfi-asm
Emit DWARF unwind info as compiler generated ".eh_frame" section
instead of using GAS ".cfi_*" directives.
-fno-eliminate-unused-debug-types
Normally, when producing DWARF output, GCC avoids producing debug
symbol output for types that are nowhere used in the source file
being compiled. Sometimes it is useful to have GCC emit
debugging information for all types declared in a compilation
unit, regardless of whether or not they are actually used in that
compilation unit, for example if, in the debugger, you want to
cast a value to a type that is not actually used in your program
(but is declared). More often, however, this results in a
significant amount of wasted space.
Options That Control Optimization
These options control various sorts of optimizations.
Without any optimization option, the compiler's goal is to reduce the
cost of compilation and to make debugging produce the expected
results. Statements are independent: if you stop the program with a
breakpoint between statements, you can then assign a new value to any
variable or change the program counter to any other statement in the
function and get exactly the results you expect from the source code.
Turning on optimization flags makes the compiler attempt to improve
the performance and/or code size at the expense of compilation time
and possibly the ability to debug the program.
The compiler performs optimization based on the knowledge it has of
the program. Compiling multiple files at once to a single output
file mode allows the compiler to use information gained from all of
the files when compiling each of them.
Not all optimizations are controlled directly by a flag. Only
optimizations that have a flag are listed in this section.
Most optimizations are only enabled if an -O level is set on the
command line. Otherwise they are disabled, even if individual
optimization flags are specified.
Depending on the target and how GCC was configured, a slightly
different set of optimizations may be enabled at each -O level than
those listed here. You can invoke GCC with -Q --help=optimizers to
find out the exact set of optimizations that are enabled at each
level.
-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and a
lot more memory for a large function.
With -O, the compiler tries to reduce code size and execution
time, without performing any optimizations that take a great deal
of compilation time.
-O turns on the following optimization flags:
-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments
-fcompare-elim -fcprop-registers -fdce -fdefer-pop
-fdelayed-branch -fdse -fforward-propagate
-fguess-branch-probability -fif-conversion2 -fif-conversion
-finline-functions-called-once -fipa-pure-const -fipa-profile
-fipa-reference -fmerge-constants -fmove-loop-invariants
-freorder-blocks -fshrink-wrap -fshrink-wrap-separate
-fsplit-wide-types -fssa-backprop -fssa-phiopt -ftree-bit-ccp
-ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop
-ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop
-ftree-fre -ftree-phiprop -ftree-sink -ftree-slsr -ftree-sra
-ftree-pta -ftree-ter -funit-at-a-time
-O also turns on -fomit-frame-pointer on machines where doing so
does not interfere with debugging.
-O2 Optimize even more. GCC performs nearly all supported
optimizations that do not involve a space-speed tradeoff. As
compared to -O, this option increases both compilation time and
the performance of the generated code.
-O2 turns on all optimization flags specified by -O. It also
turns on the following optimization flags: -fthread-jumps
-falign-functions -falign-jumps -falign-loops -falign-labels
-fcaller-saves -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fdelete-null-pointer-checks -fdevirtualize
-fdevirtualize-speculatively -fexpensive-optimizations -fgcse
-fgcse-lm -fhoist-adjacent-loads -finline-small-functions
-findirect-inlining -fipa-cp -fipa-bit-cp -fipa-vrp -fipa-sra
-fipa-icf -fisolate-erroneous-paths-dereference -flra-remat
-foptimize-sibling-calls -foptimize-strlen -fpartial-inlining
-fpeephole2 -freorder-blocks-algorithm=stc
-freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -fsched-interblock -fsched-spec
-fschedule-insns -fschedule-insns2 -fstore-merging
-fstrict-aliasing -fstrict-overflow -ftree-builtin-call-dce
-ftree-switch-conversion -ftree-tail-merge -fcode-hoisting
-ftree-pre -ftree-vrp -fipa-ra
Please note the warning under -fgcse about invoking -O2 on
programs that use computed gotos.
-O3 Optimize yet more. -O3 turns on all optimizations specified by
-O2 and also turns on the -finline-functions, -funswitch-loops,
-fpredictive-commoning, -fgcse-after-reload,
-ftree-loop-vectorize, -ftree-loop-distribute-patterns,
-fsplit-paths -ftree-slp-vectorize, -fvect-cost-model,
-ftree-partial-pre, -fpeel-loops and -fipa-cp-clone options.
-O0 Reduce compilation time and make debugging produce the expected
results. This is the default.
-Os Optimize for size. -Os enables all -O2 optimizations that do not
typically increase code size. It also performs further
optimizations designed to reduce code size.
-Os disables the following optimization flags: -falign-functions
-falign-jumps -falign-loops -falign-labels -freorder-blocks
-freorder-blocks-algorithm=stc -freorder-blocks-and-partition
-fprefetch-loop-arrays
-Ofast
Disregard strict standards compliance. -Ofast enables all -O3
optimizations. It also enables optimizations that are not valid
for all standard-compliant programs. It turns on -ffast-math and
the Fortran-specific -fno-protect-parens and -fstack-arrays.
-Og Optimize debugging experience. -Og enables optimizations that do
not interfere with debugging. It should be the optimization level
of choice for the standard edit-compile-debug cycle, offering a
reasonable level of optimization while maintaining fast
compilation and a good debugging experience.
If you use multiple -O options, with or without level numbers, the
last such option is the one that is effective.
Options of the form -fflag specify machine-independent flags. Most
flags have both positive and negative forms; the negative form of
-ffoo is -fno-foo. In the table below, only one of the forms is
listed---the one you typically use. You can figure out the other
form by either removing no- or adding it.
The following options control specific optimizations. They are
either activated by -O options or are related to ones that are. You
can use the following flags in the rare cases when "fine-tuning" of
optimizations to be performed is desired.
-fno-defer-pop
Always pop the arguments to each function call as soon as that
function returns. For machines that must pop arguments after a
function call, the compiler normally lets arguments accumulate on
the stack for several function calls and pops them all at once.
Disabled at levels -O, -O2, -O3, -Os.
-fforward-propagate
Perform a forward propagation pass on RTL. The pass tries to
combine two instructions and checks if the result can be
simplified. If loop unrolling is active, two passes are
performed and the second is scheduled after loop unrolling.
This option is enabled by default at optimization levels -O, -O2,
-O3, -Os.
-ffp-contract=style
-ffp-contract=off disables floating-point expression contraction.
-ffp-contract=fast enables floating-point expression contraction
such as forming of fused multiply-add operations if the target
has native support for them. -ffp-contract=on enables floating-
point expression contraction if allowed by the language standard.
This is currently not implemented and treated equal to
-ffp-contract=off.
The default is -ffp-contract=fast.
-fomit-frame-pointer
Don't keep the frame pointer in a register for functions that
don't need one. This avoids the instructions to save, set up and
restore frame pointers; it also makes an extra register available
in many functions. It also makes debugging impossible on some
machines.
On some machines, such as the VAX, this flag has no effect,
because the standard calling sequence automatically handles the
frame pointer and nothing is saved by pretending it doesn't
exist. The machine-description macro "FRAME_POINTER_REQUIRED"
controls whether a target machine supports this flag.
The default setting (when not optimizing for size) for 32-bit
GNU/Linux x86 and 32-bit Darwin x86 targets is
-fomit-frame-pointer. You can configure GCC with the
--enable-frame-pointer configure option to change the default.
Enabled at levels -O, -O2, -O3, -Os.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-foptimize-strlen
Optimize various standard C string functions (e.g. "strlen",
"strchr" or "strcpy") and their "_FORTIFY_SOURCE" counterparts
into faster alternatives.
Enabled at levels -O2, -O3.
-fno-inline
Do not expand any functions inline apart from those marked with
the "always_inline" attribute. This is the default when not
optimizing.
Single functions can be exempted from inlining by marking them
with the "noinline" attribute.
-finline-small-functions
Integrate functions into their callers when their body is smaller
than expected function call code (so overall size of program gets
smaller). The compiler heuristically decides which functions are
simple enough to be worth integrating in this way. This inlining
applies to all functions, even those not declared inline.
Enabled at level -O2.
-findirect-inlining
Inline also indirect calls that are discovered to be known at
compile time thanks to previous inlining. This option has any
effect only when inlining itself is turned on by the
-finline-functions or -finline-small-functions options.
Enabled at level -O2.
-finline-functions
Consider all functions for inlining, even if they are not
declared inline. The compiler heuristically decides which
functions are worth integrating in this way.
If all calls to a given function are integrated, and the function
is declared "static", then the function is normally not output as
assembler code in its own right.
Enabled at level -O3.
-finline-functions-called-once
Consider all "static" functions called once for inlining into
their caller even if they are not marked "inline". If a call to
a given function is integrated, then the function is not output
as assembler code in its own right.
Enabled at levels -O1, -O2, -O3 and -Os.
-fearly-inlining
Inline functions marked by "always_inline" and functions whose
body seems smaller than the function call overhead early before
doing -fprofile-generate instrumentation and real inlining pass.
Doing so makes profiling significantly cheaper and usually
inlining faster on programs having large chains of nested wrapper
functions.
Enabled by default.
-fipa-sra
Perform interprocedural scalar replacement of aggregates, removal
of unused parameters and replacement of parameters passed by
reference by parameters passed by value.
Enabled at levels -O2, -O3 and -Os.
-finline-limit=n
By default, GCC limits the size of functions that can be inlined.
This flag allows coarse control of this limit. n is the size of
functions that can be inlined in number of pseudo instructions.
Inlining is actually controlled by a number of parameters, which
may be specified individually by using --param name=value. The
-finline-limit=n option sets some of these parameters as follows:
max-inline-insns-single
is set to n/2.
max-inline-insns-auto
is set to n/2.
See below for a documentation of the individual parameters
controlling inlining and for the defaults of these parameters.
Note: there may be no value to -finline-limit that results in
default behavior.
Note: pseudo instruction represents, in this particular context,
an abstract measurement of function's size. In no way does it
represent a count of assembly instructions and as such its exact
meaning might change from one release to an another.
-fno-keep-inline-dllexport
This is a more fine-grained version of -fkeep-inline-functions,
which applies only to functions that are declared using the
"dllexport" attribute or declspec.
-fkeep-inline-functions
In C, emit "static" functions that are declared "inline" into the
object file, even if the function has been inlined into all of
its callers. This switch does not affect functions using the
"extern inline" extension in GNU C90. In C++, emit any and all
inline functions into the object file.
-fkeep-static-functions
Emit "static" functions into the object file, even if the
function is never used.
-fkeep-static-consts
Emit variables declared "static const" when optimization isn't
turned on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the
compiler to check if a variable is referenced, regardless of
whether or not optimization is turned on, use the
-fno-keep-static-consts option.
-fmerge-constants
Attempt to merge identical constants (string constants and
floating-point constants) across compilation units.
This option is the default for optimized compilation if the
assembler and linker support it. Use -fno-merge-constants to
inhibit this behavior.
Enabled at levels -O, -O2, -O3, -Os.
-fmerge-all-constants
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to
-fmerge-constants this considers e.g. even constant initialized
arrays or initialized constant variables with integral or
floating-point types. Languages like C or C++ require each
variable, including multiple instances of the same variable in
recursive calls, to have distinct locations, so using this option
results in non-conforming behavior.
-fmodulo-sched
Perform swing modulo scheduling immediately before the first
scheduling pass. This pass looks at innermost loops and reorders
their instructions by overlapping different iterations.
-fmodulo-sched-allow-regmoves
Perform more aggressive SMS-based modulo scheduling with register
moves allowed. By setting this flag certain anti-dependences
edges are deleted, which triggers the generation of reg-moves
based on the life-range analysis. This option is effective only
with -fmodulo-sched enabled.
-fno-branch-count-reg
Avoid running a pass scanning for opportunities to use "decrement
and branch" instructions on a count register instead of
generating sequences of instructions that decrement a register,
compare it against zero, and then branch based upon the result.
This option is only meaningful on architectures that support such
instructions, which include x86, PowerPC, IA-64 and S/390. Note
that the -fno-branch-count-reg option doesn't remove the
decrement and branch instructions from the generated instruction
stream introduced by other optimization passes.
Enabled by default at -O1 and higher.
The default is -fbranch-count-reg.
-fno-function-cse
Do not put function addresses in registers; make each instruction
that calls a constant function contain the function's address
explicitly.
This option results in less efficient code, but some strange
hacks that alter the assembler output may be confused by the
optimizations performed when this option is not used.
The default is -ffunction-cse
-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts
variables that are initialized to zero into BSS. This can save
space in the resulting code.
This option turns off this behavior because some programs
explicitly rely on variables going to the data section---e.g., so
that the resulting executable can find the beginning of that
section and/or make assumptions based on that.
The default is -fzero-initialized-in-bss.
-fthread-jumps
Perform optimizations that check to see if a jump branches to a
location where another comparison subsumed by the first is found.
If so, the first branch is redirected to either the destination
of the second branch or a point immediately following it,
depending on whether the condition is known to be true or false.
Enabled at levels -O2, -O3, -Os.
-fsplit-wide-types
When using a type that occupies multiple registers, such as "long
long" on a 32-bit system, split the registers apart and allocate
them independently. This normally generates better code for
those types, but may make debugging more difficult.
Enabled at levels -O, -O2, -O3, -Os.
-fcse-follow-jumps
In common subexpression elimination (CSE), scan through jump
instructions when the target of the jump is not reached by any
other path. For example, when CSE encounters an "if" statement
with an "else" clause, CSE follows the jump when the condition
tested is false.
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to follow
jumps that conditionally skip over blocks. When CSE encounters a
simple "if" statement with no else clause, -fcse-skip-blocks
causes CSE to follow the jump around the body of the "if".
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations
are performed.
Enabled at levels -O2, -O3, -Os.
-fgcse
Perform a global common subexpression elimination pass. This
pass also performs global constant and copy propagation.
Note: When compiling a program using computed gotos, a GCC
extension, you may get better run-time performance if you disable
the global common subexpression elimination pass by adding
-fno-gcse to the command line.
Enabled at levels -O2, -O3, -Os.
-fgcse-lm
When -fgcse-lm is enabled, global common subexpression
elimination attempts to move loads that are only killed by stores
into themselves. This allows a loop containing a load/store
sequence to be changed to a load outside the loop, and a
copy/store within the loop.
Enabled by default when -fgcse is enabled.
-fgcse-sm
When -fgcse-sm is enabled, a store motion pass is run after
global common subexpression elimination. This pass attempts to
move stores out of loops. When used in conjunction with
-fgcse-lm, loops containing a load/store sequence can be changed
to a load before the loop and a store after the loop.
Not enabled at any optimization level.
-fgcse-las
When -fgcse-las is enabled, the global common subexpression
elimination pass eliminates redundant loads that come after
stores to the same memory location (both partial and full
redundancies).
Not enabled at any optimization level.
-fgcse-after-reload
When -fgcse-after-reload is enabled, a redundant load elimination
pass is performed after reload. The purpose of this pass is to
clean up redundant spilling.
-faggressive-loop-optimizations
This option tells the loop optimizer to use language constraints
to derive bounds for the number of iterations of a loop. This
assumes that loop code does not invoke undefined behavior by for
example causing signed integer overflows or out-of-bound array
accesses. The bounds for the number of iterations of a loop are
used to guide loop unrolling and peeling and loop exit test
optimizations. This option is enabled by default.
-funconstrained-commons
This option tells the compiler that variables declared in common
blocks (e.g. Fortran) may later be overridden with longer
trailing arrays. This prevents certain optimizations that depend
on knowing the array bounds.
-fcrossjumping
Perform cross-jumping transformation. This transformation
unifies equivalent code and saves code size. The resulting code
may or may not perform better than without cross-jumping.
Enabled at levels -O2, -O3, -Os.
-fauto-inc-dec
Combine increments or decrements of addresses with memory
accesses. This pass is always skipped on architectures that do
not have instructions to support this. Enabled by default at -O
and higher on architectures that support this.
-fdce
Perform dead code elimination (DCE) on RTL. Enabled by default
at -O and higher.
-fdse
Perform dead store elimination (DSE) on RTL. Enabled by default
at -O and higher.
-fif-conversion
Attempt to transform conditional jumps into branch-less
equivalents. This includes use of conditional moves, min, max,
set flags and abs instructions, and some tricks doable by
standard arithmetics. The use of conditional execution on chips
where it is available is controlled by -fif-conversion2.
Enabled at levels -O, -O2, -O3, -Os.
-fif-conversion2
Use conditional execution (where available) to transform
conditional jumps into branch-less equivalents.
Enabled at levels -O, -O2, -O3, -Os.
-fdeclone-ctor-dtor
The C++ ABI requires multiple entry points for constructors and
destructors: one for a base subobject, one for a complete object,
and one for a virtual destructor that calls operator delete
afterwards. For a hierarchy with virtual bases, the base and
complete variants are clones, which means two copies of the
function. With this option, the base and complete variants are
changed to be thunks that call a common implementation.
Enabled by -Os.
-fdelete-null-pointer-checks
Assume that programs cannot safely dereference null pointers, and
that no code or data element resides at address zero. This
option enables simple constant folding optimizations at all
optimization levels. In addition, other optimization passes in
GCC use this flag to control global dataflow analyses that
eliminate useless checks for null pointers; these assume that a
memory access to address zero always results in a trap, so that
if a pointer is checked after it has already been dereferenced,
it cannot be null.
Note however that in some environments this assumption is not
true. Use -fno-delete-null-pointer-checks to disable this
optimization for programs that depend on that behavior.
This option is enabled by default on most targets. On Nios II
ELF, it defaults to off. On AVR and CR16, this option is
completely disabled.
Passes that use the dataflow information are enabled
independently at different optimization levels.
-fdevirtualize
Attempt to convert calls to virtual functions to direct calls.
This is done both within a procedure and interprocedurally as
part of indirect inlining (-findirect-inlining) and
interprocedural constant propagation (-fipa-cp). Enabled at
levels -O2, -O3, -Os.
-fdevirtualize-speculatively
Attempt to convert calls to virtual functions to speculative
direct calls. Based on the analysis of the type inheritance
graph, determine for a given call the set of likely targets. If
the set is small, preferably of size 1, change the call into a
conditional deciding between direct and indirect calls. The
speculative calls enable more optimizations, such as inlining.
When they seem useless after further optimization, they are
converted back into original form.
-fdevirtualize-at-ltrans
Stream extra information needed for aggressive devirtualization
when running the link-time optimizer in local transformation
mode. This option enables more devirtualization but
significantly increases the size of streamed data. For this
reason it is disabled by default.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively
expensive.
Enabled at levels -O2, -O3, -Os.
-free
Attempt to remove redundant extension instructions. This is
especially helpful for the x86-64 architecture, which implicitly
zero-extends in 64-bit registers after writing to their lower
32-bit half.
Enabled for Alpha, AArch64 and x86 at levels -O2, -O3, -Os.
-fno-lifetime-dse
In C++ the value of an object is only affected by changes within
its lifetime: when the constructor begins, the object has an
indeterminate value, and any changes during the lifetime of the
object are dead when the object is destroyed. Normally dead
store elimination will take advantage of this; if your code
relies on the value of the object storage persisting beyond the
lifetime of the object, you can use this flag to disable this
optimization. To preserve stores before the constructor starts
(e.g. because your operator new clears the object storage) but
still treat the object as dead after the destructor you, can use
-flifetime-dse=1. The default behavior can be explicitly
selected with -flifetime-dse=2. -flifetime-dse=0 is equivalent
to -fno-lifetime-dse.
-flive-range-shrinkage
Attempt to decrease register pressure through register live range
shrinkage. This is helpful for fast processors with small or
moderate size register sets.
-fira-algorithm=algorithm
Use the specified coloring algorithm for the integrated register
allocator. The algorithm argument can be priority, which
specifies Chow's priority coloring, or CB, which specifies
Chaitin-Briggs coloring. Chaitin-Briggs coloring is not
implemented for all architectures, but for those targets that do
support it, it is the default because it generates better code.
-fira-region=region
Use specified regions for the integrated register allocator. The
region argument should be one of the following:
all Use all loops as register allocation regions. This can give
the best results for machines with a small and/or irregular
register set.
mixed
Use all loops except for loops with small register pressure
as the regions. This value usually gives the best results in
most cases and for most architectures, and is enabled by
default when compiling with optimization for speed (-O, -O2,
...).
one Use all functions as a single region. This typically results
in the smallest code size, and is enabled by default for -Os
or -O0.
-fira-hoist-pressure
Use IRA to evaluate register pressure in the code hoisting pass
for decisions to hoist expressions. This option usually results
in smaller code, but it can slow the compiler down.
This option is enabled at level -Os for all targets.
-fira-loop-pressure
Use IRA to evaluate register pressure in loops for decisions to
move loop invariants. This option usually results in generation
of faster and smaller code on machines with large register files
(>= 32 registers), but it can slow the compiler down.
This option is enabled at level -O3 for some targets.
-fno-ira-share-save-slots
Disable sharing of stack slots used for saving call-used hard
registers living through a call. Each hard register gets a
separate stack slot, and as a result function stack frames are
larger.
-fno-ira-share-spill-slots
Disable sharing of stack slots allocated for pseudo-registers.
Each pseudo-register that does not get a hard register gets a
separate stack slot, and as a result function stack frames are
larger.
-flra-remat
Enable CFG-sensitive rematerialization in LRA. Instead of
loading values of spilled pseudos, LRA tries to rematerialize
(recalculate) values if it is profitable.
Enabled at levels -O2, -O3, -Os.
-fdelayed-branch
If supported for the target machine, attempt to reorder
instructions to exploit instruction slots available after delayed
branch instructions.
Enabled at levels -O, -O2, -O3, -Os.
-fschedule-insns
If supported for the target machine, attempt to reorder
instructions to eliminate execution stalls due to required data
being unavailable. This helps machines that have slow floating
point or memory load instructions by allowing other instructions
to be issued until the result of the load or floating-point
instruction is required.
Enabled at levels -O2, -O3.
-fschedule-insns2
Similar to -fschedule-insns, but requests an additional pass of
instruction scheduling after register allocation has been done.
This is especially useful on machines with a relatively small
number of registers and where memory load instructions take more
than one cycle.
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock
Don't schedule instructions across basic blocks. This is
normally enabled by default when scheduling before register
allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fno-sched-spec
Don't allow speculative motion of non-load instructions. This is
normally enabled by default when scheduling before register
allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-pressure
Enable register pressure sensitive insn scheduling before
register allocation. This only makes sense when scheduling
before register allocation is enabled, i.e. with -fschedule-insns
or at -O2 or higher. Usage of this option can improve the
generated code and decrease its size by preventing register
pressure increase above the number of available hard registers
and subsequent spills in register allocation.
-fsched-spec-load
Allow speculative motion of some load instructions. This only
makes sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.
-fsched-spec-load-dangerous
Allow speculative motion of more load instructions. This only
makes sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.
-fsched-stalled-insns
-fsched-stalled-insns=n
Define how many insns (if any) can be moved prematurely from the
queue of stalled insns into the ready list during the second
scheduling pass. -fno-sched-stalled-insns means that no insns
are moved prematurely, -fsched-stalled-insns=0 means there is no
limit on how many queued insns can be moved prematurely.
-fsched-stalled-insns without a value is equivalent to
-fsched-stalled-insns=1.
-fsched-stalled-insns-dep
-fsched-stalled-insns-dep=n
Define how many insn groups (cycles) are examined for a
dependency on a stalled insn that is a candidate for premature
removal from the queue of stalled insns. This has an effect only
during the second scheduling pass, and only if
-fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is
equivalent to -fsched-stalled-insns-dep=0.
-fsched-stalled-insns-dep without a value is equivalent to
-fsched-stalled-insns-dep=1.
-fsched2-use-superblocks
When scheduling after register allocation, use superblock
scheduling. This allows motion across basic block boundaries,
resulting in faster schedules. This option is experimental, as
not all machine descriptions used by GCC model the CPU closely
enough to avoid unreliable results from the algorithm.
This only makes sense when scheduling after register allocation,
i.e. with -fschedule-insns2 or at -O2 or higher.
-fsched-group-heuristic
Enable the group heuristic in the scheduler. This heuristic
favors the instruction that belongs to a schedule group. This is
enabled by default when scheduling is enabled, i.e. with
-fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-fsched-critical-path-heuristic
Enable the critical-path heuristic in the scheduler. This
heuristic favors instructions on the critical path. This is
enabled by default when scheduling is enabled, i.e. with
-fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-fsched-spec-insn-heuristic
Enable the speculative instruction heuristic in the scheduler.
This heuristic favors speculative instructions with greater
dependency weakness. This is enabled by default when scheduling
is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or
at -O2 or higher.
-fsched-rank-heuristic
Enable the rank heuristic in the scheduler. This heuristic
favors the instruction belonging to a basic block with greater
size or frequency. This is enabled by default when scheduling is
enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at
-O2 or higher.
-fsched-last-insn-heuristic
Enable the last-instruction heuristic in the scheduler. This
heuristic favors the instruction that is less dependent on the
last instruction scheduled. This is enabled by default when
scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
-fsched-dep-count-heuristic
Enable the dependent-count heuristic in the scheduler. This
heuristic favors the instruction that has more instructions
depending on it. This is enabled by default when scheduling is
enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at
-O2 or higher.
-freschedule-modulo-scheduled-loops
Modulo scheduling is performed before traditional scheduling. If
a loop is modulo scheduled, later scheduling passes may change
its schedule. Use this option to control that behavior.
-fselective-scheduling
Schedule instructions using selective scheduling algorithm.
Selective scheduling runs instead of the first scheduler pass.
-fselective-scheduling2
Schedule instructions using selective scheduling algorithm.
Selective scheduling runs instead of the second scheduler pass.
-fsel-sched-pipelining
Enable software pipelining of innermost loops during selective
scheduling. This option has no effect unless one of
-fselective-scheduling or -fselective-scheduling2 is turned on.
-fsel-sched-pipelining-outer-loops
When pipelining loops during selective scheduling, also pipeline
outer loops. This option has no effect unless
-fsel-sched-pipelining is turned on.
-fsemantic-interposition
Some object formats, like ELF, allow interposing of symbols by
the dynamic linker. This means that for symbols exported from
the DSO, the compiler cannot perform interprocedural propagation,
inlining and other optimizations in anticipation that the
function or variable in question may change. While this feature
is useful, for example, to rewrite memory allocation functions by
a debugging implementation, it is expensive in the terms of code
quality. With -fno-semantic-interposition the compiler assumes
that if interposition happens for functions the overwriting
function will have precisely the same semantics (and side
effects). Similarly if interposition happens for variables, the
constructor of the variable will be the same. The flag has no
effect for functions explicitly declared inline (where it is
never allowed for interposition to change semantics) and for
symbols explicitly declared weak.
-fshrink-wrap
Emit function prologues only before parts of the function that
need it, rather than at the top of the function. This flag is
enabled by default at -O and higher.
-fshrink-wrap-separate
Shrink-wrap separate parts of the prologue and epilogue
separately, so that those parts are only executed when needed.
This option is on by default, but has no effect unless
-fshrink-wrap is also turned on and the target supports this.
-fcaller-saves
Enable allocation of values to registers that are clobbered by
function calls, by emitting extra instructions to save and
restore the registers around such calls. Such allocation is done
only when it seems to result in better code.
This option is always enabled by default on certain machines,
usually those which have no call-preserved registers to use
instead.
Enabled at levels -O2, -O3, -Os.
-fcombine-stack-adjustments
Tracks stack adjustments (pushes and pops) and stack memory
references and then tries to find ways to combine them.
Enabled by default at -O1 and higher.
-fipa-ra
Use caller save registers for allocation if those registers are
not used by any called function. In that case it is not
necessary to save and restore them around calls. This is only
possible if called functions are part of same compilation unit as
current function and they are compiled before it.
Enabled at levels -O2, -O3, -Os, however the option is disabled
if generated code will be instrumented for profiling (-p, or -pg)
or if callee's register usage cannot be known exactly (this
happens on targets that do not expose prologues and epilogues in
RTL).
-fconserve-stack
Attempt to minimize stack usage. The compiler attempts to use
less stack space, even if that makes the program slower. This
option implies setting the large-stack-frame parameter to 100 and
the large-stack-frame-growth parameter to 400.
-ftree-reassoc
Perform reassociation on trees. This flag is enabled by default
at -O and higher.
-fcode-hoisting
Perform code hoisting. Code hoisting tries to move the
evaluation of expressions executed on all paths to the function
exit as early as possible. This is especially useful as a code
size optimization, but it often helps for code speed as well.
This flag is enabled by default at -O2 and higher.
-ftree-pre
Perform partial redundancy elimination (PRE) on trees. This flag
is enabled by default at -O2 and -O3.
-ftree-partial-pre
Make partial redundancy elimination (PRE) more aggressive. This
flag is enabled by default at -O3.
-ftree-forwprop
Perform forward propagation on trees. This flag is enabled by
default at -O and higher.
-ftree-fre
Perform full redundancy elimination (FRE) on trees. The
difference between FRE and PRE is that FRE only considers
expressions that are computed on all paths leading to the
redundant computation. This analysis is faster than PRE, though
it exposes fewer redundancies. This flag is enabled by default
at -O and higher.
-ftree-phiprop
Perform hoisting of loads from conditional pointers on trees.
This pass is enabled by default at -O and higher.
-fhoist-adjacent-loads
Speculatively hoist loads from both branches of an if-then-else
if the loads are from adjacent locations in the same structure
and the target architecture has a conditional move instruction.
This flag is enabled by default at -O2 and higher.
-ftree-copy-prop
Perform copy propagation on trees. This pass eliminates
unnecessary copy operations. This flag is enabled by default at
-O and higher.
-fipa-pure-const
Discover which functions are pure or constant. Enabled by
default at -O and higher.
-fipa-reference
Discover which static variables do not escape the compilation
unit. Enabled by default at -O and higher.
-fipa-pta
Perform interprocedural pointer analysis and interprocedural
modification and reference analysis. This option can cause
excessive memory and compile-time usage on large compilation
units. It is not enabled by default at any optimization level.
-fipa-profile
Perform interprocedural profile propagation. The functions
called only from cold functions are marked as cold. Also
functions executed once (such as "cold", "noreturn", static
constructors or destructors) are identified. Cold functions and
loop less parts of functions executed once are then optimized for
size. Enabled by default at -O and higher.
-fipa-cp
Perform interprocedural constant propagation. This optimization
analyzes the program to determine when values passed to functions
are constants and then optimizes accordingly. This optimization
can substantially increase performance if the application has
constants passed to functions. This flag is enabled by default
at -O2, -Os and -O3.
-fipa-cp-clone
Perform function cloning to make interprocedural constant
propagation stronger. When enabled, interprocedural constant
propagation performs function cloning when externally visible
function can be called with constant arguments. Because this
optimization can create multiple copies of functions, it may
significantly increase code size (see --param
ipcp-unit-growth=value). This flag is enabled by default at -O3.
-fipa-bit-cp
When enabled, perform interprocedural bitwise constant
propagation. This flag is enabled by default at -O2. It requires
that -fipa-cp is enabled.
-fipa-vrp
When enabled, perform interprocedural propagation of value
ranges. This flag is enabled by default at -O2. It requires that
-fipa-cp is enabled.
-fipa-icf
Perform Identical Code Folding for functions and read-only
variables. The optimization reduces code size and may disturb
unwind stacks by replacing a function by equivalent one with a
different name. The optimization works more effectively with
link-time optimization enabled.
Nevertheless the behavior is similar to Gold Linker ICF
optimization, GCC ICF works on different levels and thus the
optimizations are not same - there are equivalences that are
found only by GCC and equivalences found only by Gold.
This flag is enabled by default at -O2 and -Os.
-fisolate-erroneous-paths-dereference
Detect paths that trigger erroneous or undefined behavior due to
dereferencing a null pointer. Isolate those paths from the main
control flow and turn the statement with erroneous or undefined
behavior into a trap. This flag is enabled by default at -O2 and
higher and depends on -fdelete-null-pointer-checks also being
enabled.
-fisolate-erroneous-paths-attribute
Detect paths that trigger erroneous or undefined behavior due a
null value being used in a way forbidden by a "returns_nonnull"
or "nonnull" attribute. Isolate those paths from the main
control flow and turn the statement with erroneous or undefined
behavior into a trap. This is not currently enabled, but may be
enabled by -O2 in the future.
-ftree-sink
Perform forward store motion on trees. This flag is enabled by
default at -O and higher.
-ftree-bit-ccp
Perform sparse conditional bit constant propagation on trees and
propagate pointer alignment information. This pass only operates
on local scalar variables and is enabled by default at -O and
higher. It requires that -ftree-ccp is enabled.
-ftree-ccp
Perform sparse conditional constant propagation (CCP) on trees.
This pass only operates on local scalar variables and is enabled
by default at -O and higher.
-fssa-backprop
Propagate information about uses of a value up the definition
chain in order to simplify the definitions. For example, this
pass strips sign operations if the sign of a value never matters.
The flag is enabled by default at -O and higher.
-fssa-phiopt
Perform pattern matching on SSA PHI nodes to optimize conditional
code. This pass is enabled by default at -O and higher.
-ftree-switch-conversion
Perform conversion of simple initializations in a switch to
initializations from a scalar array. This flag is enabled by
default at -O2 and higher.
-ftree-tail-merge
Look for identical code sequences. When found, replace one with
a jump to the other. This optimization is known as tail merging
or cross jumping. This flag is enabled by default at -O2 and
higher. The compilation time in this pass can be limited using
max-tail-merge-comparisons parameter and max-tail-merge-
iterations parameter.
-ftree-dce
Perform dead code elimination (DCE) on trees. This flag is
enabled by default at -O and higher.
-ftree-builtin-call-dce
Perform conditional dead code elimination (DCE) for calls to
built-in functions that may set "errno" but are otherwise side-
effect free. This flag is enabled by default at -O2 and higher
if -Os is not also specified.
-ftree-dominator-opts
Perform a variety of simple scalar cleanups (constant/copy
propagation, redundancy elimination, range propagation and
expression simplification) based on a dominator tree traversal.
This also performs jump threading (to reduce jumps to jumps).
This flag is enabled by default at -O and higher.
-ftree-dse
Perform dead store elimination (DSE) on trees. A dead store is a
store into a memory location that is later overwritten by another
store without any intervening loads. In this case the earlier
store can be deleted. This flag is enabled by default at -O and
higher.
-ftree-ch
Perform loop header copying on trees. This is beneficial since
it increases effectiveness of code motion optimizations. It also
saves one jump. This flag is enabled by default at -O and
higher. It is not enabled for -Os, since it usually increases
code size.
-ftree-loop-optimize
Perform loop optimizations on trees. This flag is enabled by
default at -O and higher.
-ftree-loop-linear
-floop-interchange
-floop-strip-mine
-floop-block
-floop-unroll-and-jam
Perform loop nest optimizations. Same as -floop-nest-optimize.
To use this code transformation, GCC has to be configured with
--with-isl to enable the Graphite loop transformation
infrastructure.
-fgraphite-identity
Enable the identity transformation for graphite. For every SCoP
we generate the polyhedral representation and transform it back
to gimple. Using -fgraphite-identity we can check the costs or
benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation.
Some minimal optimizations are also performed by the code
generator isl, like index splitting and dead code elimination in
loops.
-floop-nest-optimize
Enable the isl based loop nest optimizer. This is a generic loop
nest optimizer based on the Pluto optimization algorithms. It
calculates a loop structure optimized for data-locality and
parallelism. This option is experimental.
-floop-parallelize-all
Use the Graphite data dependence analysis to identify loops that
can be parallelized. Parallelize all the loops that can be
analyzed to not contain loop carried dependences without checking
that it is profitable to parallelize the loops.
-ftree-coalesce-vars
While transforming the program out of the SSA representation,
attempt to reduce copying by coalescing versions of different
user-defined variables, instead of just compiler temporaries.
This may severely limit the ability to debug an optimized program
compiled with -fno-var-tracking-assignments. In the negated
form, this flag prevents SSA coalescing of user variables. This
option is enabled by default if optimization is enabled, and it
does very little otherwise.
-ftree-loop-if-convert
Attempt to transform conditional jumps in the innermost loops to
branch-less equivalents. The intent is to remove control-flow
from the innermost loops in order to improve the ability of the
vectorization pass to handle these loops. This is enabled by
default if vectorization is enabled.
-ftree-loop-distribution
Perform loop distribution. This flag can improve cache
performance on big loop bodies and allow further loop
optimizations, like parallelization or vectorization, to take
place. For example, the loop
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDO
is transformed to
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDO
-ftree-loop-distribute-patterns
Perform loop distribution of patterns that can be code generated
with calls to a library. This flag is enabled by default at -O3.
This pass distributes the initialization loops and generates a
call to memset zero. For example, the loop
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDO
is transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDO
and the initialization loop is transformed into a call to memset
zero.
-ftree-loop-im
Perform loop invariant motion on trees. This pass moves only
invariants that are hard to handle at RTL level (function calls,
operations that expand to nontrivial sequences of insns). With
-funswitch-loops it also moves operands of conditions that are
invariant out of the loop, so that we can use just trivial
invariantness analysis in loop unswitching. The pass also
includes store motion.
-ftree-loop-ivcanon
Create a canonical counter for number of iterations in loops for
which determining number of iterations requires complicated
analysis. Later optimizations then may determine the number
easily. Useful especially in connection with unrolling.
-fivopts
Perform induction variable optimizations (strength reduction,
induction variable merging and induction variable elimination) on
trees.
-ftree-parallelize-loops=n
Parallelize loops, i.e., split their iteration space to run in n
threads. This is only possible for loops whose iterations are
independent and can be arbitrarily reordered. The optimization
is only profitable on multiprocessor machines, for loops that are
CPU-intensive, rather than constrained e.g. by memory bandwidth.
This option implies -pthread, and thus is only supported on
targets that have support for -pthread.
-ftree-pta
Perform function-local points-to analysis on trees. This flag is
enabled by default at -O and higher.
-ftree-sra
Perform scalar replacement of aggregates. This pass replaces
structure references with scalars to prevent committing
structures to memory too early. This flag is enabled by default
at -O and higher.
-fstore-merging
Perform merging of narrow stores to consecutive memory addresses.
This pass merges contiguous stores of immediate values narrower
than a word into fewer wider stores to reduce the number of
instructions. This is enabled by default at -O2 and higher as
well as -Os.
-ftree-ter
Perform temporary expression replacement during the SSA->normal
phase. Single use/single def temporaries are replaced at their
use location with their defining expression. This results in
non-GIMPLE code, but gives the expanders much more complex trees
to work on resulting in better RTL generation. This is enabled
by default at -O and higher.
-ftree-slsr
Perform straight-line strength reduction on trees. This
recognizes related expressions involving multiplications and
replaces them by less expensive calculations when possible. This
is enabled by default at -O and higher.
-ftree-vectorize
Perform vectorization on trees. This flag enables
-ftree-loop-vectorize and -ftree-slp-vectorize if not explicitly
specified.
-ftree-loop-vectorize
Perform loop vectorization on trees. This flag is enabled by
default at -O3 and when -ftree-vectorize is enabled.
-ftree-slp-vectorize
Perform basic block vectorization on trees. This flag is enabled
by default at -O3 and when -ftree-vectorize is enabled.
-fvect-cost-model=model
Alter the cost model used for vectorization. The model argument
should be one of unlimited, dynamic or cheap. With the unlimited
model the vectorized code-path is assumed to be profitable while
with the dynamic model a runtime check guards the vectorized
code-path to enable it only for iteration counts that will likely
execute faster than when executing the original scalar loop. The
cheap model disables vectorization of loops where doing so would
be cost prohibitive for example due to required runtime checks
for data dependence or alignment but otherwise is equal to the
dynamic model. The default cost model depends on other
optimization flags and is either dynamic or cheap.
-fsimd-cost-model=model
Alter the cost model used for vectorization of loops marked with
the OpenMP or Cilk Plus simd directive. The model argument
should be one of unlimited, dynamic, cheap. All values of model
have the same meaning as described in -fvect-cost-model and by
default a cost model defined with -fvect-cost-model is used.
-ftree-vrp
Perform Value Range Propagation on trees. This is similar to the
constant propagation pass, but instead of values, ranges of
values are propagated. This allows the optimizers to remove
unnecessary range checks like array bound checks and null pointer
checks. This is enabled by default at -O2 and higher. Null
pointer check elimination is only done if
-fdelete-null-pointer-checks is enabled.
-fsplit-paths
Split paths leading to loop backedges. This can improve dead
code elimination and common subexpression elimination. This is
enabled by default at -O2 and above.
-fsplit-ivs-in-unroller
Enables expression of values of induction variables in later
iterations of the unrolled loop using the value in the first
iteration. This breaks long dependency chains, thus improving
efficiency of the scheduling passes.
A combination of -fweb and CSE is often sufficient to obtain the
same effect. However, that is not reliable in cases where the
loop body is more complicated than a single basic block. It also
does not work at all on some architectures due to restrictions in
the CSE pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller
With this option, the compiler creates multiple copies of some
local variables when unrolling a loop, which can result in
superior code.
-fpartial-inlining
Inline parts of functions. This option has any effect only when
inlining itself is turned on by the -finline-functions or
-finline-small-functions options.
Enabled at level -O2.
-fpredictive-commoning
Perform predictive commoning optimization, i.e., reusing
computations (especially memory loads and stores) performed in
previous iterations of loops.
This option is enabled at level -O3.
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to
prefetch memory to improve the performance of loops that access
large arrays.
This option may generate better or worse code; results are highly
dependent on the structure of loops within the source code.
Disabled at level -Os.
-fno-printf-return-value
Do not substitute constants for known return value of formatted
output functions such as "sprintf", "snprintf", "vsprintf", and
"vsnprintf" (but not "printf" of "fprintf"). This transformation
allows GCC to optimize or even eliminate branches based on the
known return value of these functions called with arguments that
are either constant, or whose values are known to be in a range
that makes determining the exact return value possible. For
example, when -fprintf-return-value is in effect, both the branch
and the body of the "if" statement (but not the call to
"snprint") can be optimized away when "i" is a 32-bit or smaller
integer because the return value is guaranteed to be at most 8.
char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
...
The -fprintf-return-value option relies on other optimizations
and yields best results with -O2. It works in tandem with the
-Wformat-overflow and -Wformat-truncation options. The
-fprintf-return-value option is enabled by default.
-fno-peephole
-fno-peephole2
Disable any machine-specific peephole optimizations. The
difference between -fno-peephole and -fno-peephole2 is in how
they are implemented in the compiler; some targets use one, some
use the other, a few use both.
-fpeephole is enabled by default. -fpeephole2 enabled at levels
-O2, -O3, -Os.
-fno-guess-branch-probability
Do not guess branch probabilities using heuristics.
GCC uses heuristics to guess branch probabilities if they are not
provided by profiling feedback (-fprofile-arcs). These
heuristics are based on the control flow graph. If some branch
probabilities are specified by "__builtin_expect", then the
heuristics are used to guess branch probabilities for the rest of
the control flow graph, taking the "__builtin_expect" info into
account. The interactions between the heuristics and
"__builtin_expect" can be complex, and in some cases, it may be
useful to disable the heuristics so that the effects of
"__builtin_expect" are easier to understand.
The default is -fguess-branch-probability at levels -O, -O2, -O3,
-Os.
-freorder-blocks
Reorder basic blocks in the compiled function in order to reduce
number of taken branches and improve code locality.
Enabled at levels -O, -O2, -O3, -Os.
-freorder-blocks-algorithm=algorithm
Use the specified algorithm for basic block reordering. The
algorithm argument can be simple, which does not increase code
size (except sometimes due to secondary effects like alignment),
or stc, the "software trace cache" algorithm, which tries to put
all often executed code together, minimizing the number of
branches executed by making extra copies of code.
The default is simple at levels -O, -Os, and stc at levels -O2,
-O3.
-freorder-blocks-and-partition
In addition to reordering basic blocks in the compiled function,
in order to reduce number of taken branches, partitions hot and
cold basic blocks into separate sections of the assembly and .o
files, to improve paging and cache locality performance.
This optimization is automatically turned off in the presence of
exception handling, for linkonce sections, for functions with a
user-defined section attribute and on any architecture that does
not support named sections.
Enabled for x86 at levels -O2, -O3.
-freorder-functions
Reorder functions in the object file in order to improve code
locality. This is implemented by using special subsections
".text.hot" for most frequently executed functions and
".text.unlikely" for unlikely executed functions. Reordering is
done by the linker so object file format must support named
sections and linker must place them in a reasonable way.
Also profile feedback must be available to make this option
effective. See -fprofile-arcs for details.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing
Allow the compiler to assume the strictest aliasing rules
applicable to the language being compiled. For C (and C++), this
activates optimizations based on the type of expressions. In
particular, an object of one type is assumed never to reside at
the same address as an object of a different type, unless the
types are almost the same. For example, an "unsigned int" can
alias an "int", but not a "void*" or a "double". A character
type may alias any other type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the
one most recently written to (called "type-punning") is common.
Even with -fstrict-aliasing, type-punning is allowed, provided
the memory is accessed through the union type. So, the code
above works as expected. However, this code might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Similarly, access by taking the address, casting the resulting
pointer and dereferencing the result has undefined behavior, even
if the cast uses a union type, e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}
The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.
-fstrict-overflow
Allow the compiler to assume strict signed overflow rules,
depending on the language being compiled. For C (and C++) this
means that overflow when doing arithmetic with signed numbers is
undefined, which means that the compiler may assume that it does
not happen. This permits various optimizations. For example,
the compiler assumes that an expression like "i + 10 > i" is
always true for signed "i". This assumption is only valid if
signed overflow is undefined, as the expression is false if "i +
10" overflows when using twos complement arithmetic. When this
option is in effect any attempt to determine whether an operation
on signed numbers overflows must be written carefully to not
actually involve overflow.
This option also allows the compiler to assume strict pointer
semantics: given a pointer to an object, if adding an offset to
that pointer does not produce a pointer to the same object, the
addition is undefined. This permits the compiler to conclude
that "p + u > p" is always true for a pointer "p" and unsigned
integer "u". This assumption is only valid because pointer
wraparound is undefined, as the expression is false if "p + u"
overflows using twos complement arithmetic.
See also the -fwrapv option. Using -fwrapv means that integer
signed overflow is fully defined: it wraps. When -fwrapv is
used, there is no difference between -fstrict-overflow and
-fno-strict-overflow for integers. With -fwrapv certain types of
overflow are permitted. For example, if the compiler gets an
overflow when doing arithmetic on constants, the overflowed value
can still be used with -fwrapv, but not otherwise.
The -fstrict-overflow option is enabled at levels -O2, -O3, -Os.
-falign-functions
-falign-functions=n
Align the start of functions to the next power-of-two greater
than n, skipping up to n bytes. For instance,
-falign-functions=32 aligns functions to the next 32-byte
boundary, but -falign-functions=24 aligns to the next 32-byte
boundary only if this can be done by skipping 23 bytes or less.
-fno-align-functions and -falign-functions=1 are equivalent and
mean that functions are not aligned.
Some assemblers only support this flag when n is a power of two;
in that case, it is rounded up.
If n is not specified or is zero, use a machine-dependent
default.
Enabled at levels -O2, -O3.
-flimit-function-alignment
If this option is enabled, the compiler tries to avoid
unnecessarily overaligning functions. It attempts to instruct the
assembler to align by the amount specified by -falign-functions,
but not to skip more bytes than the size of the function.
-falign-labels
-falign-labels=n
Align all branch targets to a power-of-two boundary, skipping up
to n bytes like -falign-functions. This option can easily make
code slower, because it must insert dummy operations for when the
branch target is reached in the usual flow of the code.
-fno-align-labels and -falign-labels=1 are equivalent and mean
that labels are not aligned.
If -falign-loops or -falign-jumps are applicable and are greater
than this value, then their values are used instead.
If n is not specified or is zero, use a machine-dependent default
which is very likely to be 1, meaning no alignment.
Enabled at levels -O2, -O3.
-falign-loops
-falign-loops=n
Align loops to a power-of-two boundary, skipping up to n bytes
like -falign-functions. If the loops are executed many times,
this makes up for any execution of the dummy operations.
-fno-align-loops and -falign-loops=1 are equivalent and mean that
loops are not aligned.
If n is not specified or is zero, use a machine-dependent
default.
Enabled at levels -O2, -O3.
-falign-jumps
-falign-jumps=n
Align branch targets to a power-of-two boundary, for branch
targets where the targets can only be reached by jumping,
skipping up to n bytes like -falign-functions. In this case, no
dummy operations need be executed.
-fno-align-jumps and -falign-jumps=1 are equivalent and mean that
loops are not aligned.
If n is not specified or is zero, use a machine-dependent
default.
Enabled at levels -O2, -O3.
-funit-at-a-time
This option is left for compatibility reasons. -funit-at-a-time
has no effect, while -fno-unit-at-a-time implies
-fno-toplevel-reorder and -fno-section-anchors.
Enabled by default.
-fno-toplevel-reorder
Do not reorder top-level functions, variables, and "asm"
statements. Output them in the same order that they appear in
the input file. When this option is used, unreferenced static
variables are not removed. This option is intended to support
existing code that relies on a particular ordering. For new
code, it is better to use attributes when possible.
Enabled at level -O0. When disabled explicitly, it also implies
-fno-section-anchors, which is otherwise enabled at -O0 on some
targets.
-fweb
Constructs webs as commonly used for register allocation purposes
and assign each web individual pseudo register. This allows the
register allocation pass to operate on pseudos directly, but also
strengthens several other optimization passes, such as CSE, loop
optimizer and trivial dead code remover. It can, however, make
debugging impossible, since variables no longer stay in a "home
register".
Enabled by default with -funroll-loops.
-fwhole-program
Assume that the current compilation unit represents the whole
program being compiled. All public functions and variables with
the exception of "main" and those merged by attribute
"externally_visible" become static functions and in effect are
optimized more aggressively by interprocedural optimizers.
This option should not be used in combination with -flto.
Instead relying on a linker plugin should provide safer and more
precise information.
-flto[=n]
This option runs the standard link-time optimizer. When invoked
with source code, it generates GIMPLE (one of GCC's internal
representations) and writes it to special ELF sections in the
object file. When the object files are linked together, all the
function bodies are read from these ELF sections and instantiated
as if they had been part of the same translation unit.
To use the link-time optimizer, -flto and optimization options
should be specified at compile time and during the final link.
It is recommended that you compile all the files participating in
the same link with the same options and also specify those
options at link time. For example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o
The first two invocations to GCC save a bytecode representation
of GIMPLE into special ELF sections inside foo.o and bar.o. The
final invocation reads the GIMPLE bytecode from foo.o and bar.o,
merges the two files into a single internal image, and compiles
the result as usual. Since both foo.o and bar.o are merged into
a single image, this causes all the interprocedural analyses and
optimizations in GCC to work across the two files as if they were
a single one. This means, for example, that the inliner is able
to inline functions in bar.o into functions in foo.o and vice-
versa.
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.c
The above generates bytecode for foo.c and bar.c, merges them
together into a single GIMPLE representation and optimizes them
as usual to produce myprog.
The only important thing to keep in mind is that to enable link-
time optimizations you need to use the GCC driver to perform the
link step. GCC then automatically performs link-time
optimization if any of the objects involved were compiled with
the -flto command-line option. You generally should specify the
optimization options to be used for link-time optimization though
GCC tries to be clever at guessing an optimization level to use
from the options used at compile time if you fail to specify one
at link time. You can always override the automatic decision to
do link-time optimization by passing -fno-lto to the link
command.
To make whole program optimization effective, it is necessary to
make certain whole program assumptions. The compiler needs to
know what functions and variables can be accessed by libraries
and runtime outside of the link-time optimized unit. When
supported by the linker, the linker plugin (see
-fuse-linker-plugin) passes information to the compiler about
used and externally visible symbols. When the linker plugin is
not available, -fwhole-program should be used to allow the
compiler to make these assumptions, which leads to more
aggressive optimization decisions.
When -fuse-linker-plugin is not enabled, when a file is compiled
with -flto, the generated object file is larger than a regular
object file because it contains GIMPLE bytecodes and the usual
final code (see -ffat-lto-objects. This means that object files
with LTO information can be linked as normal object files; if
-fno-lto is passed to the linker, no interprocedural
optimizations are applied. Note that when -fno-fat-lto-objects
is enabled the compile stage is faster but you cannot perform a
regular, non-LTO link on them.
Additionally, the optimization flags used to compile individual
files are not necessarily related to those used at link time.
For instance,
gcc -c -O0 -ffat-lto-objects -flto foo.c
gcc -c -O0 -ffat-lto-objects -flto bar.c
gcc -o myprog -O3 foo.o bar.o
This produces individual object files with unoptimized assembler
code, but the resulting binary myprog is optimized at -O3. If,
instead, the final binary is generated with -fno-lto, then myprog
is not optimized.
When producing the final binary, GCC only applies link-time
optimizations to those files that contain bytecode. Therefore,
you can mix and match object files and libraries with GIMPLE
bytecodes and final object code. GCC automatically selects which
files to optimize in LTO mode and which files to link without
further processing.
There are some code generation flags preserved by GCC when
generating bytecodes, as they need to be used during the final
link stage. Generally options specified at link time override
those specified at compile time.
If you do not specify an optimization level option -O at link
time, then GCC uses the highest optimization level used when
compiling the object files.
Currently, the following options and their settings are taken
from the first object file that explicitly specifies them: -fPIC,
-fpic, -fpie, -fcommon, -fexceptions, -fnon-call-exceptions,
-fgnu-tm and all the -m target flags.
Certain ABI-changing flags are required to match in all
compilation units, and trying to override this at link time with
a conflicting value is ignored. This includes options such as
-freg-struct-return and -fpcc-struct-return.
Other options such as -ffp-contract, -fno-strict-overflow,
-fwrapv, -fno-trapv or -fno-strict-aliasing are passed through to
the link stage and merged conservatively for conflicting
translation units. Specifically -fno-strict-overflow, -fwrapv
and -fno-trapv take precedence; and for example -ffp-contract=off
takes precedence over -ffp-contract=fast. You can override them
at link time.
If LTO encounters objects with C linkage declared with
incompatible types in separate translation units to be linked
together (undefined behavior according to ISO C99 6.2.7), a non-
fatal diagnostic may be issued. The behavior is still undefined
at run time. Similar diagnostics may be raised for other
languages.
Another feature of LTO is that it is possible to apply
interprocedural optimizations on files written in different
languages:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
Notice that the final link is done with g++ to get the C++
runtime libraries and -lgfortran is added to get the Fortran
runtime libraries. In general, when mixing languages in LTO
mode, you should use the same link command options as when mixing
languages in a regular (non-LTO) compilation.
If object files containing GIMPLE bytecode are stored in a
library archive, say libfoo.a, it is possible to extract and use
them in an LTO link if you are using a linker with plugin
support. To create static libraries suitable for LTO, use gcc-ar
and gcc-ranlib instead of ar and ranlib; to show the symbols of
object files with GIMPLE bytecode, use gcc-nm. Those commands
require that ar, ranlib and nm have been compiled with plugin
support. At link time, use the the flag -fuse-linker-plugin to
ensure that the library participates in the LTO optimization
process:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
With the linker plugin enabled, the linker extracts the needed
GIMPLE files from libfoo.a and passes them on to the running GCC
to make them part of the aggregated GIMPLE image to be optimized.
If you are not using a linker with plugin support and/or do not
enable the linker plugin, then the objects inside libfoo.a are
extracted and linked as usual, but they do not participate in the
LTO optimization process. In order to make a static library
suitable for both LTO optimization and usual linkage, compile its
object files with -flto -ffat-lto-objects.
Link-time optimizations do not require the presence of the whole
program to operate. If the program does not require any symbols
to be exported, it is possible to combine -flto and
-fwhole-program to allow the interprocedural optimizers to use
more aggressive assumptions which may lead to improved
optimization opportunities. Use of -fwhole-program is not needed
when linker plugin is active (see -fuse-linker-plugin).
The current implementation of LTO makes no attempt to generate
bytecode that is portable between different types of hosts. The
bytecode files are versioned and there is a strict version check,
so bytecode files generated in one version of GCC do not work
with an older or newer version of GCC.
Link-time optimization does not work well with generation of
debugging information. Combining -flto with -g is currently
experimental and expected to produce unexpected results.
If you specify the optional n, the optimization and code
generation done at link time is executed in parallel using n
parallel jobs by utilizing an installed make program. The
environment variable MAKE may be used to override the program
used. The default value for n is 1.
You can also specify -flto=jobserver to use GNU make's job server
mode to determine the number of parallel jobs. This is useful
when the Makefile calling GCC is already executing in parallel.
You must prepend a + to the command recipe in the parent Makefile
for this to work. This option likely only works if MAKE is GNU
make.
-flto-partition=alg
Specify the partitioning algorithm used by the link-time
optimizer. The value is either 1to1 to specify a partitioning
mirroring the original source files or balanced to specify
partitioning into equally sized chunks (whenever possible) or max
to create new partition for every symbol where possible.
Specifying none as an algorithm disables partitioning and
streaming completely. The default value is balanced. While 1to1
can be used as an workaround for various code ordering issues,
the max partitioning is intended for internal testing only. The
value one specifies that exactly one partition should be used
while the value none bypasses partitioning and executes the link-
time optimization step directly from the WPA phase.
-flto-odr-type-merging
Enable streaming of mangled types names of C++ types and their
unification at link time. This increases size of LTO object
files, but enables diagnostics about One Definition Rule
violations.
-flto-compression-level=n
This option specifies the level of compression used for
intermediate language written to LTO object files, and is only
meaningful in conjunction with LTO mode (-flto). Valid values
are 0 (no compression) to 9 (maximum compression). Values
outside this range are clamped to either 0 or 9. If the option
is not given, a default balanced compression setting is used.
-fuse-linker-plugin
Enables the use of a linker plugin during link-time optimization.
This option relies on plugin support in the linker, which is
available in gold or in GNU ld 2.21 or newer.
This option enables the extraction of object files with GIMPLE
bytecode out of library archives. This improves the quality of
optimization by exposing more code to the link-time optimizer.
This information specifies what symbols can be accessed
externally (by non-LTO object or during dynamic linking).
Resulting code quality improvements on binaries (and shared
libraries that use hidden visibility) are similar to
-fwhole-program. See -flto for a description of the effect of
this flag and how to use it.
This option is enabled by default when LTO support in GCC is
enabled and GCC was configured for use with a linker supporting
plugins (GNU ld 2.21 or newer or gold).
-ffat-lto-objects
Fat LTO objects are object files that contain both the
intermediate language and the object code. This makes them usable
for both LTO linking and normal linking. This option is effective
only when compiling with -flto and is ignored at link time.
-fno-fat-lto-objects improves compilation time over plain LTO,
but requires the complete toolchain to be aware of LTO. It
requires a linker with linker plugin support for basic
functionality. Additionally, nm, ar and ranlib need to support
linker plugins to allow a full-featured build environment
(capable of building static libraries etc). GCC provides the
gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to
these tools. With non fat LTO makefiles need to be modified to
use them.
The default is -fno-fat-lto-objects on targets with linker plugin
support.
-fcompare-elim
After register allocation and post-register allocation
instruction splitting, identify arithmetic instructions that
compute processor flags similar to a comparison operation based
on that arithmetic. If possible, eliminate the explicit
comparison operation.
This pass only applies to certain targets that cannot explicitly
represent the comparison operation before register allocation is
complete.
Enabled at levels -O, -O2, -O3, -Os.
-fcprop-registers
After register allocation and post-register allocation
instruction splitting, perform a copy-propagation pass to try to
reduce scheduling dependencies and occasionally eliminate the
copy.
Enabled at levels -O, -O2, -O3, -Os.
-fprofile-correction
Profiles collected using an instrumented binary for multi-
threaded programs may be inconsistent due to missed counter
updates. When this option is specified, GCC uses heuristics to
correct or smooth out such inconsistencies. By default, GCC emits
an error message when an inconsistent profile is detected.
-fprofile-use
-fprofile-use=path
Enable profile feedback-directed optimizations, and the following
optimizations which are generally profitable only with profile
feedback available: -fbranch-probabilities, -fvpt,
-funroll-loops, -fpeel-loops, -ftracer, -ftree-vectorize, and
ftree-loop-distribute-patterns.
Before you can use this option, you must first generate profiling
information.
By default, GCC emits an error message if the feedback profiles
do not match the source code. This error can be turned into a
warning by using -Wcoverage-mismatch. Note this may result in
poorly optimized code.
If path is specified, GCC looks at the path to find the profile
feedback data files. See -fprofile-dir.
-fauto-profile
-fauto-profile=path
Enable sampling-based feedback-directed optimizations, and the
following optimizations which are generally profitable only with
profile feedback available: -fbranch-probabilities, -fvpt,
-funroll-loops, -fpeel-loops, -ftracer, -ftree-vectorize,
-finline-functions, -fipa-cp, -fipa-cp-clone,
-fpredictive-commoning, -funswitch-loops, -fgcse-after-reload,
and -ftree-loop-distribute-patterns.
path is the name of a file containing AutoFDO profile
information. If omitted, it defaults to fbdata.afdo in the
current directory.
Producing an AutoFDO profile data file requires running your
program with the perf utility on a supported GNU/Linux target
system. For more information, see
<https://perf.wiki.kernel.org/ >.
E.g.
perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_program
Then use the create_gcov tool to convert the raw profile data to
a format that can be used by GCC. You must also supply the
unstripped binary for your program to this tool. See
<https://github.com/google/autofdo >.
E.g.
create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdo
The following options control compiler behavior regarding floating-
point arithmetic. These options trade off between speed and
correctness. All must be specifically enabled.
-ffloat-store
Do not store floating-point variables in registers, and inhibit
other options that might change whether a floating-point value is
taken from a register or memory.
This option prevents undesirable excess precision on machines
such as the 68000 where the floating registers (of the 68881)
keep more precision than a "double" is supposed to have.
Similarly for the x86 architecture. For most programs, the
excess precision does only good, but a few programs rely on the
precise definition of IEEE floating point. Use -ffloat-store for
such programs, after modifying them to store all pertinent
intermediate computations into variables.
-fexcess-precision=style
This option allows further control over excess precision on
machines where floating-point operations occur in a format with
more precision or range than the IEEE standard and interchange
floating-point types. By default, -fexcess-precision=fast is in
effect; this means that operations may be carried out in a wider
precision than the types specified in the source if that would
result in faster code, and it is unpredictable when rounding to
the types specified in the source code takes place. When
compiling C, if -fexcess-precision=standard is specified then
excess precision follows the rules specified in ISO C99; in
particular, both casts and assignments cause values to be rounded
to their semantic types (whereas -ffloat-store only affects
assignments). This option is enabled by default for C if a
strict conformance option such as -std=c99 is used. -ffast-math
enables -fexcess-precision=fast by default regardless of whether
a strict conformance option is used.
-fexcess-precision=standard is not implemented for languages
other than C. On the x86, it has no effect if -mfpmath=sse or
-mfpmath=sse+387 is specified; in the former case, IEEE semantics
apply without excess precision, and in the latter, rounding is
unpredictable.
-ffast-math
Sets the options -fno-math-errno, -funsafe-math-optimizations,
-ffinite-math-only, -fno-rounding-math, -fno-signaling-nans,
-fcx-limited-range and -fexcess-precision=fast.
This option causes the preprocessor macro "__FAST_MATH__" to be
defined.
This option is not turned on by any -O option besides -Ofast
since it can result in incorrect output for programs that depend
on an exact implementation of IEEE or ISO rules/specifications
for math functions. It may, however, yield faster code for
programs that do not require the guarantees of these
specifications.
-fno-math-errno
Do not set "errno" after calling math functions that are executed
with a single instruction, e.g., "sqrt". A program that relies
on IEEE exceptions for math error handling may want to use this
flag for speed while maintaining IEEE arithmetic compatibility.
This option is not turned on by any -O option since it can result
in incorrect output for programs that depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that
do not require the guarantees of these specifications.
The default is -fmath-errno.
On Darwin systems, the math library never sets "errno". There is
therefore no reason for the compiler to consider the possibility
that it might, and -fno-math-errno is the default.
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume
that arguments and results are valid and (b) may violate IEEE or
ANSI standards. When used at link time, it may include libraries
or startup files that change the default FPU control word or
other similar optimizations.
This option is not turned on by any -O option since it can result
in incorrect output for programs that depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that
do not require the guarantees of these specifications. Enables
-fno-signed-zeros, -fno-trapping-math, -fassociative-math and
-freciprocal-math.
The default is -fno-unsafe-math-optimizations.
-fassociative-math
Allow re-association of operands in series of floating-point
operations. This violates the ISO C and C++ language standard by
possibly changing computation result. NOTE: re-ordering may
change the sign of zero as well as ignore NaNs and inhibit or
create underflow or overflow (and thus cannot be used on code
that relies on rounding behavior like "(x + 2**52) - 2**52". May
also reorder floating-point comparisons and thus may not be used
when ordered comparisons are required. This option requires that
both -fno-signed-zeros and -fno-trapping-math be in effect.
Moreover, it doesn't make much sense with -frounding-math. For
Fortran the option is automatically enabled when both
-fno-signed-zeros and -fno-trapping-math are in effect.
The default is -fno-associative-math.
-freciprocal-math
Allow the reciprocal of a value to be used instead of dividing by
the value if this enables optimizations. For example "x / y" can
be replaced with "x * (1/y)", which is useful if "(1/y)" is
subject to common subexpression elimination. Note that this
loses precision and increases the number of flops operating on
the value.
The default is -fno-reciprocal-math.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume
that arguments and results are not NaNs or +-Infs.
This option is not turned on by any -O option since it can result
in incorrect output for programs that depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that
do not require the guarantees of these specifications.
The default is -fno-finite-math-only.
-fno-signed-zeros
Allow optimizations for floating-point arithmetic that ignore the
signedness of zero. IEEE arithmetic specifies the behavior of
distinct +0.0 and -0.0 values, which then prohibits
simplification of expressions such as x+0.0 or 0.0*x (even with
-ffinite-math-only). This option implies that the sign of a zero
result isn't significant.
The default is -fsigned-zeros.
-fno-trapping-math
Compile code assuming that floating-point operations cannot
generate user-visible traps. These traps include division by
zero, overflow, underflow, inexact result and invalid operation.
This option requires that -fno-signaling-nans be in effect.
Setting this option may allow faster code if one relies on "non-
stop" IEEE arithmetic, for example.
This option should never be turned on by any -O option since it
can result in incorrect output for programs that depend on an
exact implementation of IEEE or ISO rules/specifications for math
functions.
The default is -ftrapping-math.
-frounding-math
Disable transformations and optimizations that assume default
floating-point rounding behavior. This is round-to-zero for all
floating point to integer conversions, and round-to-nearest for
all other arithmetic truncations. This option should be
specified for programs that change the FP rounding mode
dynamically, or that may be executed with a non-default rounding
mode. This option disables constant folding of floating-point
expressions at compile time (which may be affected by rounding
mode) and arithmetic transformations that are unsafe in the
presence of sign-dependent rounding modes.
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee to
disable all GCC optimizations that are affected by rounding mode.
Future versions of GCC may provide finer control of this setting
using C99's "FENV_ACCESS" pragma. This command-line option will
be used to specify the default state for "FENV_ACCESS".
-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may generate user-
visible traps during floating-point operations. Setting this
option disables optimizations that may change the number of
exceptions visible with signaling NaNs. This option implies
-ftrapping-math.
This option causes the preprocessor macro "__SUPPORT_SNAN__" to
be defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to
disable all GCC optimizations that affect signaling NaN behavior.
-fno-fp-int-builtin-inexact
Do not allow the built-in functions "ceil", "floor", "round" and
"trunc", and their "float" and "long double" variants, to
generate code that raises the "inexact" floating-point exception
for noninteger arguments. ISO C99 and C11 allow these functions
to raise the "inexact" exception, but ISO/IEC TS 18661-1:2014,
the C bindings to IEEE 754-2008, does not allow these functions
to do so.
The default is -ffp-int-builtin-inexact, allowing the exception
to be raised. This option does nothing unless -ftrapping-math is
in effect.
Even if -fno-fp-int-builtin-inexact is used, if the functions
generate a call to a library function then the "inexact"
exception may be raised if the library implementation does not
follow TS 18661.
-fsingle-precision-constant
Treat floating-point constants as single precision instead of
implicitly converting them to double-precision constants.
-fcx-limited-range
When enabled, this option states that a range reduction step is
not needed when performing complex division. Also, there is no
checking whether the result of a complex multiplication or
division is "NaN + I*NaN", with an attempt to rescue the
situation in that case. The default is -fno-cx-limited-range,
but is enabled by -ffast-math.
This option controls the default setting of the ISO C99
"CX_LIMITED_RANGE" pragma. Nevertheless, the option applies to
all languages.
-fcx-fortran-rules
Complex multiplication and division follow Fortran rules. Range
reduction is done as part of complex division, but there is no
checking whether the result of a complex multiplication or
division is "NaN + I*NaN", with an attempt to rescue the
situation in that case.
The default is -fno-cx-fortran-rules.
The following options control optimizations that may improve
performance, but are not enabled by any -O options. This section
includes experimental options that may produce broken code.
-fbranch-probabilities
After running a program compiled with -fprofile-arcs, you can
compile it a second time using -fbranch-probabilities, to improve
optimizations based on the number of times each branch was taken.
When a program compiled with -fprofile-arcs exits, it saves arc
execution counts to a file called sourcename.gcda for each source
file. The information in this data file is very dependent on the
structure of the generated code, so you must use the same source
code and the same optimization options for both compilations.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each
JUMP_INSN and CALL_INSN. These can be used to improve
optimization. Currently, they are only used in one place: in
reorg.c, instead of guessing which path a branch is most likely
to take, the REG_BR_PROB values are used to exactly determine
which path is taken more often.
-fprofile-values
If combined with -fprofile-arcs, it adds code so that some data
about values of expressions in the program is gathered.
With -fbranch-probabilities, it reads back the data gathered from
profiling values of expressions for usage in optimizations.
Enabled with -fprofile-generate and -fprofile-use.
-fprofile-reorder-functions
Function reordering based on profile instrumentation collects
first time of execution of a function and orders these functions
in ascending order.
Enabled with -fprofile-use.
-fvpt
If combined with -fprofile-arcs, this option instructs the
compiler to add code to gather information about values of
expressions.
With -fbranch-probabilities, it reads back the data gathered and
actually performs the optimizations based on them. Currently the
optimizations include specialization of division operations using
the knowledge about the value of the denominator.
-frename-registers
Attempt to avoid false dependencies in scheduled code by making
use of registers left over after register allocation. This
optimization most benefits processors with lots of registers.
Depending on the debug information format adopted by the target,
however, it can make debugging impossible, since variables no
longer stay in a "home register".
Enabled by default with -funroll-loops.
-fschedule-fusion
Performs a target dependent pass over the instruction stream to
schedule instructions of same type together because target
machine can execute them more efficiently if they are adjacent to
each other in the instruction flow.
Enabled at levels -O2, -O3, -Os.
-ftracer
Perform tail duplication to enlarge superblock size. This
transformation simplifies the control flow of the function
allowing other optimizations to do a better job.
Enabled with -fprofile-use.
-funroll-loops
Unroll loops whose number of iterations can be determined at
compile time or upon entry to the loop. -funroll-loops implies
-frerun-cse-after-loop, -fweb and -frename-registers. It also
turns on complete loop peeling (i.e. complete removal of loops
with a small constant number of iterations). This option makes
code larger, and may or may not make it run faster.
Enabled with -fprofile-use.
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain
when the loop is entered. This usually makes programs run more
slowly. -funroll-all-loops implies the same options as
-funroll-loops.
-fpeel-loops
Peels loops for which there is enough information that they do
not roll much (from profile feedback or static analysis). It
also turns on complete loop peeling (i.e. complete removal of
loops with small constant number of iterations).
Enabled with -O3 and/or -fprofile-use.
-fmove-loop-invariants
Enables the loop invariant motion pass in the RTL loop optimizer.
Enabled at level -O1
-fsplit-loops
Split a loop into two if it contains a condition that's always
true for one side of the iteration space and false for the other.
-funswitch-loops
Move branches with loop invariant conditions out of the loop,
with duplicates of the loop on both branches (modified according
to result of the condition).
-ffunction-sections
-fdata-sections
Place each function or data item into its own section in the
output file if the target supports arbitrary sections. The name
of the function or the name of the data item determines the
section's name in the output file.
Use these options on systems where the linker can perform
optimizations to improve locality of reference in the instruction
space. Most systems using the ELF object format and SPARC
processors running Solaris 2 have linkers with such
optimizations. AIX may have these optimizations in the future.
Only use these options when there are significant benefits from
doing so. When you specify these options, the assembler and
linker create larger object and executable files and are also
slower. You cannot use gprof on all systems if you specify this
option, and you may have problems with debugging if you specify
both this option and -g.
-fbranch-target-load-optimize
Perform branch target register load optimization before prologue
/ epilogue threading. The use of target registers can typically
be exposed only during reload, thus hoisting loads out of loops
and doing inter-block scheduling needs a separate optimization
pass.
-fbranch-target-load-optimize2
Perform branch target register load optimization after prologue /
epilogue threading.
-fbtr-bb-exclusive
When performing branch target register load optimization, don't
reuse branch target registers within any basic block.
-fstdarg-opt
Optimize the prologue of variadic argument functions with respect
to usage of those arguments.
-fsection-anchors
Try to reduce the number of symbolic address calculations by
using shared "anchor" symbols to address nearby objects. This
transformation can help to reduce the number of GOT entries and
GOT accesses on some targets.
For example, the implementation of the following function "foo":
static int a, b, c;
int foo (void) { return a + b + c; }
usually calculates the addresses of all three variables, but if
you compile it with -fsection-anchors, it accesses the variables
from a common anchor point instead. The effect is similar to the
following pseudocode (which isn't valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
Not all targets support this option.
--param name=value
In some places, GCC uses various constants to control the amount
of optimization that is done. For example, GCC does not inline
functions that contain more than a certain number of
instructions. You can control some of these constants on the
command line using the --param option.
The names of specific parameters, and the meaning of the values,
are tied to the internals of the compiler, and are subject to
change without notice in future releases.
In each case, the value is an integer. The allowable choices for
name are:
predictable-branch-outcome
When branch is predicted to be taken with probability lower
than this threshold (in percent), then it is considered well
predictable. The default is 10.
max-rtl-if-conversion-insns
RTL if-conversion tries to remove conditional branches around
a block and replace them with conditionally executed
instructions. This parameter gives the maximum number of
instructions in a block which should be considered for if-
conversion. The default is 10, though the compiler will also
use other heuristics to decide whether if-conversion is
likely to be profitable.
max-rtl-if-conversion-predictable-cost
max-rtl-if-conversion-unpredictable-cost
RTL if-conversion will try to remove conditional branches
around a block and replace them with conditionally executed
instructions. These parameters give the maximum permissible
cost for the sequence that would be generated by if-
conversion depending on whether the branch is statically
determined to be predictable or not. The units for this
parameter are the same as those for the GCC internal seq_cost
metric. The compiler will try to provide a reasonable
default for this parameter using the BRANCH_COST target
macro.
max-crossjump-edges
The maximum number of incoming edges to consider for cross-
jumping. The algorithm used by -fcrossjumping is O(N^2) in
the number of edges incoming to each block. Increasing
values mean more aggressive optimization, making the
compilation time increase with probably small improvement in
executable size.
min-crossjump-insns
The minimum number of instructions that must be matched at
the end of two blocks before cross-jumping is performed on
them. This value is ignored in the case where all
instructions in the block being cross-jumped from are
matched. The default value is 5.
max-grow-copy-bb-insns
The maximum code size expansion factor when copying basic
blocks instead of jumping. The expansion is relative to a
jump instruction. The default value is 8.
max-goto-duplication-insns
The maximum number of instructions to duplicate to a block
that jumps to a computed goto. To avoid O(N^2) behavior in a
number of passes, GCC factors computed gotos early in the
compilation process, and unfactors them as late as possible.
Only computed jumps at the end of a basic blocks with no more
than max-goto-duplication-insns are unfactored. The default
value is 8.
max-delay-slot-insn-search
The maximum number of instructions to consider when looking
for an instruction to fill a delay slot. If more than this
arbitrary number of instructions are searched, the time
savings from filling the delay slot are minimal, so stop
searching. Increasing values mean more aggressive
optimization, making the compilation time increase with
probably small improvement in execution time.
max-delay-slot-live-search
When trying to fill delay slots, the maximum number of
instructions to consider when searching for a block with
valid live register information. Increasing this arbitrarily
chosen value means more aggressive optimization, increasing
the compilation time. This parameter should be removed when
the delay slot code is rewritten to maintain the control-flow
graph.
max-gcse-memory
The approximate maximum amount of memory that can be
allocated in order to perform the global common subexpression
elimination optimization. If more memory than specified is
required, the optimization is not done.
max-gcse-insertion-ratio
If the ratio of expression insertions to deletions is larger
than this value for any expression, then RTL PRE inserts or
removes the expression and thus leaves partially redundant
computations in the instruction stream. The default value is
20.
max-pending-list-length
The maximum number of pending dependencies scheduling allows
before flushing the current state and starting over. Large
functions with few branches or calls can create excessively
large lists which needlessly consume memory and resources.
max-modulo-backtrack-attempts
The maximum number of backtrack attempts the scheduler should
make when modulo scheduling a loop. Larger values can
exponentially increase compilation time.
max-inline-insns-single
Several parameters control the tree inliner used in GCC.
This number sets the maximum number of instructions (counted
in GCC's internal representation) in a single function that
the tree inliner considers for inlining. This only affects
functions declared inline and methods implemented in a class
declaration (C++). The default value is 400.
max-inline-insns-auto
When you use -finline-functions (included in -O3), a lot of
functions that would otherwise not be considered for inlining
by the compiler are investigated. To those functions, a
different (more restrictive) limit compared to functions
declared inline can be applied. The default value is 40.
inline-min-speedup
When estimated performance improvement of caller + callee
runtime exceeds this threshold (in percent), the function can
be inlined regardless of the limit on --param max-inline-
insns-single and --param max-inline-insns-auto.
large-function-insns
The limit specifying really large functions. For functions
larger than this limit after inlining, inlining is
constrained by --param large-function-growth. This parameter
is useful primarily to avoid extreme compilation time caused
by non-linear algorithms used by the back end. The default
value is 2700.
large-function-growth
Specifies maximal growth of large function caused by inlining
in percents. The default value is 100 which limits large
function growth to 2.0 times the original size.
large-unit-insns
The limit specifying large translation unit. Growth caused
by inlining of units larger than this limit is limited by
--param inline-unit-growth. For small units this might be
too tight. For example, consider a unit consisting of
function A that is inline and B that just calls A three
times. If B is small relative to A, the growth of unit is
300\% and yet such inlining is very sane. For very large
units consisting of small inlineable functions, however, the
overall unit growth limit is needed to avoid exponential
explosion of code size. Thus for smaller units, the size is
increased to --param large-unit-insns before applying --param
inline-unit-growth. The default is 10000.
inline-unit-growth
Specifies maximal overall growth of the compilation unit
caused by inlining. The default value is 20 which limits
unit growth to 1.2 times the original size. Cold functions
(either marked cold via an attribute or by profile feedback)
are not accounted into the unit size.
ipcp-unit-growth
Specifies maximal overall growth of the compilation unit
caused by interprocedural constant propagation. The default
value is 10 which limits unit growth to 1.1 times the
original size.
large-stack-frame
The limit specifying large stack frames. While inlining the
algorithm is trying to not grow past this limit too much.
The default value is 256 bytes.
large-stack-frame-growth
Specifies maximal growth of large stack frames caused by
inlining in percents. The default value is 1000 which limits
large stack frame growth to 11 times the original size.
max-inline-insns-recursive
max-inline-insns-recursive-auto
Specifies the maximum number of instructions an out-of-line
copy of a self-recursive inline function can grow into by
performing recursive inlining.
--param max-inline-insns-recursive applies to functions
declared inline. For functions not declared inline,
recursive inlining happens only when -finline-functions
(included in -O3) is enabled; --param max-inline-insns-
recursive-auto applies instead. The default value is 450.
max-inline-recursive-depth
max-inline-recursive-depth-auto
Specifies the maximum recursion depth used for recursive
inlining.
--param max-inline-recursive-depth applies to functions
declared inline. For functions not declared inline,
recursive inlining happens only when -finline-functions
(included in -O3) is enabled; --param max-inline-recursive-
depth-auto applies instead. The default value is 8.
min-inline-recursive-probability
Recursive inlining is profitable only for function having
deep recursion in average and can hurt for function having
little recursion depth by increasing the prologue size or
complexity of function body to other optimizers.
When profile feedback is available (see -fprofile-generate)
the actual recursion depth can be guessed from the
probability that function recurses via a given call
expression. This parameter limits inlining only to call
expressions whose probability exceeds the given threshold (in
percents). The default value is 10.
early-inlining-insns
Specify growth that the early inliner can make. In effect it
increases the amount of inlining for code having a large
abstraction penalty. The default value is 14.
max-early-inliner-iterations
Limit of iterations of the early inliner. This basically
bounds the number of nested indirect calls the early inliner
can resolve. Deeper chains are still handled by late
inlining.
comdat-sharing-probability
Probability (in percent) that C++ inline function with comdat
visibility are shared across multiple compilation units. The
default value is 20.
profile-func-internal-id
A parameter to control whether to use function internal id in
profile database lookup. If the value is 0, the compiler uses
an id that is based on function assembler name and filename,
which makes old profile data more tolerant to source changes
such as function reordering etc. The default value is 0.
min-vect-loop-bound
The minimum number of iterations under which loops are not
vectorized when -ftree-vectorize is used. The number of
iterations after vectorization needs to be greater than the
value specified by this option to allow vectorization. The
default value is 0.
gcse-cost-distance-ratio
Scaling factor in calculation of maximum distance an
expression can be moved by GCSE optimizations. This is
currently supported only in the code hoisting pass. The
bigger the ratio, the more aggressive code hoisting is with
simple expressions, i.e., the expressions that have cost less
than gcse-unrestricted-cost. Specifying 0 disables hoisting
of simple expressions. The default value is 10.
gcse-unrestricted-cost
Cost, roughly measured as the cost of a single typical
machine instruction, at which GCSE optimizations do not
constrain the distance an expression can travel. This is
currently supported only in the code hoisting pass. The
lesser the cost, the more aggressive code hoisting is.
Specifying 0 allows all expressions to travel unrestricted
distances. The default value is 3.
max-hoist-depth
The depth of search in the dominator tree for expressions to
hoist. This is used to avoid quadratic behavior in hoisting
algorithm. The value of 0 does not limit on the search, but
may slow down compilation of huge functions. The default
value is 30.
max-tail-merge-comparisons
The maximum amount of similar bbs to compare a bb with. This
is used to avoid quadratic behavior in tree tail merging.
The default value is 10.
max-tail-merge-iterations
The maximum amount of iterations of the pass over the
function. This is used to limit compilation time in tree
tail merging. The default value is 2.
store-merging-allow-unaligned
Allow the store merging pass to introduce unaligned stores if
it is legal to do so. The default value is 1.
max-stores-to-merge
The maximum number of stores to attempt to merge into wider
stores in the store merging pass. The minimum value is 2 and
the default is 64.
max-unrolled-insns
The maximum number of instructions that a loop may have to be
unrolled. If a loop is unrolled, this parameter also
determines how many times the loop code is unrolled.
max-average-unrolled-insns
The maximum number of instructions biased by probabilities of
their execution that a loop may have to be unrolled. If a
loop is unrolled, this parameter also determines how many
times the loop code is unrolled.
max-unroll-times
The maximum number of unrollings of a single loop.
max-peeled-insns
The maximum number of instructions that a loop may have to be
peeled. If a loop is peeled, this parameter also determines
how many times the loop code is peeled.
max-peel-times
The maximum number of peelings of a single loop.
max-peel-branches
The maximum number of branches on the hot path through the
peeled sequence.
max-completely-peeled-insns
The maximum number of insns of a completely peeled loop.
max-completely-peel-times
The maximum number of iterations of a loop to be suitable for
complete peeling.
max-completely-peel-loop-nest-depth
The maximum depth of a loop nest suitable for complete
peeling.
max-unswitch-insns
The maximum number of insns of an unswitched loop.
max-unswitch-level
The maximum number of branches unswitched in a single loop.
max-loop-headers-insns
The maximum number of insns in loop header duplicated by he
copy loop headers pass.
lim-expensive
The minimum cost of an expensive expression in the loop
invariant motion.
iv-consider-all-candidates-bound
Bound on number of candidates for induction variables, below
which all candidates are considered for each use in induction
variable optimizations. If there are more candidates than
this, only the most relevant ones are considered to avoid
quadratic time complexity.
iv-max-considered-uses
The induction variable optimizations give up on loops that
contain more induction variable uses.
iv-always-prune-cand-set-bound
If the number of candidates in the set is smaller than this
value, always try to remove unnecessary ivs from the set when
adding a new one.
avg-loop-niter
Average number of iterations of a loop.
dse-max-object-size
Maximum size (in bytes) of objects tracked bytewise by dead
store elimination. Larger values may result in larger
compilation times.
scev-max-expr-size
Bound on size of expressions used in the scalar evolutions
analyzer. Large expressions slow the analyzer.
scev-max-expr-complexity
Bound on the complexity of the expressions in the scalar
evolutions analyzer. Complex expressions slow the analyzer.
max-tree-if-conversion-phi-args
Maximum number of arguments in a PHI supported by TREE if
conversion unless the loop is marked with simd pragma.
vect-max-version-for-alignment-checks
The maximum number of run-time checks that can be performed
when doing loop versioning for alignment in the vectorizer.
vect-max-version-for-alias-checks
The maximum number of run-time checks that can be performed
when doing loop versioning for alias in the vectorizer.
vect-max-peeling-for-alignment
The maximum number of loop peels to enhance access alignment
for vectorizer. Value -1 means no limit.
max-iterations-to-track
The maximum number of iterations of a loop the brute-force
algorithm for analysis of the number of iterations of the
loop tries to evaluate.
hot-bb-count-ws-permille
A basic block profile count is considered hot if it
contributes to the given permillage (i.e. 0...1000) of the
entire profiled execution.
hot-bb-frequency-fraction
Select fraction of the entry block frequency of executions of
basic block in function given basic block needs to have to be
considered hot.
max-predicted-iterations
The maximum number of loop iterations we predict statically.
This is useful in cases where a function contains a single
loop with known bound and another loop with unknown bound.
The known number of iterations is predicted correctly, while
the unknown number of iterations average to roughly 10. This
means that the loop without bounds appears artificially cold
relative to the other one.
builtin-expect-probability
Control the probability of the expression having the
specified value. This parameter takes a percentage (i.e. 0
... 100) as input. The default probability of 90 is obtained
empirically.
align-threshold
Select fraction of the maximal frequency of executions of a
basic block in a function to align the basic block.
align-loop-iterations
A loop expected to iterate at least the selected number of
iterations is aligned.
tracer-dynamic-coverage
tracer-dynamic-coverage-feedback
This value is used to limit superblock formation once the
given percentage of executed instructions is covered. This
limits unnecessary code size expansion.
The tracer-dynamic-coverage-feedback parameter is used only
when profile feedback is available. The real profiles (as
opposed to statically estimated ones) are much less balanced
allowing the threshold to be larger value.
tracer-max-code-growth
Stop tail duplication once code growth has reached given
percentage. This is a rather artificial limit, as most of
the duplicates are eliminated later in cross jumping, so it
may be set to much higher values than is the desired code
growth.
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of best edge
is less than this threshold (in percent).
tracer-min-branch-probability
tracer-min-branch-probability-feedback
Stop forward growth if the best edge has probability lower
than this threshold.
Similarly to tracer-dynamic-coverage two parameters are
provided. tracer-min-branch-probability-feedback is used for
compilation with profile feedback and tracer-min-branch-
probability compilation without. The value for compilation
with profile feedback needs to be more conservative (higher)
in order to make tracer effective.
max-cse-path-length
The maximum number of basic blocks on path that CSE
considers. The default is 10.
max-cse-insns
The maximum number of instructions CSE processes before
flushing. The default is 1000.
ggc-min-expand
GCC uses a garbage collector to manage its own memory
allocation. This parameter specifies the minimum percentage
by which the garbage collector's heap should be allowed to
expand between collections. Tuning this may improve
compilation speed; it has no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of
100% when RAM >= 1GB. If "getrlimit" is available, the
notion of "RAM" is the smallest of actual RAM and
"RLIMIT_DATA" or "RLIMIT_AS". If GCC is not able to
calculate RAM on a particular platform, the lower bound of
30% is used. Setting this parameter and ggc-min-heapsize to
zero causes a full collection to occur at every opportunity.
This is extremely slow, but can be useful for debugging.
ggc-min-heapsize
Minimum size of the garbage collector's heap before it begins
bothering to collect garbage. The first collection occurs
after the heap expands by ggc-min-expand% beyond ggc-min-
heapsize. Again, tuning this may improve compilation speed,
and has no effect on code generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit
that tries to ensure that RLIMIT_DATA or RLIMIT_AS are not
exceeded, but with a lower bound of 4096 (four megabytes) and
an upper bound of 131072 (128 megabytes). If GCC is not able
to calculate RAM on a particular platform, the lower bound is
used. Setting this parameter very large effectively disables
garbage collection. Setting this parameter and ggc-min-
expand to zero causes a full collection to occur at every
opportunity.
max-reload-search-insns
The maximum number of instruction reload should look backward
for equivalent register. Increasing values mean more
aggressive optimization, making the compilation time increase
with probably slightly better performance. The default value
is 100.
max-cselib-memory-locations
The maximum number of memory locations cselib should take
into account. Increasing values mean more aggressive
optimization, making the compilation time increase with
probably slightly better performance. The default value is
500.
max-sched-ready-insns
The maximum number of instructions ready to be issued the
scheduler should consider at any given time during the first
scheduling pass. Increasing values mean more thorough
searches, making the compilation time increase with probably
little benefit. The default value is 100.
max-sched-region-blocks
The maximum number of blocks in a region to be considered for
interblock scheduling. The default value is 10.
max-pipeline-region-blocks
The maximum number of blocks in a region to be considered for
pipelining in the selective scheduler. The default value is
15.
max-sched-region-insns
The maximum number of insns in a region to be considered for
interblock scheduling. The default value is 100.
max-pipeline-region-insns
The maximum number of insns in a region to be considered for
pipelining in the selective scheduler. The default value is
200.
min-spec-prob
The minimum probability (in percents) of reaching a source
block for interblock speculative scheduling. The default
value is 40.
max-sched-extend-regions-iters
The maximum number of iterations through CFG to extend
regions. A value of 0 (the default) disables region
extensions.
max-sched-insn-conflict-delay
The maximum conflict delay for an insn to be considered for
speculative motion. The default value is 3.
sched-spec-prob-cutoff
The minimal probability of speculation success (in percents),
so that speculative insns are scheduled. The default value
is 40.
sched-state-edge-prob-cutoff
The minimum probability an edge must have for the scheduler
to save its state across it. The default value is 10.
sched-mem-true-dep-cost
Minimal distance (in CPU cycles) between store and load
targeting same memory locations. The default value is 1.
selsched-max-lookahead
The maximum size of the lookahead window of selective
scheduling. It is a depth of search for available
instructions. The default value is 50.
selsched-max-sched-times
The maximum number of times that an instruction is scheduled
during selective scheduling. This is the limit on the number
of iterations through which the instruction may be pipelined.
The default value is 2.
selsched-insns-to-rename
The maximum number of best instructions in the ready list
that are considered for renaming in the selective scheduler.
The default value is 2.
sms-min-sc
The minimum value of stage count that swing modulo scheduler
generates. The default value is 2.
max-last-value-rtl
The maximum size measured as number of RTLs that can be
recorded in an expression in combiner for a pseudo register
as last known value of that register. The default is 10000.
max-combine-insns
The maximum number of instructions the RTL combiner tries to
combine. The default value is 2 at -Og and 4 otherwise.
integer-share-limit
Small integer constants can use a shared data structure,
reducing the compiler's memory usage and increasing its
speed. This sets the maximum value of a shared integer
constant. The default value is 256.
ssp-buffer-size
The minimum size of buffers (i.e. arrays) that receive stack
smashing protection when -fstack-protection is used.
min-size-for-stack-sharing
The minimum size of variables taking part in stack slot
sharing when not optimizing. The default value is 32.
max-jump-thread-duplication-stmts
Maximum number of statements allowed in a block that needs to
be duplicated when threading jumps.
max-fields-for-field-sensitive
Maximum number of fields in a structure treated in a field
sensitive manner during pointer analysis. The default is
zero for -O0 and -O1, and 100 for -Os, -O2, and -O3.
prefetch-latency
Estimate on average number of instructions that are executed
before prefetch finishes. The distance prefetched ahead is
proportional to this constant. Increasing this number may
also lead to less streams being prefetched (see simultaneous-
prefetches).
simultaneous-prefetches
Maximum number of prefetches that can run at the same time.
l1-cache-line-size
The size of cache line in L1 cache, in bytes.
l1-cache-size
The size of L1 cache, in kilobytes.
l2-cache-size
The size of L2 cache, in kilobytes.
min-insn-to-prefetch-ratio
The minimum ratio between the number of instructions and the
number of prefetches to enable prefetching in a loop.
prefetch-min-insn-to-mem-ratio
The minimum ratio between the number of instructions and the
number of memory references to enable prefetching in a loop.
use-canonical-types
Whether the compiler should use the "canonical" type system.
By default, this should always be 1, which uses a more
efficient internal mechanism for comparing types in C++ and
Objective-C++. However, if bugs in the canonical type system
are causing compilation failures, set this value to 0 to
disable canonical types.
switch-conversion-max-branch-ratio
Switch initialization conversion refuses to create arrays
that are bigger than switch-conversion-max-branch-ratio times
the number of branches in the switch.
max-partial-antic-length
Maximum length of the partial antic set computed during the
tree partial redundancy elimination optimization (-ftree-pre)
when optimizing at -O3 and above. For some sorts of source
code the enhanced partial redundancy elimination optimization
can run away, consuming all of the memory available on the
host machine. This parameter sets a limit on the length of
the sets that are computed, which prevents the runaway
behavior. Setting a value of 0 for this parameter allows an
unlimited set length.
sccvn-max-scc-size
Maximum size of a strongly connected component (SCC) during
SCCVN processing. If this limit is hit, SCCVN processing for
the whole function is not done and optimizations depending on
it are disabled. The default maximum SCC size is 10000.
sccvn-max-alias-queries-per-access
Maximum number of alias-oracle queries we perform when
looking for redundancies for loads and stores. If this limit
is hit the search is aborted and the load or store is not
considered redundant. The number of queries is
algorithmically limited to the number of stores on all paths
from the load to the function entry. The default maximum
number of queries is 1000.
ira-max-loops-num
IRA uses regional register allocation by default. If a
function contains more loops than the number given by this
parameter, only at most the given number of the most
frequently-executed loops form regions for regional register
allocation. The default value of the parameter is 100.
ira-max-conflict-table-size
Although IRA uses a sophisticated algorithm to compress the
conflict table, the table can still require excessive amounts
of memory for huge functions. If the conflict table for a
function could be more than the size in MB given by this
parameter, the register allocator instead uses a faster,
simpler, and lower-quality algorithm that does not require
building a pseudo-register conflict table. The default value
of the parameter is 2000.
ira-loop-reserved-regs
IRA can be used to evaluate more accurate register pressure
in loops for decisions to move loop invariants (see -O3).
The number of available registers reserved for some other
purposes is given by this parameter. The default value of
the parameter is 2, which is the minimal number of registers
needed by typical instructions. This value is the best found
from numerous experiments.
lra-inheritance-ebb-probability-cutoff
LRA tries to reuse values reloaded in registers in subsequent
insns. This optimization is called inheritance. EBB is used
as a region to do this optimization. The parameter defines a
minimal fall-through edge probability in percentage used to
add BB to inheritance EBB in LRA. The default value of the
parameter is 40. The value was chosen from numerous runs of
SPEC2000 on x86-64.
loop-invariant-max-bbs-in-loop
Loop invariant motion can be very expensive, both in
compilation time and in amount of needed compile-time memory,
with very large loops. Loops with more basic blocks than
this parameter won't have loop invariant motion optimization
performed on them. The default value of the parameter is
1000 for -O1 and 10000 for -O2 and above.
loop-max-datarefs-for-datadeps
Building data dependencies is expensive for very large loops.
This parameter limits the number of data references in loops
that are considered for data dependence analysis. These
large loops are no handled by the optimizations using loop
data dependencies. The default value is 1000.
max-vartrack-size
Sets a maximum number of hash table slots to use during
variable tracking dataflow analysis of any function. If this
limit is exceeded with variable tracking at assignments
enabled, analysis for that function is retried without it,
after removing all debug insns from the function. If the
limit is exceeded even without debug insns, var tracking
analysis is completely disabled for the function. Setting
the parameter to zero makes it unlimited.
max-vartrack-expr-depth
Sets a maximum number of recursion levels when attempting to
map variable names or debug temporaries to value expressions.
This trades compilation time for more complete debug
information. If this is set too low, value expressions that
are available and could be represented in debug information
may end up not being used; setting this higher may enable the
compiler to find more complex debug expressions, but compile
time and memory use may grow. The default is 12.
min-nondebug-insn-uid
Use uids starting at this parameter for nondebug insns. The
range below the parameter is reserved exclusively for debug
insns created by -fvar-tracking-assignments, but debug insns
may get (non-overlapping) uids above it if the reserved range
is exhausted.
ipa-sra-ptr-growth-factor
IPA-SRA replaces a pointer to an aggregate with one or more
new parameters only when their cumulative size is less or
equal to ipa-sra-ptr-growth-factor times the size of the
original pointer parameter.
sra-max-scalarization-size-Ospeed
sra-max-scalarization-size-Osize
The two Scalar Reduction of Aggregates passes (SRA and IPA-
SRA) aim to replace scalar parts of aggregates with uses of
independent scalar variables. These parameters control the
maximum size, in storage units, of aggregate which is
considered for replacement when compiling for speed (sra-max-
scalarization-size-Ospeed) or size (sra-max-scalarization-
size-Osize) respectively.
tm-max-aggregate-size
When making copies of thread-local variables in a
transaction, this parameter specifies the size in bytes after
which variables are saved with the logging functions as
opposed to save/restore code sequence pairs. This option
only applies when using -fgnu-tm.
graphite-max-nb-scop-params
To avoid exponential effects in the Graphite loop transforms,
the number of parameters in a Static Control Part (SCoP) is
bounded. The default value is 10 parameters. A variable
whose value is unknown at compilation time and defined
outside a SCoP is a parameter of the SCoP.
graphite-max-bbs-per-function
To avoid exponential effects in the detection of SCoPs, the
size of the functions analyzed by Graphite is bounded. The
default value is 100 basic blocks.
loop-block-tile-size
Loop blocking or strip mining transforms, enabled with
-floop-block or -floop-strip-mine, strip mine each loop in
the loop nest by a given number of iterations. The strip
length can be changed using the loop-block-tile-size
parameter. The default value is 51 iterations.
loop-unroll-jam-size
Specify the unroll factor for the -floop-unroll-and-jam
option. The default value is 4.
loop-unroll-jam-depth
Specify the dimension to be unrolled (counting from the most
inner loop) for the -floop-unroll-and-jam. The default
value is 2.
ipa-cp-value-list-size
IPA-CP attempts to track all possible values and types passed
to a function's parameter in order to propagate them and
perform devirtualization. ipa-cp-value-list-size is the
maximum number of values and types it stores per one formal
parameter of a function.
ipa-cp-eval-threshold
IPA-CP calculates its own score of cloning profitability
heuristics and performs those cloning opportunities with
scores that exceed ipa-cp-eval-threshold.
ipa-cp-recursion-penalty
Percentage penalty the recursive functions will receive when
they are evaluated for cloning.
ipa-cp-single-call-penalty
Percentage penalty functions containing a single call to
another function will receive when they are evaluated for
cloning.
ipa-max-agg-items
IPA-CP is also capable to propagate a number of scalar values
passed in an aggregate. ipa-max-agg-items controls the
maximum number of such values per one parameter.
ipa-cp-loop-hint-bonus
When IPA-CP determines that a cloning candidate would make
the number of iterations of a loop known, it adds a bonus of
ipa-cp-loop-hint-bonus to the profitability score of the
candidate.
ipa-cp-array-index-hint-bonus
When IPA-CP determines that a cloning candidate would make
the index of an array access known, it adds a bonus of ipa-
cp-array-index-hint-bonus to the profitability score of the
candidate.
ipa-max-aa-steps
During its analysis of function bodies, IPA-CP employs alias
analysis in order to track values pointed to by function
parameters. In order not spend too much time analyzing huge
functions, it gives up and consider all memory clobbered
after examining ipa-max-aa-steps statements modifying memory.
lto-partitions
Specify desired number of partitions produced during WHOPR
compilation. The number of partitions should exceed the
number of CPUs used for compilation. The default value is
32.
lto-min-partition
Size of minimal partition for WHOPR (in estimated
instructions). This prevents expenses of splitting very
small programs into too many partitions.
lto-max-partition
Size of max partition for WHOPR (in estimated instructions).
to provide an upper bound for individual size of partition.
Meant to be used only with balanced partitioning.
cxx-max-namespaces-for-diagnostic-help
The maximum number of namespaces to consult for suggestions
when C++ name lookup fails for an identifier. The default is
1000.
sink-frequency-threshold
The maximum relative execution frequency (in percents) of the
target block relative to a statement's original block to
allow statement sinking of a statement. Larger numbers
result in more aggressive statement sinking. The default
value is 75. A small positive adjustment is applied for
statements with memory operands as those are even more
profitable so sink.
max-stores-to-sink
The maximum number of conditional store pairs that can be
sunk. Set to 0 if either vectorization (-ftree-vectorize) or
if-conversion (-ftree-loop-if-convert) is disabled. The
default is 2.
allow-store-data-races
Allow optimizers to introduce new data races on stores. Set
to 1 to allow, otherwise to 0. This option is enabled by
default at optimization level -Ofast.
case-values-threshold
The smallest number of different values for which it is best
to use a jump-table instead of a tree of conditional
branches. If the value is 0, use the default for the
machine. The default is 0.
tree-reassoc-width
Set the maximum number of instructions executed in parallel
in reassociated tree. This parameter overrides target
dependent heuristics used by default if has non zero value.
sched-pressure-algorithm
Choose between the two available implementations of
-fsched-pressure. Algorithm 1 is the original implementation
and is the more likely to prevent instructions from being
reordered. Algorithm 2 was designed to be a compromise
between the relatively conservative approach taken by
algorithm 1 and the rather aggressive approach taken by the
default scheduler. It relies more heavily on having a
regular register file and accurate register pressure classes.
See haifa-sched.c in the GCC sources for more details.
The default choice depends on the target.
max-slsr-cand-scan
Set the maximum number of existing candidates that are
considered when seeking a basis for a new straight-line
strength reduction candidate.
asan-globals
Enable buffer overflow detection for global objects. This
kind of protection is enabled by default if you are using
-fsanitize=address option. To disable global objects
protection use --param asan-globals=0.
asan-stack
Enable buffer overflow detection for stack objects. This
kind of protection is enabled by default when using
-fsanitize=address. To disable stack protection use --param
asan-stack=0 option.
asan-instrument-reads
Enable buffer overflow detection for memory reads. This kind
of protection is enabled by default when using
-fsanitize=address. To disable memory reads protection use
--param asan-instrument-reads=0.
asan-instrument-writes
Enable buffer overflow detection for memory writes. This
kind of protection is enabled by default when using
-fsanitize=address. To disable memory writes protection use
--param asan-instrument-writes=0 option.
asan-memintrin
Enable detection for built-in functions. This kind of
protection is enabled by default when using
-fsanitize=address. To disable built-in functions protection
use --param asan-memintrin=0.
asan-use-after-return
Enable detection of use-after-return. This kind of
protection is enabled by default when using the
-fsanitize=address option. To disable it use --param
asan-use-after-return=0.
Note: By default the check is disabled at run time. To
enable it, add "detect_stack_use_after_return=1" to the
environment variable ASAN_OPTIONS.
asan-instrumentation-with-call-threshold
If number of memory accesses in function being instrumented
is greater or equal to this number, use callbacks instead of
inline checks. E.g. to disable inline code use --param
asan-instrumentation-with-call-threshold=0.
use-after-scope-direct-emission-threshold
If the size of a local variable in bytes is smaller or equal
to this number, directly poison (or unpoison) shadow memory
instead of using run-time callbacks. The default value is
256.
chkp-max-ctor-size
Static constructors generated by Pointer Bounds Checker may
become very large and significantly increase compile time at
optimization level -O1 and higher. This parameter is a
maximum number of statements in a single generated
constructor. Default value is 5000.
max-fsm-thread-path-insns
Maximum number of instructions to copy when duplicating
blocks on a finite state automaton jump thread path. The
default is 100.
max-fsm-thread-length
Maximum number of basic blocks on a finite state automaton
jump thread path. The default is 10.
max-fsm-thread-paths
Maximum number of new jump thread paths to create for a
finite state automaton. The default is 50.
parloops-chunk-size
Chunk size of omp schedule for loops parallelized by
parloops. The default is 0.
parloops-schedule
Schedule type of omp schedule for loops parallelized by
parloops (static, dynamic, guided, auto, runtime). The
default is static.
max-ssa-name-query-depth
Maximum depth of recursion when querying properties of SSA
names in things like fold routines. One level of recursion
corresponds to following a use-def chain.
hsa-gen-debug-stores
Enable emission of special debug stores within HSA kernels
which are then read and reported by libgomp plugin.
Generation of these stores is disabled by default, use
--param hsa-gen-debug-stores=1 to enable it.
max-speculative-devirt-maydefs
The maximum number of may-defs we analyze when looking for a
must-def specifying the dynamic type of an object that
invokes a virtual call we may be able to devirtualize
speculatively.
max-vrp-switch-assertions
The maximum number of assertions to add along the default
edge of a switch statement during VRP. The default is 10.
Program Instrumentation Options
GCC supports a number of command-line options that control adding
run-time instrumentation to the code it normally generates. For
example, one purpose of instrumentation is collect profiling
statistics for use in finding program hot spots, code coverage
analysis, or profile-guided optimizations. Another class of program
instrumentation is adding run-time checking to detect programming
errors like invalid pointer dereferences or out-of-bounds array
accesses, as well as deliberately hostile attacks such as stack
smashing or C++ vtable hijacking. There is also a general hook which
can be used to implement other forms of tracing or function-level
instrumentation for debug or program analysis purposes.
-p Generate extra code to write profile information suitable for the
analysis program prof. You must use this option when compiling
the source files you want data about, and you must also use it
when linking.
-pg Generate extra code to write profile information suitable for the
analysis program gprof. You must use this option when compiling
the source files you want data about, and you must also use it
when linking.
-fprofile-arcs
Add code so that program flow arcs are instrumented. During
execution the program records how many times each branch and call
is executed and how many times it is taken or returns. On
targets that support constructors with priority support,
profiling properly handles constructors, destructors and C++
constructors (and destructors) of classes which are used as a
type of a global variable.
When the compiled program exits it saves this data to a file
called auxname.gcda for each source file. The data may be used
for profile-directed optimizations (-fbranch-probabilities), or
for test coverage analysis (-ftest-coverage). Each object file's
auxname is generated from the name of the output file, if
explicitly specified and it is not the final executable,
otherwise it is the basename of the source file. In both cases
any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or
dir/foo.gcda for output file specified as -o dir/foo.o).
--coverage
This option is used to compile and link code instrumented for
coverage analysis. The option is a synonym for -fprofile-arcs
-ftest-coverage (when compiling) and -lgcov (when linking). See
the documentation for those options for more details.
* Compile the source files with -fprofile-arcs plus
optimization and code generation options. For test coverage
analysis, use the additional -ftest-coverage option. You do
not need to profile every source file in a program.
* Link your object files with -lgcov or -fprofile-arcs (the
latter implies the former).
* Run the program on a representative workload to generate the
arc profile information. This may be repeated any number of
times. You can run concurrent instances of your program, and
provided that the file system supports locking, the data
files will be correctly updated. Also "fork" calls are
detected and correctly handled (double counting will not
happen).
* For profile-directed optimizations, compile the source files
again with the same optimization and code generation options
plus -fbranch-probabilities.
* For test coverage analysis, use gcov to produce human
readable information from the .gcno and .gcda files. Refer
to the gcov documentation for further information.
With -fprofile-arcs, for each function of your program GCC
creates a program flow graph, then finds a spanning tree for the
graph. Only arcs that are not on the spanning tree have to be
instrumented: the compiler adds code to count the number of times
that these arcs are executed. When an arc is the only exit or
only entrance to a block, the instrumentation code can be added
to the block; otherwise, a new basic block must be created to
hold the instrumentation code.
-ftest-coverage
Produce a notes file that the gcov code-coverage utility can use
to show program coverage. Each source file's note file is called
auxname.gcno. Refer to the -fprofile-arcs option above for a
description of auxname and instructions on how to generate test
coverage data. Coverage data matches the source files more
closely if you do not optimize.
-fprofile-dir=path
Set the directory to search for the profile data files in to
path. This option affects only the profile data generated by
-fprofile-generate, -ftest-coverage, -fprofile-arcs and used by
-fprofile-use and -fbranch-probabilities and its related options.
Both absolute and relative paths can be used. By default, GCC
uses the current directory as path, thus the profile data file
appears in the same directory as the object file.
-fprofile-generate
-fprofile-generate=path
Enable options usually used for instrumenting application to
produce profile useful for later recompilation with profile
feedback based optimization. You must use -fprofile-generate
both when compiling and when linking your program.
The following options are enabled: -fprofile-arcs,
-fprofile-values, -fvpt.
If path is specified, GCC looks at the path to find the profile
feedback data files. See -fprofile-dir.
To optimize the program based on the collected profile
information, use -fprofile-use.
-fprofile-update=method
Alter the update method for an application instrumented for
profile feedback based optimization. The method argument should
be one of single, atomic or prefer-atomic. The first one is
useful for single-threaded applications, while the second one
prevents profile corruption by emitting thread-safe code.
Warning: When an application does not properly join all threads
(or creates an detached thread), a profile file can be still
corrupted.
Using prefer-atomic would be transformed either to atomic, when
supported by a target, or to single otherwise. The GCC driver
automatically selects prefer-atomic when -pthread is present in
the command line.
-fsanitize=address
Enable AddressSanitizer, a fast memory error detector. Memory
access instructions are instrumented to detect out-of-bounds and
use-after-free bugs. The option enables
-fsanitize-address-use-after-scope. See
<https://github.com/google/sanitizers/wiki/AddressSanitizer > for
more details. The run-time behavior can be influenced using the
ASAN_OPTIONS environment variable. When set to "help=1", the
available options are shown at startup of the instrumented
program. See
<https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags >
for a list of supported options. The option cannot be combined
with -fsanitize=thread and/or -fcheck-pointer-bounds.
-fsanitize=kernel-address
Enable AddressSanitizer for Linux kernel. See
<https://github.com/google/kasan/wiki > for more details. The
option cannot be combined with -fcheck-pointer-bounds.
-fsanitize=thread
Enable ThreadSanitizer, a fast data race detector. Memory access
instructions are instrumented to detect data race bugs. See
<https://github.com/google/sanitizers/wiki#threadsanitizer > for
more details. The run-time behavior can be influenced using the
TSAN_OPTIONS environment variable; see
<https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags >
for a list of supported options. The option cannot be combined
with -fsanitize=address, -fsanitize=leak and/or
-fcheck-pointer-bounds.
Note that sanitized atomic builtins cannot throw exceptions when
operating on invalid memory addresses with non-call exceptions
(-fnon-call-exceptions).
-fsanitize=leak
Enable LeakSanitizer, a memory leak detector. This option only
matters for linking of executables and the executable is linked
against a library that overrides "malloc" and other allocator
functions. See
<https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer >
for more details. The run-time behavior can be influenced using
the LSAN_OPTIONS environment variable. The option cannot be
combined with -fsanitize=thread.
-fsanitize=undefined
Enable UndefinedBehaviorSanitizer, a fast undefined behavior
detector. Various computations are instrumented to detect
undefined behavior at runtime. Current suboptions are:
-fsanitize=shift
This option enables checking that the result of a shift
operation is not undefined. Note that what exactly is
considered undefined differs slightly between C and C++, as
well as between ISO C90 and C99, etc. This option has two
suboptions, -fsanitize=shift-base and
-fsanitize=shift-exponent.
-fsanitize=shift-exponent
This option enables checking that the second argument of a
shift operation is not negative and is smaller than the
precision of the promoted first argument.
-fsanitize=shift-base
If the second argument of a shift operation is within range,
check that the result of a shift operation is not undefined.
Note that what exactly is considered undefined differs
slightly between C and C++, as well as between ISO C90 and
C99, etc.
-fsanitize=integer-divide-by-zero
Detect integer division by zero as well as "INT_MIN / -1"
division.
-fsanitize=unreachable
With this option, the compiler turns the
"__builtin_unreachable" call into a diagnostics message call
instead. When reaching the "__builtin_unreachable" call, the
behavior is undefined.
-fsanitize=vla-bound
This option instructs the compiler to check that the size of
a variable length array is positive.
-fsanitize=null
This option enables pointer checking. Particularly, the
application built with this option turned on will issue an
error message when it tries to dereference a NULL pointer, or
if a reference (possibly an rvalue reference) is bound to a
NULL pointer, or if a method is invoked on an object pointed
by a NULL pointer.
-fsanitize=return
This option enables return statement checking. Programs
built with this option turned on will issue an error message
when the end of a non-void function is reached without
actually returning a value. This option works in C++ only.
-fsanitize=signed-integer-overflow
This option enables signed integer overflow checking. We
check that the result of "+", "*", and both unary and binary
"-" does not overflow in the signed arithmetics. Note,
integer promotion rules must be taken into account. That is,
the following is not an overflow:
signed char a = SCHAR_MAX;
a++;
-fsanitize=bounds
This option enables instrumentation of array bounds. Various
out of bounds accesses are detected. Flexible array members,
flexible array member-like arrays, and initializers of
variables with static storage are not instrumented. The
option cannot be combined with -fcheck-pointer-bounds.
-fsanitize=bounds-strict
This option enables strict instrumentation of array bounds.
Most out of bounds accesses are detected, including flexible
array members and flexible array member-like arrays.
Initializers of variables with static storage are not
instrumented. The option cannot be combined with
-fcheck-pointer-bounds.
-fsanitize=alignment
This option enables checking of alignment of pointers when
they are dereferenced, or when a reference is bound to
insufficiently aligned target, or when a method or
constructor is invoked on insufficiently aligned object.
-fsanitize=object-size
This option enables instrumentation of memory references
using the "__builtin_object_size" function. Various out of
bounds pointer accesses are detected.
-fsanitize=float-divide-by-zero
Detect floating-point division by zero. Unlike other similar
options, -fsanitize=float-divide-by-zero is not enabled by
-fsanitize=undefined, since floating-point division by zero
can be a legitimate way of obtaining infinities and NaNs.
-fsanitize=float-cast-overflow
This option enables floating-point type to integer conversion
checking. We check that the result of the conversion does
not overflow. Unlike other similar options,
-fsanitize=float-cast-overflow is not enabled by
-fsanitize=undefined. This option does not work well with
"FE_INVALID" exceptions enabled.
-fsanitize=nonnull-attribute
This option enables instrumentation of calls, checking
whether null values are not passed to arguments marked as
requiring a non-null value by the "nonnull" function
attribute.
-fsanitize=returns-nonnull-attribute
This option enables instrumentation of return statements in
functions marked with "returns_nonnull" function attribute,
to detect returning of null values from such functions.
-fsanitize=bool
This option enables instrumentation of loads from bool. If a
value other than 0/1 is loaded, a run-time error is issued.
-fsanitize=enum
This option enables instrumentation of loads from an enum
type. If a value outside the range of values for the enum
type is loaded, a run-time error is issued.
-fsanitize=vptr
This option enables instrumentation of C++ member function
calls, member accesses and some conversions between pointers
to base and derived classes, to verify the referenced object
has the correct dynamic type.
While -ftrapv causes traps for signed overflows to be emitted,
-fsanitize=undefined gives a diagnostic message. This currently
works only for the C family of languages.
-fno-sanitize=all
This option disables all previously enabled sanitizers.
-fsanitize=all is not allowed, as some sanitizers cannot be used
together.
-fasan-shadow-offset=number
This option forces GCC to use custom shadow offset in
AddressSanitizer checks. It is useful for experimenting with
different shadow memory layouts in Kernel AddressSanitizer.
-fsanitize-sections=s1,s2,...
Sanitize global variables in selected user-defined sections. si
may contain wildcards.
-fsanitize-recover[=opts]
-fsanitize-recover= controls error recovery mode for sanitizers
mentioned in comma-separated list of opts. Enabling this option
for a sanitizer component causes it to attempt to continue
running the program as if no error happened. This means multiple
runtime errors can be reported in a single program run, and the
exit code of the program may indicate success even when errors
have been reported. The -fno-sanitize-recover= option can be
used to alter this behavior: only the first detected error is
reported and program then exits with a non-zero exit code.
Currently this feature only works for -fsanitize=undefined (and
its suboptions except for -fsanitize=unreachable and
-fsanitize=return), -fsanitize=float-cast-overflow,
-fsanitize=float-divide-by-zero, -fsanitize=bounds-strict,
-fsanitize=kernel-address and -fsanitize=address. For these
sanitizers error recovery is turned on by default, except
-fsanitize=address, for which this feature is experimental.
-fsanitize-recover=all and -fno-sanitize-recover=all is also
accepted, the former enables recovery for all sanitizers that
support it, the latter disables recovery for all sanitizers that
support it.
Even if a recovery mode is turned on the compiler side, it needs
to be also enabled on the runtime library side, otherwise the
failures are still fatal. The runtime library defaults to
"halt_on_error=0" for ThreadSanitizer and
UndefinedBehaviorSanitizer, while default value for
AddressSanitizer is "halt_on_error=1". This can be overridden
through setting the "halt_on_error" flag in the corresponding
environment variable.
Syntax without an explicit opts parameter is deprecated. It is
equivalent to specifying an opts list of:
undefined,float-cast-overflow,float-divide-by-zero,bounds-strict
-fsanitize-address-use-after-scope
Enable sanitization of local variables to detect use-after-scope
bugs. The option sets -fstack-reuse to none.
-fsanitize-undefined-trap-on-error
The -fsanitize-undefined-trap-on-error option instructs the
compiler to report undefined behavior using "__builtin_trap"
rather than a "libubsan" library routine. The advantage of this
is that the "libubsan" library is not needed and is not linked
in, so this is usable even in freestanding environments.
-fsanitize-coverage=trace-pc
Enable coverage-guided fuzzing code instrumentation. Inserts a
call to "__sanitizer_cov_trace_pc" into every basic block.
-fbounds-check
For front ends that support it, generate additional code to check
that indices used to access arrays are within the declared range.
This is currently only supported by the Fortran front end, where
this option defaults to false.
-fcheck-pointer-bounds
Enable Pointer Bounds Checker instrumentation. Each memory
reference is instrumented with checks of the pointer used for
memory access against bounds associated with that pointer.
Currently there is only an implementation for Intel MPX
available, thus x86 GNU/Linux target and -mmpx are required to
enable this feature. MPX-based instrumentation requires a
runtime library to enable MPX in hardware and handle bounds
violation signals. By default when -fcheck-pointer-bounds and
-mmpx options are used to link a program, the GCC driver links
against the libmpx and libmpxwrappers libraries. Bounds checking
on calls to dynamic libraries requires a linker with -z bndplt
support; if GCC was configured with a linker without support for
this option (including the Gold linker and older versions of ld),
a warning is given if you link with -mmpx without also specifying
-static, since the overall effectiveness of the bounds checking
protection is reduced. See also -static-libmpxwrappers.
MPX-based instrumentation may be used for debugging and also may
be included in production code to increase program security.
Depending on usage, you may have different requirements for the
runtime library. The current version of the MPX runtime library
is more oriented for use as a debugging tool. MPX runtime
library usage implies -lpthread. See also -static-libmpx. The
runtime library behavior can be influenced using various
CHKP_RT_* environment variables. See
<https://gcc.gnu.org/wiki/Intel%20MPX%20support%20in%20the%20GCC%20compiler >
for more details.
Generated instrumentation may be controlled by various -fchkp-*
options and by the "bnd_variable_size" structure field attribute
and "bnd_legacy", and "bnd_instrument" function attributes. GCC
also provides a number of built-in functions for controlling the
Pointer Bounds Checker.
-fchkp-check-incomplete-type
Generate pointer bounds checks for variables with incomplete
type. Enabled by default.
-fchkp-narrow-bounds
Controls bounds used by Pointer Bounds Checker for pointers to
object fields. If narrowing is enabled then field bounds are
used. Otherwise object bounds are used. See also
-fchkp-narrow-to-innermost-array and
-fchkp-first-field-has-own-bounds. Enabled by default.
-fchkp-first-field-has-own-bounds
Forces Pointer Bounds Checker to use narrowed bounds for the
address of the first field in the structure. By default a
pointer to the first field has the same bounds as a pointer to
the whole structure.
-fchkp-flexible-struct-trailing-arrays
Forces Pointer Bounds Checker to treat all trailing arrays in
structures as possibly flexible. By default only array fields
with zero length or that are marked with attribute
bnd_variable_size are treated as flexible.
-fchkp-narrow-to-innermost-array
Forces Pointer Bounds Checker to use bounds of the innermost
arrays in case of nested static array access. By default this
option is disabled and bounds of the outermost array are used.
-fchkp-optimize
Enables Pointer Bounds Checker optimizations. Enabled by default
at optimization levels -O, -O2, -O3.
-fchkp-use-fast-string-functions
Enables use of *_nobnd versions of string functions (not copying
bounds) by Pointer Bounds Checker. Disabled by default.
-fchkp-use-nochk-string-functions
Enables use of *_nochk versions of string functions (not checking
bounds) by Pointer Bounds Checker. Disabled by default.
-fchkp-use-static-bounds
Allow Pointer Bounds Checker to generate static bounds holding
bounds of static variables. Enabled by default.
-fchkp-use-static-const-bounds
Use statically-initialized bounds for constant bounds instead of
generating them each time they are required. By default enabled
when -fchkp-use-static-bounds is enabled.
-fchkp-treat-zero-dynamic-size-as-infinite
With this option, objects with incomplete type whose dynamically-
obtained size is zero are treated as having infinite size instead
by Pointer Bounds Checker. This option may be helpful if a
program is linked with a library missing size information for
some symbols. Disabled by default.
-fchkp-check-read
Instructs Pointer Bounds Checker to generate checks for all read
accesses to memory. Enabled by default.
-fchkp-check-write
Instructs Pointer Bounds Checker to generate checks for all write
accesses to memory. Enabled by default.
-fchkp-store-bounds
Instructs Pointer Bounds Checker to generate bounds stores for
pointer writes. Enabled by default.
-fchkp-instrument-calls
Instructs Pointer Bounds Checker to pass pointer bounds to calls.
Enabled by default.
-fchkp-instrument-marked-only
Instructs Pointer Bounds Checker to instrument only functions
marked with the "bnd_instrument" attribute. Disabled by default.
-fchkp-use-wrappers
Allows Pointer Bounds Checker to replace calls to built-in
functions with calls to wrapper functions. When
-fchkp-use-wrappers is used to link a program, the GCC driver
automatically links against libmpxwrappers. See also
-static-libmpxwrappers. Enabled by default.
-fstack-protector
Emit extra code to check for buffer overflows, such as stack
smashing attacks. This is done by adding a guard variable to
functions with vulnerable objects. This includes functions that
call "alloca", and functions with buffers larger than 8 bytes.
The guards are initialized when a function is entered and then
checked when the function exits. If a guard check fails, an
error message is printed and the program exits.
-fstack-protector-all
Like -fstack-protector except that all functions are protected.
-fstack-protector-strong
Like -fstack-protector but includes additional functions to be
protected --- those that have local array definitions, or have
references to local frame addresses.
-fstack-protector-explicit
Like -fstack-protector but only protects those functions which
have the "stack_protect" attribute.
-fstack-check
Generate code to verify that you do not go beyond the boundary of
the stack. You should specify this flag if you are running in an
environment with multiple threads, but you only rarely need to
specify it in a single-threaded environment since stack overflow
is automatically detected on nearly all systems if there is only
one stack.
Note that this switch does not actually cause checking to be
done; the operating system or the language runtime must do that.
The switch causes generation of code to ensure that they see the
stack being extended.
You can additionally specify a string parameter: no means no
checking, generic means force the use of old-style checking,
specific means use the best checking method and is equivalent to
bare -fstack-check.
Old-style checking is a generic mechanism that requires no
specific target support in the compiler but comes with the
following drawbacks:
1. Modified allocation strategy for large objects: they are
always allocated dynamically if their size exceeds a fixed
threshold.
2. Fixed limit on the size of the static frame of functions:
when it is topped by a particular function, stack checking is
not reliable and a warning is issued by the compiler.
3. Inefficiency: because of both the modified allocation
strategy and the generic implementation, code performance is
hampered.
Note that old-style stack checking is also the fallback method
for specific if no target support has been added in the compiler.
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
Generate code to ensure that the stack does not grow beyond a
certain value, either the value of a register or the address of a
symbol. If a larger stack is required, a signal is raised at run
time. For most targets, the signal is raised before the stack
overruns the boundary, so it is possible to catch the signal
without taking special precautions.
For instance, if the stack starts at absolute address 0x80000000
and grows downwards, you can use the flags
-fstack-limit-symbol=__stack_limit and
-Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of
128KB. Note that this may only work with the GNU linker.
You can locally override stack limit checking by using the
"no_stack_limit" function attribute.
-fsplit-stack
Generate code to automatically split the stack before it
overflows. The resulting program has a discontiguous stack which
can only overflow if the program is unable to allocate any more
memory. This is most useful when running threaded programs, as
it is no longer necessary to calculate a good stack size to use
for each thread. This is currently only implemented for the x86
targets running GNU/Linux.
When code compiled with -fsplit-stack calls code compiled without
-fsplit-stack, there may not be much stack space available for
the latter code to run. If compiling all code, including library
code, with -fsplit-stack is not an option, then the linker can
fix up these calls so that the code compiled without
-fsplit-stack always has a large stack. Support for this is
implemented in the gold linker in GNU binutils release 2.21 and
later.
-fvtable-verify=[std|preinit|none]
This option is only available when compiling C++ code. It turns
on (or off, if using -fvtable-verify=none) the security feature
that verifies at run time, for every virtual call, that the
vtable pointer through which the call is made is valid for the
type of the object, and has not been corrupted or overwritten.
If an invalid vtable pointer is detected at run time, an error is
reported and execution of the program is immediately halted.
This option causes run-time data structures to be built at
program startup, which are used for verifying the vtable
pointers. The options std and preinit control the timing of when
these data structures are built. In both cases the data
structures are built before execution reaches "main". Using
-fvtable-verify=std causes the data structures to be built after
shared libraries have been loaded and initialized.
-fvtable-verify=preinit causes them to be built before shared
libraries have been loaded and initialized.
If this option appears multiple times in the command line with
different values specified, none takes highest priority over both
std and preinit; preinit takes priority over std.
-fvtv-debug
When used in conjunction with -fvtable-verify=std or
-fvtable-verify=preinit, causes debug versions of the runtime
functions for the vtable verification feature to be called. This
flag also causes the compiler to log information about which
vtable pointers it finds for each class. This information is
written to a file named vtv_set_ptr_data.log in the directory
named by the environment variable VTV_LOGS_DIR if that is defined
or the current working directory otherwise.
Note: This feature appends data to the log file. If you want a
fresh log file, be sure to delete any existing one.
-fvtv-counts
This is a debugging flag. When used in conjunction with
-fvtable-verify=std or -fvtable-verify=preinit, this causes the
compiler to keep track of the total number of virtual calls it
encounters and the number of verifications it inserts. It also
counts the number of calls to certain run-time library functions
that it inserts and logs this information for each compilation
unit. The compiler writes this information to a file named
vtv_count_data.log in the directory named by the environment
variable VTV_LOGS_DIR if that is defined or the current working
directory otherwise. It also counts the size of the vtable
pointer sets for each class, and writes this information to
vtv_class_set_sizes.log in the same directory.
Note: This feature appends data to the log files. To get fresh
log files, be sure to delete any existing ones.
-finstrument-functions
Generate instrumentation calls for entry and exit to functions.
Just after function entry and just before function exit, the
following profiling functions are called with the address of the
current function and its call site. (On some platforms,
"__builtin_return_address" does not work beyond the current
function, so the call site information may not be available to
the profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current
function, which may be looked up exactly in the symbol table.
This instrumentation is also done for functions expanded inline
in other functions. The profiling calls indicate where,
conceptually, the inline function is entered and exited. This
means that addressable versions of such functions must be
available. If all your uses of a function are expanded inline,
this may mean an additional expansion of code size. If you use
"extern inline" in your C code, an addressable version of such
functions must be provided. (This is normally the case anyway,
but if you get lucky and the optimizer always expands the
functions inline, you might have gotten away without providing
static copies.)
A function may be given the attribute "no_instrument_function",
in which case this instrumentation is not done. This can be
used, for example, for the profiling functions listed above,
high-priority interrupt routines, and any functions from which
the profiling functions cannot safely be called (perhaps signal
handlers, if the profiling routines generate output or allocate
memory).
-finstrument-functions-exclude-file-list=file,file,...
Set the list of functions that are excluded from instrumentation
(see the description of -finstrument-functions). If the file
that contains a function definition matches with one of file,
then that function is not instrumented. The match is done on
substrings: if the file parameter is a substring of the file
name, it is considered to be a match.
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sys
excludes any inline function defined in files whose pathnames
contain /bits/stl or include/sys.
If, for some reason, you want to include letter , in one of sym,
write ,. For example,
-finstrument-functions-exclude-file-list=',,tmp' (note the single
quote surrounding the option).
-finstrument-functions-exclude-function-list=sym,sym,...
This is similar to -finstrument-functions-exclude-file-list, but
this option sets the list of function names to be excluded from
instrumentation. The function name to be matched is its user-
visible name, such as "vector<int> blah(const vector<int> &)",
not the internal mangled name (e.g.,
"_Z4blahRSt6vectorIiSaIiEE"). The match is done on substrings:
if the sym parameter is a substring of the function name, it is
considered to be a match. For C99 and C++ extended identifiers,
the function name must be given in UTF-8, not using universal
character names.
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C
source file before actual compilation.
If you use the -E option, nothing is done except preprocessing. Some
of these options make sense only together with -E because they cause
the preprocessor output to be unsuitable for actual compilation.
In addition to the options listed here, there are a number of options
to control search paths for include files documented in Directory
Options. Options to control preprocessor diagnostics are listed in
Warning Options.
-D name
Predefine name as a macro, with definition 1.
-D name=definition
The contents of definition are tokenized and processed as if they
appeared during translation phase three in a #define directive.
In particular, the definition is truncated by embedded newline
characters.
If you are invoking the preprocessor from a shell or shell-like
program you may need to use the shell's quoting syntax to protect
characters such as spaces that have a meaning in the shell
syntax.
If you wish to define a function-like macro on the command line,
write its argument list with surrounding parentheses before the
equals sign (if any). Parentheses are meaningful to most shells,
so you should quote the option. With sh and csh,
-D'name(args...)=definition' works.
-D and -U options are processed in the order they are given on
the command line. All -imacros file and -include file options
are processed after all -D and -U options.
-U name
Cancel any previous definition of name, either built in or
provided with a -D option.
-include file
Process file as if "#include "file"" appeared as the first line
of the primary source file. However, the first directory
searched for file is the preprocessor's working directory instead
of the directory containing the main source file. If not found
there, it is searched for in the remainder of the "#include
"..."" search chain as normal.
If multiple -include options are given, the files are included in
the order they appear on the command line.
-imacros file
Exactly like -include, except that any output produced by
scanning file is thrown away. Macros it defines remain defined.
This allows you to acquire all the macros from a header without
also processing its declarations.
All files specified by -imacros are processed before all files
specified by -include.
-undef
Do not predefine any system-specific or GCC-specific macros. The
standard predefined macros remain defined.
-pthread
Define additional macros required for using the POSIX threads
library. You should use this option consistently for both
compilation and linking. This option is supported on GNU/Linux
targets, most other Unix derivatives, and also on x86 Cygwin and
MinGW targets.
-M Instead of outputting the result of preprocessing, output a rule
suitable for make describing the dependencies of the main source
file. The preprocessor outputs one make rule containing the
object file name for that source file, a colon, and the names of
all the included files, including those coming from -include or
-imacros command-line options.
Unless specified explicitly (with -MT or -MQ), the object file
name consists of the name of the source file with any suffix
replaced with object file suffix and with any leading directory
parts removed. If there are many included files then the rule is
split into several lines using \-newline. The rule has no
commands.
This option does not suppress the preprocessor's debug output,
such as -dM. To avoid mixing such debug output with the
dependency rules you should explicitly specify the dependency
output file with -MF, or use an environment variable like
DEPENDENCIES_OUTPUT. Debug output is still sent to the regular
output stream as normal.
Passing -M to the driver implies -E, and suppresses warnings with
an implicit -w.
-MM Like -M but do not mention header files that are found in system
header directories, nor header files that are included, directly
or indirectly, from such a header.
This implies that the choice of angle brackets or double quotes
in an #include directive does not in itself determine whether
that header appears in -MM dependency output.
-MF file
When used with -M or -MM, specifies a file to write the
dependencies to. If no -MF switch is given the preprocessor
sends the rules to the same place it would send preprocessed
output.
When used with the driver options -MD or -MMD, -MF overrides the
default dependency output file.
-MG In conjunction with an option such as -M requesting dependency
generation, -MG assumes missing header files are generated files
and adds them to the dependency list without raising an error.
The dependency filename is taken directly from the "#include"
directive without prepending any path. -MG also suppresses
preprocessed output, as a missing header file renders this
useless.
This feature is used in automatic updating of makefiles.
-MP This option instructs CPP to add a phony target for each
dependency other than the main file, causing each to depend on
nothing. These dummy rules work around errors make gives if you
remove header files without updating the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
-MT target
Change the target of the rule emitted by dependency generation.
By default CPP takes the name of the main input file, deletes any
directory components and any file suffix such as .c, and appends
the platform's usual object suffix. The result is the target.
An -MT option sets the target to be exactly the string you
specify. If you want multiple targets, you can specify them as a
single argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are special to
Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given
with -MQ.
-MD -MD is equivalent to -M -MF file, except that -E is not implied.
The driver determines file based on whether an -o option is
given. If it is, the driver uses its argument but with a suffix
of .d, otherwise it takes the name of the input file, removes any
directory components and suffix, and applies a .d suffix.
If -MD is used in conjunction with -E, any -o switch is
understood to specify the dependency output file, but if used
without -E, each -o is understood to specify a target object
file.
Since -E is not implied, -MD can be used to generate a dependency
output file as a side-effect of the compilation process.
-MMD
Like -MD except mention only user header files, not system header
files.
-fpreprocessed
Indicate to the preprocessor that the input file has already been
preprocessed. This suppresses things like macro expansion,
trigraph conversion, escaped newline splicing, and processing of
most directives. The preprocessor still recognizes and removes
comments, so that you can pass a file preprocessed with -C to the
compiler without problems. In this mode the integrated
preprocessor is little more than a tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of the
extensions .i, .ii or .mi. These are the extensions that GCC
uses for preprocessed files created by -save-temps.
-fdirectives-only
When preprocessing, handle directives, but do not expand macros.
The option's behavior depends on the -E and -fpreprocessed
options.
With -E, preprocessing is limited to the handling of directives
such as "#define", "#ifdef", and "#error". Other preprocessor
operations, such as macro expansion and trigraph conversion are
not performed. In addition, the -dD option is implicitly
enabled.
With -fpreprocessed, predefinition of command line and most
builtin macros is disabled. Macros such as "__LINE__", which are
contextually dependent, are handled normally. This enables
compilation of files previously preprocessed with "-E
-fdirectives-only".
With both -E and -fpreprocessed, the rules for -fpreprocessed
take precedence. This enables full preprocessing of files
previously preprocessed with "-E -fdirectives-only".
-fdollars-in-identifiers
Accept $ in identifiers.
-fextended-identifiers
Accept universal character names in identifiers. This option is
enabled by default for C99 (and later C standard versions) and
C++.
-fno-canonical-system-headers
When preprocessing, do not shorten system header paths with
canonicalization.
-ftabstop=width
Set the distance between tab stops. This helps the preprocessor
report correct column numbers in warnings or errors, even if tabs
appear on the line. If the value is less than 1 or greater than
100, the option is ignored. The default is 8.
-ftrack-macro-expansion[=level]
Track locations of tokens across macro expansions. This allows
the compiler to emit diagnostic about the current macro expansion
stack when a compilation error occurs in a macro expansion. Using
this option makes the preprocessor and the compiler consume more
memory. The level parameter can be used to choose the level of
precision of token location tracking thus decreasing the memory
consumption if necessary. Value 0 of level de-activates this
option. Value 1 tracks tokens locations in a degraded mode for
the sake of minimal memory overhead. In this mode all tokens
resulting from the expansion of an argument of a function-like
macro have the same location. Value 2 tracks tokens locations
completely. This value is the most memory hungry. When this
option is given no argument, the default parameter value is 2.
Note that "-ftrack-macro-expansion=2" is activated by default.
-fexec-charset=charset
Set the execution character set, used for string and character
constants. The default is UTF-8. charset can be any encoding
supported by the system's "iconv" library routine.
-fwide-exec-charset=charset
Set the wide execution character set, used for wide string and
character constants. The default is UTF-32 or UTF-16, whichever
corresponds to the width of "wchar_t". As with -fexec-charset,
charset can be any encoding supported by the system's "iconv"
library routine; however, you will have problems with encodings
that do not fit exactly in "wchar_t".
-finput-charset=charset
Set the input character set, used for translation from the
character set of the input file to the source character set used
by GCC. If the locale does not specify, or GCC cannot get this
information from the locale, the default is UTF-8. This can be
overridden by either the locale or this command-line option.
Currently the command-line option takes precedence if there's a
conflict. charset can be any encoding supported by the system's
"iconv" library routine.
-fpch-deps
When using precompiled headers, this flag causes the dependency-
output flags to also list the files from the precompiled header's
dependencies. If not specified, only the precompiled header are
listed and not the files that were used to create it, because
those files are not consulted when a precompiled header is used.
-fpch-preprocess
This option allows use of a precompiled header together with -E.
It inserts a special "#pragma", "#pragma GCC pch_preprocess
"filename"" in the output to mark the place where the precompiled
header was found, and its filename. When -fpreprocessed is in
use, GCC recognizes this "#pragma" and loads the PCH.
This option is off by default, because the resulting preprocessed
output is only really suitable as input to GCC. It is switched
on by -save-temps.
You should not write this "#pragma" in your own code, but it is
safe to edit the filename if the PCH file is available in a
different location. The filename may be absolute or it may be
relative to GCC's current directory.
-fworking-directory
Enable generation of linemarkers in the preprocessor output that
let the compiler know the current working directory at the time
of preprocessing. When this option is enabled, the preprocessor
emits, after the initial linemarker, a second linemarker with the
current working directory followed by two slashes. GCC uses this
directory, when it's present in the preprocessed input, as the
directory emitted as the current working directory in some
debugging information formats. This option is implicitly enabled
if debugging information is enabled, but this can be inhibited
with the negated form -fno-working-directory. If the -P flag is
present in the command line, this option has no effect, since no
"#line" directives are emitted whatsoever.
-A predicate=answer
Make an assertion with the predicate predicate and answer answer.
This form is preferred to the older form -A predicate(answer),
which is still supported, because it does not use shell special
characters.
-A -predicate=answer
Cancel an assertion with the predicate predicate and answer
answer.
-C Do not discard comments. All comments are passed through to the
output file, except for comments in processed directives, which
are deleted along with the directive.
You should be prepared for side effects when using -C; it causes
the preprocessor to treat comments as tokens in their own right.
For example, comments appearing at the start of what would be a
directive line have the effect of turning that line into an
ordinary source line, since the first token on the line is no
longer a #.
-CC Do not discard comments, including during macro expansion. This
is like -C, except that comments contained within macros are also
passed through to the output file where the macro is expanded.
In addition to the side-effects of the -C option, the -CC option
causes all C++-style comments inside a macro to be converted to
C-style comments. This is to prevent later use of that macro
from inadvertently commenting out the remainder of the source
line.
The -CC option is generally used to support lint comments.
-P Inhibit generation of linemarkers in the output from the
preprocessor. This might be useful when running the preprocessor
on something that is not C code, and will be sent to a program
which might be confused by the linemarkers.
-traditional
-traditional-cpp
Try to imitate the behavior of pre-standard C preprocessors, as
opposed to ISO C preprocessors. See the GNU CPP manual for
details.
Note that GCC does not otherwise attempt to emulate a pre-
standard C compiler, and these options are only supported with
the -E switch, or when invoking CPP explicitly.
-trigraphs
Support ISO C trigraphs. These are three-character sequences,
all starting with ??, that are defined by ISO C to stand for
single characters. For example, ??/ stands for \, so '??/n' is a
character constant for a newline.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
By default, GCC ignores trigraphs, but in standard-conforming
modes it converts them. See the -std and -ansi options.
-remap
Enable special code to work around file systems which only permit
very short file names, such as MS-DOS.
-H Print the name of each header file used, in addition to other
normal activities. Each name is indented to show how deep in the
#include stack it is. Precompiled header files are also printed,
even if they are found to be invalid; an invalid precompiled
header file is printed with ...x and a valid one with ...! .
-dletters
Says to make debugging dumps during compilation as specified by
letters. The flags documented here are those relevant to the
preprocessor. Other letters are interpreted by the compiler
proper, or reserved for future versions of GCC, and so are
silently ignored. If you specify letters whose behavior
conflicts, the result is undefined.
-dM Instead of the normal output, generate a list of #define
directives for all the macros defined during the execution of
the preprocessor, including predefined macros. This gives
you a way of finding out what is predefined in your version
of the preprocessor. Assuming you have no file foo.h, the
command
touch foo.h; cpp -dM foo.h
shows all the predefined macros.
If you use -dM without the -E option, -dM is interpreted as a
synonym for -fdump-rtl-mach.
-dD Like -dM except in two respects: it does not include the
predefined macros, and it outputs both the #define directives
and the result of preprocessing. Both kinds of output go to
the standard output file.
-dN Like -dD, but emit only the macro names, not their
expansions.
-dI Output #include directives in addition to the result of
preprocessing.
-dU Like -dD except that only macros that are expanded, or whose
definedness is tested in preprocessor directives, are output;
the output is delayed until the use or test of the macro; and
#undef directives are also output for macros tested but
undefined at the time.
-fdebug-cpp
This option is only useful for debugging GCC. When used from CPP
or with -E, it dumps debugging information about location maps.
Every token in the output is preceded by the dump of the map its
location belongs to.
When used from GCC without -E, this option has no effect.
-Wp,option
You can use -Wp,option to bypass the compiler driver and pass
option directly through to the preprocessor. If option contains
commas, it is split into multiple options at the commas.
However, many options are modified, translated or interpreted by
the compiler driver before being passed to the preprocessor, and
-Wp forcibly bypasses this phase. The preprocessor's direct
interface is undocumented and subject to change, so whenever
possible you should avoid using -Wp and let the driver handle the
options instead.
-Xpreprocessor option
Pass option as an option to the preprocessor. You can use this
to supply system-specific preprocessor options that GCC does not
recognize.
If you want to pass an option that takes an argument, you must
use -Xpreprocessor twice, once for the option and once for the
argument.
-no-integrated-cpp
Perform preprocessing as a separate pass before compilation. By
default, GCC performs preprocessing as an integrated part of
input tokenization and parsing. If this option is provided, the
appropriate language front end (cc1, cc1plus, or cc1obj for C,
C++, and Objective-C, respectively) is instead invoked twice,
once for preprocessing only and once for actual compilation of
the preprocessed input. This option may be useful in conjunction
with the -B or -wrapper options to specify an alternate
preprocessor or perform additional processing of the program
source between normal preprocessing and compilation.
Passing Options to the Assembler
You can pass options to the assembler.
-Wa,option
Pass option as an option to the assembler. If option contains
commas, it is split into multiple options at the commas.
-Xassembler option
Pass option as an option to the assembler. You can use this to
supply system-specific assembler options that GCC does not
recognize.
If you want to pass an option that takes an argument, you must
use -Xassembler twice, once for the option and once for the
argument.
Options for Linking
These options come into play when the compiler links object files
into an executable output file. They are meaningless if the compiler
is not doing a link step.
object-file-name
A file name that does not end in a special recognized suffix is
considered to name an object file or library. (Object files are
distinguished from libraries by the linker according to the file
contents.) If linking is done, these object files are used as
input to the linker.
-c
-S
-E If any of these options is used, then the linker is not run, and
object file names should not be used as arguments.
-fuse-ld=bfd
Use the bfd linker instead of the default linker.
-fuse-ld=gold
Use the gold linker instead of the default linker.
-llibrary
-l library
Search the library named library when linking. (The second
alternative with the library as a separate argument is only for
POSIX compliance and is not recommended.)
It makes a difference where in the command you write this option;
the linker searches and processes libraries and object files in
the order they are specified. Thus, foo.o -lz bar.o searches
library z after file foo.o but before bar.o. If bar.o refers to
functions in z, those functions may not be loaded.
The linker searches a standard list of directories for the
library, which is actually a file named liblibrary.a. The linker
then uses this file as if it had been specified precisely by
name.
The directories searched include several standard system
directories plus any that you specify with -L.
Normally the files found this way are library files---archive
files whose members are object files. The linker handles an
archive file by scanning through it for members which define
symbols that have so far been referenced but not defined. But if
the file that is found is an ordinary object file, it is linked
in the usual fashion. The only difference between using an -l
option and specifying a file name is that -l surrounds library
with lib and .a and searches several directories.
-lobjc
You need this special case of the -l option in order to link an
Objective-C or Objective-C++ program.
-nostartfiles
Do not use the standard system startup files when linking. The
standard system libraries are used normally, unless -nostdlib or
-nodefaultlibs is used.
-nodefaultlibs
Do not use the standard system libraries when linking. Only the
libraries you specify are passed to the linker, and options
specifying linkage of the system libraries, such as
-static-libgcc or -shared-libgcc, are ignored. The standard
startup files are used normally, unless -nostartfiles is used.
The compiler may generate calls to "memcmp", "memset", "memcpy"
and "memmove". These entries are usually resolved by entries in
libc. These entry points should be supplied through some other
mechanism when this option is specified.
-nostdlib
Do not use the standard system startup files or libraries when
linking. No startup files and only the libraries you specify are
passed to the linker, and options specifying linkage of the
system libraries, such as -static-libgcc or -shared-libgcc, are
ignored.
The compiler may generate calls to "memcmp", "memset", "memcpy"
and "memmove". These entries are usually resolved by entries in
libc. These entry points should be supplied through some other
mechanism when this option is specified.
One of the standard libraries bypassed by -nostdlib and
-nodefaultlibs is libgcc.a, a library of internal subroutines
which GCC uses to overcome shortcomings of particular machines,
or special needs for some languages.
In most cases, you need libgcc.a even when you want to avoid
other standard libraries. In other words, when you specify
-nostdlib or -nodefaultlibs you should usually specify -lgcc as
well. This ensures that you have no unresolved references to
internal GCC library subroutines. (An example of such an
internal subroutine is "__main", used to ensure C++ constructors
are called.)
-pie
Produce a position independent executable on targets that support
it. For predictable results, you must also specify the same set
of options used for compilation (-fpie, -fPIE, or model
suboptions) when you specify this linker option.
-no-pie
Don't produce a position independent executable.
-pthread
Link with the POSIX threads library. This option is supported on
GNU/Linux targets, most other Unix derivatives, and also on x86
Cygwin and MinGW targets. On some targets this option also sets
flags for the preprocessor, so it should be used consistently for
both compilation and linking.
-rdynamic
Pass the flag -export-dynamic to the ELF linker, on targets that
support it. This instructs the linker to add all symbols, not
only used ones, to the dynamic symbol table. This option is
needed for some uses of "dlopen" or to allow obtaining backtraces
from within a program.
-s Remove all symbol table and relocation information from the
executable.
-static
On systems that support dynamic linking, this prevents linking
with the shared libraries. On other systems, this option has no
effect.
-shared
Produce a shared object which can then be linked with other
objects to form an executable. Not all systems support this
option. For predictable results, you must also specify the same
set of options used for compilation (-fpic, -fPIC, or model
suboptions) when you specify this linker option.[1]
-shared-libgcc
-static-libgcc
On systems that provide libgcc as a shared library, these options
force the use of either the shared or static version,
respectively. If no shared version of libgcc was built when the
compiler was configured, these options have no effect.
There are several situations in which an application should use
the shared libgcc instead of the static version. The most common
of these is when the application wishes to throw and catch
exceptions across different shared libraries. In that case, each
of the libraries as well as the application itself should use the
shared libgcc.
Therefore, the G++ and driver automatically adds -shared-libgcc
whenever you build a shared library or a main executable,
because C++
programs typically use exceptions, so this is the right thing to
do.
If, instead, you use the GCC driver to create shared libraries,
you may find that they are not always linked with the shared
libgcc. If GCC finds, at its configuration time, that you have a
non-GNU linker or a GNU linker that does not support option
--eh-frame-hdr, it links the shared version of libgcc into shared
libraries by default. Otherwise, it takes advantage of the
linker and optimizes away the linking with the shared version of
libgcc, linking with the static version of libgcc by default.
This allows exceptions to propagate through such shared
libraries, without incurring relocation costs at library load
time.
However, if a library or main executable is supposed to throw or
catch exceptions, you must link it using the G++ driver, as
appropriate for the languages used in the program, or using the
option -shared-libgcc, such that it is linked with the shared
libgcc.
-static-libasan
When the -fsanitize=address option is used to link a program, the
GCC driver automatically links against libasan. If libasan is
available as a shared library, and the -static option is not
used, then this links against the shared version of libasan. The
-static-libasan option directs the GCC driver to link libasan
statically, without necessarily linking other libraries
statically.
-static-libtsan
When the -fsanitize=thread option is used to link a program, the
GCC driver automatically links against libtsan. If libtsan is
available as a shared library, and the -static option is not
used, then this links against the shared version of libtsan. The
-static-libtsan option directs the GCC driver to link libtsan
statically, without necessarily linking other libraries
statically.
-static-liblsan
When the -fsanitize=leak option is used to link a program, the
GCC driver automatically links against liblsan. If liblsan is
available as a shared library, and the -static option is not
used, then this links against the shared version of liblsan. The
-static-liblsan option directs the GCC driver to link liblsan
statically, without necessarily linking other libraries
statically.
-static-libubsan
When the -fsanitize=undefined option is used to link a program,
the GCC driver automatically links against libubsan. If libubsan
is available as a shared library, and the -static option is not
used, then this links against the shared version of libubsan.
The -static-libubsan option directs the GCC driver to link
libubsan statically, without necessarily linking other libraries
statically.
-static-libmpx
When the -fcheck-pointer bounds and -mmpx options are used to
link a program, the GCC driver automatically links against
libmpx. If libmpx is available as a shared library, and the
-static option is not used, then this links against the shared
version of libmpx. The -static-libmpx option directs the GCC
driver to link libmpx statically, without necessarily linking
other libraries statically.
-static-libmpxwrappers
When the -fcheck-pointer bounds and -mmpx options are used to
link a program without also using -fno-chkp-use-wrappers, the GCC
driver automatically links against libmpxwrappers. If
libmpxwrappers is available as a shared library, and the -static
option is not used, then this links against the shared version of
libmpxwrappers. The -static-libmpxwrappers option directs the
GCC driver to link libmpxwrappers statically, without necessarily
linking other libraries statically.
-static-libstdc++
When the g++ program is used to link a C++ program, it normally
automatically links against libstdc++. If libstdc++ is available
as a shared library, and the -static option is not used, then
this links against the shared version of libstdc++. That is
normally fine. However, it is sometimes useful to freeze the
version of libstdc++ used by the program without going all the
way to a fully static link. The -static-libstdc++ option directs
the g++ driver to link libstdc++ statically, without necessarily
linking other libraries statically.
-symbolic
Bind references to global symbols when building a shared object.
Warn about any unresolved references (unless overridden by the
link editor option -Xlinker -z -Xlinker defs). Only a few
systems support this option.
-T script
Use script as the linker script. This option is supported by
most systems using the GNU linker. On some targets, such as
bare-board targets without an operating system, the -T option may
be required when linking to avoid references to undefined
symbols.
-Xlinker option
Pass option as an option to the linker. You can use this to
supply system-specific linker options that GCC does not
recognize.
If you want to pass an option that takes a separate argument, you
must use -Xlinker twice, once for the option and once for the
argument. For example, to pass -assert definitions, you must
write -Xlinker -assert -Xlinker definitions. It does not work to
write -Xlinker "-assert definitions", because this passes the
entire string as a single argument, which is not what the linker
expects.
When using the GNU linker, it is usually more convenient to pass
arguments to linker options using the option=value syntax than as
separate arguments. For example, you can specify -Xlinker
-Map=output.map rather than -Xlinker -Map -Xlinker output.map.
Other linkers may not support this syntax for command-line
options.
-Wl,option
Pass option as an option to the linker. If option contains
commas, it is split into multiple options at the commas. You can
use this syntax to pass an argument to the option. For example,
-Wl,-Map,output.map passes -Map output.map to the linker. When
using the GNU linker, you can also get the same effect with
-Wl,-Map=output.map.
-u symbol
Pretend the symbol symbol is undefined, to force linking of
library modules to define it. You can use -u multiple times with
different symbols to force loading of additional library modules.
-z keyword
-z is passed directly on to the linker along with the keyword
keyword. See the section in the documentation of your linker for
permitted values and their meanings.
Options for Directory Search
These options specify directories to search for header files, for
libraries and for parts of the compiler:
-I dir
-iquote dir
-isystem dir
-idirafter dir
Add the directory dir to the list of directories to be searched
for header files during preprocessing. If dir begins with =,
then the = is replaced by the sysroot prefix; see --sysroot and
-isysroot.
Directories specified with -iquote apply only to the quote form
of the directive, "#include "file"". Directories specified with
-I, -isystem, or -idirafter apply to lookup for both the
"#include "file"" and "#include <file>" directives.
You can specify any number or combination of these options on the
command line to search for header files in several directories.
The lookup order is as follows:
1. For the quote form of the include directive, the directory of
the current file is searched first.
2. For the quote form of the include directive, the directories
specified by -iquote options are searched in left-to-right
order, as they appear on the command line.
3. Directories specified with -I options are scanned in left-to-
right order.
4. Directories specified with -isystem options are scanned in
left-to-right order.
5. Standard system directories are scanned.
6. Directories specified with -idirafter options are scanned in
left-to-right order.
You can use -I to override a system header file, substituting
your own version, since these directories are searched before the
standard system header file directories. However, you should not
use this option to add directories that contain vendor-supplied
system header files; use -isystem for that.
The -isystem and -idirafter options also mark the directory as a
system directory, so that it gets the same special treatment that
is applied to the standard system directories.
If a standard system include directory, or a directory specified
with -isystem, is also specified with -I, the -I option is
ignored. The directory is still searched but as a system
directory at its normal position in the system include chain.
This is to ensure that GCC's procedure to fix buggy system
headers and the ordering for the "#include_next" directive are
not inadvertently changed. If you really need to change the
search order for system directories, use the -nostdinc and/or
-isystem options.
-I- Split the include path. This option has been deprecated. Please
use -iquote instead for -I directories before the -I- and remove
the -I- option.
Any directories specified with -I options before -I- are searched
only for headers requested with "#include "file""; they are not
searched for "#include <file>". If additional directories are
specified with -I options after the -I-, those directories are
searched for all #include directives.
In addition, -I- inhibits the use of the directory of the current
file directory as the first search directory for
"#include "file"". There is no way to override this effect of
-I-.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix options.
If the prefix represents a directory, you should include the
final /.
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with -iprefix, and
add the resulting directory to the include search path.
-iwithprefixbefore puts it in the same place -I would;
-iwithprefix puts it where -idirafter would.
-isysroot dir
This option is like the --sysroot option, but applies only to
header files (except for Darwin targets, where it applies to both
header files and libraries). See the --sysroot option for more
information.
-imultilib dir
Use dir as a subdirectory of the directory containing target-
specific C++ headers.
-nostdinc
Do not search the standard system directories for header files.
Only the directories explicitly specified with -I, -iquote,
-isystem, and/or -idirafter options (and the directory of the
current file, if appropriate) are searched.
-nostdinc++
Do not search for header files in the C++-specific standard
directories, but do still search the other standard directories.
(This option is used when building the C++ library.)
-iplugindir=dir
Set the directory to search for plugins that are passed by
-fplugin=name instead of -fplugin=path/name.so. This option is
not meant to be used by the user, but only passed by the driver.
-Ldir
Add directory dir to the list of directories to be searched for
-l.
-Bprefix
This option specifies where to find the executables, libraries,
include files, and data files of the compiler itself.
The compiler driver program runs one or more of the subprograms
cpp, cc1, as and ld. It tries prefix as a prefix for each
program it tries to run, both with and without machine/version/
for the corresponding target machine and compiler version.
For each subprogram to be run, the compiler driver first tries
the -B prefix, if any. If that name is not found, or if -B is
not specified, the driver tries two standard prefixes,
/usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those
results in a file name that is found, the unmodified program name
is searched for using the directories specified in your PATH
environment variable.
The compiler checks to see if the path provided by -B refers to a
directory, and if necessary it adds a directory separator
character at the end of the path.
-B prefixes that effectively specify directory names also apply
to libraries in the linker, because the compiler translates these
options into -L options for the linker. They also apply to
include files in the preprocessor, because the compiler
translates these options into -isystem options for the
preprocessor. In this case, the compiler appends include to the
prefix.
The runtime support file libgcc.a can also be searched for using
the -B prefix, if needed. If it is not found there, the two
standard prefixes above are tried, and that is all. The file is
left out of the link if it is not found by those means.
Another way to specify a prefix much like the -B prefix is to use
the environment variable GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is [dir/]stageN/,
where N is a number in the range 0 to 9, then it is replaced by
[dir/]include. This is to help with boot-strapping the compiler.
-no-canonical-prefixes
Do not expand any symbolic links, resolve references to /../ or
/./, or make the path absolute when generating a relative prefix.
--sysroot=dir
Use dir as the logical root directory for headers and libraries.
For example, if the compiler normally searches for headers in
/usr/include and libraries in /usr/lib, it instead searches
dir/usr/include and dir/usr/lib.
If you use both this option and the -isysroot option, then the
--sysroot option applies to libraries, but the -isysroot option
applies to header files.
The GNU linker (beginning with version 2.16) has the necessary
support for this option. If your linker does not support this
option, the header file aspect of --sysroot still works, but the
library aspect does not.
--no-sysroot-suffix
For some targets, a suffix is added to the root directory
specified with --sysroot, depending on the other options used, so
that headers may for example be found in dir/suffix/usr/include
instead of dir/usr/include. This option disables the addition of
such a suffix.
Options for Code Generation Conventions
These machine-independent options control the interface conventions
used in code generation.
Most of them have both positive and negative forms; the negative form
of -ffoo is -fno-foo. In the table below, only one of the forms is
listed---the one that is not the default. You can figure out the
other form by either removing no- or adding it.
-fstack-reuse=reuse-level
This option controls stack space reuse for user declared
local/auto variables and compiler generated temporaries.
reuse_level can be all, named_vars, or none. all enables stack
reuse for all local variables and temporaries, named_vars enables
the reuse only for user defined local variables with names, and
none disables stack reuse completely. The default value is all.
The option is needed when the program extends the lifetime of a
scoped local variable or a compiler generated temporary beyond
the end point defined by the language. When a lifetime of a
variable ends, and if the variable lives in memory, the
optimizing compiler has the freedom to reuse its stack space with
other temporaries or scoped local variables whose live range does
not overlap with it. Legacy code extending local lifetime is
likely to break with the stack reuse optimization.
For example,
int *p;
{
int local1;
p = &local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}
Another example:
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
A *ap;
void foo(const A& ar)
{
ap = &ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}
The lifetime of a compiler generated temporary is well defined by
the C++ standard. When a lifetime of a temporary ends, and if the
temporary lives in memory, the optimizing compiler has the
freedom to reuse its stack space with other temporaries or scoped
local variables whose live range does not overlap with it.
However some of the legacy code relies on the behavior of older
compilers in which temporaries' stack space is not reused, the
aggressive stack reuse can lead to runtime errors. This option is
used to control the temporary stack reuse optimization.
-ftrapv
This option generates traps for signed overflow on addition,
subtraction, multiplication operations. The options -ftrapv and
-fwrapv override each other, so using -ftrapv -fwrapv on the
command-line results in -fwrapv being effective. Note that only
active options override, so using -ftrapv -fwrapv -fno-wrapv on
the command-line results in -ftrapv being effective.
-fwrapv
This option instructs the compiler to assume that signed
arithmetic overflow of addition, subtraction and multiplication
wraps around using twos-complement representation. This flag
enables some optimizations and disables others. The options
-ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv
on the command-line results in -fwrapv being effective. Note
that only active options override, so using -ftrapv -fwrapv
-fno-wrapv on the command-line results in -ftrapv being
effective.
-fexceptions
Enable exception handling. Generates extra code needed to
propagate exceptions. For some targets, this implies GCC
generates frame unwind information for all functions, which can
produce significant data size overhead, although it does not
affect execution. If you do not specify this option, GCC enables
it by default for languages like C++ that normally require
exception handling, and disables it for languages like C that do
not normally require it. However, you may need to enable this
option when compiling C code that needs to interoperate properly
with exception handlers written in C++. You may also wish to
disable this option if you are compiling older C++ programs that
don't use exception handling.
-fnon-call-exceptions
Generate code that allows trapping instructions to throw
exceptions. Note that this requires platform-specific runtime
support that does not exist everywhere. Moreover, it only allows
trapping instructions to throw exceptions, i.e. memory references
or floating-point instructions. It does not allow exceptions to
be thrown from arbitrary signal handlers such as "SIGALRM".
-fdelete-dead-exceptions
Consider that instructions that may throw exceptions but don't
otherwise contribute to the execution of the program can be
optimized away. This option is enabled by default for the Ada
front end, as permitted by the Ada language specification.
Optimization passes that cause dead exceptions to be removed are
enabled independently at different optimization levels.
-funwind-tables
Similar to -fexceptions, except that it just generates any needed
static data, but does not affect the generated code in any other
way. You normally do not need to enable this option; instead, a
language processor that needs this handling enables it on your
behalf.
-fasynchronous-unwind-tables
Generate unwind table in DWARF format, if supported by target
machine. The table is exact at each instruction boundary, so it
can be used for stack unwinding from asynchronous events (such as
debugger or garbage collector).
-fno-gnu-unique
On systems with recent GNU assembler and C library, the C++
compiler uses the "STB_GNU_UNIQUE" binding to make sure that
definitions of template static data members and static local
variables in inline functions are unique even in the presence of
"RTLD_LOCAL"; this is necessary to avoid problems with a library
used by two different "RTLD_LOCAL" plugins depending on a
definition in one of them and therefore disagreeing with the
other one about the binding of the symbol. But this causes
"dlclose" to be ignored for affected DSOs; if your program relies
on reinitialization of a DSO via "dlclose" and "dlopen", you can
use -fno-gnu-unique.
-fpcc-struct-return
Return "short" "struct" and "union" values in memory like longer
ones, rather than in registers. This convention is less
efficient, but it has the advantage of allowing intercallability
between GCC-compiled files and files compiled with other
compilers, particularly the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends
on the target configuration macros.
Short structures and unions are those whose size and alignment
match that of some integer type.
Warning: code compiled with the -fpcc-struct-return switch is not
binary compatible with code compiled with the -freg-struct-return
switch. Use it to conform to a non-default application binary
interface.
-freg-struct-return
Return "struct" and "union" values in registers when possible.
This is more efficient for small structures than
-fpcc-struct-return.
If you specify neither -fpcc-struct-return nor
-freg-struct-return, GCC defaults to whichever convention is
standard for the target. If there is no standard convention, GCC
defaults to -fpcc-struct-return, except on targets where GCC is
the principal compiler. In those cases, we can choose the
standard, and we chose the more efficient register return
alternative.
Warning: code compiled with the -freg-struct-return switch is not
binary compatible with code compiled with the -fpcc-struct-return
switch. Use it to conform to a non-default application binary
interface.
-fshort-enums
Allocate to an "enum" type only as many bytes as it needs for the
declared range of possible values. Specifically, the "enum" type
is equivalent to the smallest integer type that has enough room.
Warning: the -fshort-enums switch causes GCC to generate code
that is not binary compatible with code generated without that
switch. Use it to conform to a non-default application binary
interface.
-fshort-wchar
Override the underlying type for "wchar_t" to be "short unsigned
int" instead of the default for the target. This option is
useful for building programs to run under WINE.
Warning: the -fshort-wchar switch causes GCC to generate code
that is not binary compatible with code generated without that
switch. Use it to conform to a non-default application binary
interface.
-fno-common
In C code, this option controls the placement of global variables
defined without an initializer, known as tentative definitions in
the C standard. Tentative definitions are distinct from
declarations of a variable with the "extern" keyword, which do
not allocate storage.
Unix C compilers have traditionally allocated storage for
uninitialized global variables in a common block. This allows
the linker to resolve all tentative definitions of the same
variable in different compilation units to the same object, or to
a non-tentative definition. This is the behavior specified by
-fcommon, and is the default for GCC on most targets. On the
other hand, this behavior is not required by ISO C, and on some
targets may carry a speed or code size penalty on variable
references.
The -fno-common option specifies that the compiler should instead
place uninitialized global variables in the data section of the
object file. This inhibits the merging of tentative definitions
by the linker so you get a multiple-definition error if the same
variable is defined in more than one compilation unit. Compiling
with -fno-common is useful on targets for which it provides
better performance, or if you wish to verify that the program
will work on other systems that always treat uninitialized
variable definitions this way.
-fno-ident
Ignore the "#ident" directive.
-finhibit-size-directive
Don't output a ".size" assembler directive, or anything else that
would cause trouble if the function is split in the middle, and
the two halves are placed at locations far apart in memory. This
option is used when compiling crtstuff.c; you should not need to
use it for anything else.
-fverbose-asm
Put extra commentary information in the generated assembly code
to make it more readable. This option is generally only of use
to those who actually need to read the generated assembly code
(perhaps while debugging the compiler itself).
-fno-verbose-asm, the default, causes the extra information to be
omitted and is useful when comparing two assembler files.
The added comments include:
* information on the compiler version and command-line options,
* the source code lines associated with the assembly
instructions, in the form FILENAME:LINENUMBER:CONTENT OF
LINE,
* hints on which high-level expressions correspond to the
various assembly instruction operands.
For example, given this C source file:
int test (int n)
{
int i;
int total = 0;
for (i = 0; i < n; i++)
total += i * i;
return total;
}
compiling to (x86_64) assembly via -S and emitting the result
direct to stdout via -o -
gcc -S test.c -fverbose-asm -Os -o -
gives output similar to this:
.file "test.c"
# GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
[...snip...]
# options passed:
[...snip...]
.text
.globl test
.type test, @function
test:
.LFB0:
.cfi_startproc
# test.c:4: int total = 0;
xorl %eax, %eax # <retval>
# test.c:6: for (i = 0; i < n; i++)
xorl %edx, %edx # i
.L2:
# test.c:6: for (i = 0; i < n; i++)
cmpl %edi, %edx # n, i
jge .L5 #,
# test.c:7: total += i * i;
movl %edx, %ecx # i, tmp92
imull %edx, %ecx # i, tmp92
# test.c:6: for (i = 0; i < n; i++)
incl %edx # i
# test.c:7: total += i * i;
addl %ecx, %eax # tmp92, <retval>
jmp .L2 #
.L5:
# test.c:10: }
ret
.cfi_endproc
.LFE0:
.size test, .-test
.ident "GCC: (GNU) 7.0.0 20160809 (experimental)"
.section .note.GNU-stack,"",@progbits
The comments are intended for humans rather than machines and
hence the precise format of the comments is subject to change.
-frecord-gcc-switches
This switch causes the command line used to invoke the compiler
to be recorded into the object file that is being created. This
switch is only implemented on some targets and the exact format
of the recording is target and binary file format dependent, but
it usually takes the form of a section containing ASCII text.
This switch is related to the -fverbose-asm switch, but that
switch only records information in the assembler output file as
comments, so it never reaches the object file. See also
-grecord-gcc-switches for another way of storing compiler options
into the object file.
-fpic
Generate position-independent code (PIC) suitable for use in a
shared library, if supported for the target machine. Such code
accesses all constant addresses through a global offset table
(GOT). The dynamic loader resolves the GOT entries when the
program starts (the dynamic loader is not part of GCC; it is part
of the operating system). If the GOT size for the linked
executable exceeds a machine-specific maximum size, you get an
error message from the linker indicating that -fpic does not
work; in that case, recompile with -fPIC instead. (These
maximums are 8k on the SPARC, 28k on AArch64 and 32k on the m68k
and RS/6000. The x86 has no such limit.)
Position-independent code requires special support, and therefore
works only on certain machines. For the x86, GCC supports PIC
for System V but not for the Sun 386i. Code generated for the
IBM RS/6000 is always position-independent.
When this flag is set, the macros "__pic__" and "__PIC__" are
defined to 1.
-fPIC
If supported for the target machine, emit position-independent
code, suitable for dynamic linking and avoiding any limit on the
size of the global offset table. This option makes a difference
on AArch64, m68k, PowerPC and SPARC.
Position-independent code requires special support, and therefore
works only on certain machines.
When this flag is set, the macros "__pic__" and "__PIC__" are
defined to 2.
-fpie
-fPIE
These options are similar to -fpic and -fPIC, but generated
position independent code can be only linked into executables.
Usually these options are used when -pie GCC option is used
during linking.
-fpie and -fPIE both define the macros "__pie__" and "__PIE__".
The macros have the value 1 for -fpie and 2 for -fPIE.
-fno-plt
Do not use the PLT for external function calls in position-
independent code. Instead, load the callee address at call sites
from the GOT and branch to it. This leads to more efficient code
by eliminating PLT stubs and exposing GOT loads to optimizations.
On architectures such as 32-bit x86 where PLT stubs expect the
GOT pointer in a specific register, this gives more register
allocation freedom to the compiler. Lazy binding requires use of
the PLT; with -fno-plt all external symbols are resolved at load
time.
Alternatively, the function attribute "noplt" can be used to
avoid calls through the PLT for specific external functions.
In position-dependent code, a few targets also convert calls to
functions that are marked to not use the PLT to use the GOT
instead.
-fno-jump-tables
Do not use jump tables for switch statements even where it would
be more efficient than other code generation strategies. This
option is of use in conjunction with -fpic or -fPIC for building
code that forms part of a dynamic linker and cannot reference the
address of a jump table. On some targets, jump tables do not
require a GOT and this option is not needed.
-ffixed-reg
Treat the register named reg as a fixed register; generated code
should never refer to it (except perhaps as a stack pointer,
frame pointer or in some other fixed role).
reg must be the name of a register. The register names accepted
are machine-specific and are defined in the "REGISTER_NAMES"
macro in the machine description macro file.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-used-reg
Treat the register named reg as an allocable register that is
clobbered by function calls. It may be allocated for temporaries
or variables that do not live across a call. Functions compiled
this way do not save and restore the register reg.
It is an error to use this flag with the frame pointer or stack
pointer. Use of this flag for other registers that have fixed
pervasive roles in the machine's execution model produces
disastrous results.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-saved-reg
Treat the register named reg as an allocable register saved by
functions. It may be allocated even for temporaries or variables
that live across a call. Functions compiled this way save and
restore the register reg if they use it.
It is an error to use this flag with the frame pointer or stack
pointer. Use of this flag for other registers that have fixed
pervasive roles in the machine's execution model produces
disastrous results.
A different sort of disaster results from the use of this flag
for a register in which function values may be returned.
This flag does not have a negative form, because it specifies a
three-way choice.
-fpack-struct[=n]
Without a value specified, pack all structure members together
without holes. When a value is specified (which must be a small
power of two), pack structure members according to this value,
representing the maximum alignment (that is, objects with default
alignment requirements larger than this are output potentially
unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code
that is not binary compatible with code generated without that
switch. Additionally, it makes the code suboptimal. Use it to
conform to a non-default application binary interface.
-fleading-underscore
This option and its counterpart, -fno-leading-underscore,
forcibly change the way C symbols are represented in the object
file. One use is to help link with legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to generate
code that is not binary compatible with code generated without
that switch. Use it to conform to a non-default application
binary interface. Not all targets provide complete support for
this switch.
-ftls-model=model
Alter the thread-local storage model to be used. The model
argument should be one of global-dynamic, local-dynamic, initial-
exec or local-exec. Note that the choice is subject to
optimization: the compiler may use a more efficient model for
symbols not visible outside of the translation unit, or if -fpic
is not given on the command line.
The default without -fpic is initial-exec; with -fpic the default
is global-dynamic.
-ftrampolines
For targets that normally need trampolines for nested functions,
always generate them instead of using descriptors. Otherwise,
for targets that do not need them, like for example HP-PA or
IA-64, do nothing.
A trampoline is a small piece of code that is created at run time
on the stack when the address of a nested function is taken, and
is used to call the nested function indirectly. Therefore, it
requires the stack to be made executable in order for the program
to work properly.
-fno-trampolines is enabled by default on a language by language
basis to let the compiler avoid generating them, if it computes
that this is safe, and replace them with descriptors.
Descriptors are made up of data only, but the generated code must
be prepared to deal with them. As of this writing,
-fno-trampolines is enabled by default only for Ada.
Moreover, code compiled with -ftrampolines and code compiled with
-fno-trampolines are not binary compatible if nested functions
are present. This option must therefore be used on a program-
wide basis and be manipulated with extreme care.
-fvisibility=[default|internal|hidden|protected]
Set the default ELF image symbol visibility to the specified
option---all symbols are marked with this unless overridden
within the code. Using this feature can very substantially
improve linking and load times of shared object libraries,
produce more optimized code, provide near-perfect API export and
prevent symbol clashes. It is strongly recommended that you use
this in any shared objects you distribute.
Despite the nomenclature, default always means public; i.e.,
available to be linked against from outside the shared object.
protected and internal are pretty useless in real-world usage so
the only other commonly used option is hidden. The default if
-fvisibility isn't specified is default, i.e., make every symbol
public.
A good explanation of the benefits offered by ensuring ELF
symbols have the correct visibility is given by "How To Write
Shared Libraries" by Ulrich Drepper (which can be found at
<https://www.akkadia.org/drepper/ >)---however a superior solution
made possible by this option to marking things hidden when the
default is public is to make the default hidden and mark things
public. This is the norm with DLLs on Windows and with
-fvisibility=hidden and "__attribute__ ((visibility("default")))"
instead of "__declspec(dllexport)" you get almost identical
semantics with identical syntax. This is a great boon to those
working with cross-platform projects.
For those adding visibility support to existing code, you may
find "#pragma GCC visibility" of use. This works by you
enclosing the declarations you wish to set visibility for with
(for example) "#pragma GCC visibility push(hidden)" and "#pragma
GCC visibility pop". Bear in mind that symbol visibility should
be viewed as part of the API interface contract and thus all new
code should always specify visibility when it is not the default;
i.e., declarations only for use within the local DSO should
always be marked explicitly as hidden as so to avoid PLT
indirection overheads---making this abundantly clear also aids
readability and self-documentation of the code. Note that due to
ISO C++ specification requirements, "operator new" and "operator
delete" must always be of default visibility.
Be aware that headers from outside your project, in particular
system headers and headers from any other library you use, may
not be expecting to be compiled with visibility other than the
default. You may need to explicitly say "#pragma GCC visibility
push(default)" before including any such headers.
"extern" declarations are not affected by -fvisibility, so a lot
of code can be recompiled with -fvisibility=hidden with no
modifications. However, this means that calls to "extern"
functions with no explicit visibility use the PLT, so it is more
effective to use "__attribute ((visibility))" and/or "#pragma GCC
visibility" to tell the compiler which "extern" declarations
should be treated as hidden.
Note that -fvisibility does affect C++ vague linkage entities.
This means that, for instance, an exception class that is be
thrown between DSOs must be explicitly marked with default
visibility so that the type_info nodes are unified between the
DSOs.
An overview of these techniques, their benefits and how to use
them is at <http://gcc.gnu.org/wiki/Visibility >.
-fstrict-volatile-bitfields
This option should be used if accesses to volatile bit-fields (or
other structure fields, although the compiler usually honors
those types anyway) should use a single access of the width of
the field's type, aligned to a natural alignment if possible.
For example, targets with memory-mapped peripheral registers
might require all such accesses to be 16 bits wide; with this
flag you can declare all peripheral bit-fields as "unsigned
short" (assuming short is 16 bits on these targets) to force GCC
to use 16-bit accesses instead of, perhaps, a more efficient
32-bit access.
If this option is disabled, the compiler uses the most efficient
instruction. In the previous example, that might be a 32-bit
load instruction, even though that accesses bytes that do not
contain any portion of the bit-field, or memory-mapped registers
unrelated to the one being updated.
In some cases, such as when the "packed" attribute is applied to
a structure field, it may not be possible to access the field
with a single read or write that is correctly aligned for the
target machine. In this case GCC falls back to generating
multiple accesses rather than code that will fault or truncate
the result at run time.
Note: Due to restrictions of the C/C++11 memory model, write
accesses are not allowed to touch non bit-field members. It is
therefore recommended to define all bits of the field's type as
bit-field members.
The default value of this option is determined by the application
binary interface for the target processor.
-fsync-libcalls
This option controls whether any out-of-line instance of the
"__sync" family of functions may be used to implement the C++11
"__atomic" family of functions.
The default value of this option is enabled, thus the only useful
form of the option is -fno-sync-libcalls. This option is used in
the implementation of the libatomic runtime library.
GCC Developer Options
This section describes command-line options that are primarily of
interest to GCC developers, including options to support compiler
testing and investigation of compiler bugs and compile-time
performance problems. This includes options that produce debug dumps
at various points in the compilation; that print statistics such as
memory use and execution time; and that print information about GCC's
configuration, such as where it searches for libraries. You should
rarely need to use any of these options for ordinary compilation and
linking tasks.
-dletters
-fdump-rtl-pass
-fdump-rtl-pass=filename
Says to make debugging dumps during compilation at times
specified by letters. This is used for debugging the RTL-based
passes of the compiler. The file names for most of the dumps are
made by appending a pass number and a word to the dumpname, and
the files are created in the directory of the output file. In
case of =filename option, the dump is output on the given file
instead of the pass numbered dump files. Note that the pass
number is assigned as passes are registered into the pass
manager. Most passes are registered in the order that they will
execute and for these passes the number corresponds to the pass
execution order. However, passes registered by plugins, passes
specific to compilation targets, or passes that are otherwise
registered after all the other passes are numbered higher than a
pass named "final", even if they are executed earlier. dumpname
is generated from the name of the output file if explicitly
specified and not an executable, otherwise it is the basename of
the source file.
Some -dletters switches have different meaning when -E is used
for preprocessing.
Debug dumps can be enabled with a -fdump-rtl switch or some -d
option letters. Here are the possible letters for use in pass
and letters, and their meanings:
-fdump-rtl-alignments
Dump after branch alignments have been computed.
-fdump-rtl-asmcons
Dump after fixing rtl statements that have unsatisfied in/out
constraints.
-fdump-rtl-auto_inc_dec
Dump after auto-inc-dec discovery. This pass is only run on
architectures that have auto inc or auto dec instructions.
-fdump-rtl-barriers
Dump after cleaning up the barrier instructions.
-fdump-rtl-bbpart
Dump after partitioning hot and cold basic blocks.
-fdump-rtl-bbro
Dump after block reordering.
-fdump-rtl-btl1
-fdump-rtl-btl2
-fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the
two branch target load optimization passes.
-fdump-rtl-bypass
Dump after jump bypassing and control flow optimizations.
-fdump-rtl-combine
Dump after the RTL instruction combination pass.
-fdump-rtl-compgotos
Dump after duplicating the computed gotos.
-fdump-rtl-ce1
-fdump-rtl-ce2
-fdump-rtl-ce3
-fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable
dumping after the three if conversion passes.
-fdump-rtl-cprop_hardreg
Dump after hard register copy propagation.
-fdump-rtl-csa
Dump after combining stack adjustments.
-fdump-rtl-cse1
-fdump-rtl-cse2
-fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the
two common subexpression elimination passes.
-fdump-rtl-dce
Dump after the standalone dead code elimination passes.
-fdump-rtl-dbr
Dump after delayed branch scheduling.
-fdump-rtl-dce1
-fdump-rtl-dce2
-fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the
two dead store elimination passes.
-fdump-rtl-eh
Dump after finalization of EH handling code.
-fdump-rtl-eh_ranges
Dump after conversion of EH handling range regions.
-fdump-rtl-expand
Dump after RTL generation.
-fdump-rtl-fwprop1
-fdump-rtl-fwprop2
-fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping
after the two forward propagation passes.
-fdump-rtl-gcse1
-fdump-rtl-gcse2
-fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after
global common subexpression elimination.
-fdump-rtl-init-regs
Dump after the initialization of the registers.
-fdump-rtl-initvals
Dump after the computation of the initial value sets.
-fdump-rtl-into_cfglayout
Dump after converting to cfglayout mode.
-fdump-rtl-ira
Dump after iterated register allocation.
-fdump-rtl-jump
Dump after the second jump optimization.
-fdump-rtl-loop2
-fdump-rtl-loop2 enables dumping after the rtl loop
optimization passes.
-fdump-rtl-mach
Dump after performing the machine dependent reorganization
pass, if that pass exists.
-fdump-rtl-mode_sw
Dump after removing redundant mode switches.
-fdump-rtl-rnreg
Dump after register renumbering.
-fdump-rtl-outof_cfglayout
Dump after converting from cfglayout mode.
-fdump-rtl-peephole2
Dump after the peephole pass.
-fdump-rtl-postreload
Dump after post-reload optimizations.
-fdump-rtl-pro_and_epilogue
Dump after generating the function prologues and epilogues.
-fdump-rtl-sched1
-fdump-rtl-sched2
-fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after
the basic block scheduling passes.
-fdump-rtl-ree
Dump after sign/zero extension elimination.
-fdump-rtl-seqabstr
Dump after common sequence discovery.
-fdump-rtl-shorten
Dump after shortening branches.
-fdump-rtl-sibling
Dump after sibling call optimizations.
-fdump-rtl-split1
-fdump-rtl-split2
-fdump-rtl-split3
-fdump-rtl-split4
-fdump-rtl-split5
These options enable dumping after five rounds of instruction
splitting.
-fdump-rtl-sms
Dump after modulo scheduling. This pass is only run on some
architectures.
-fdump-rtl-stack
Dump after conversion from GCC's "flat register file"
registers to the x87's stack-like registers. This pass is
only run on x86 variants.
-fdump-rtl-subreg1
-fdump-rtl-subreg2
-fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping
after the two subreg expansion passes.
-fdump-rtl-unshare
Dump after all rtl has been unshared.
-fdump-rtl-vartrack
Dump after variable tracking.
-fdump-rtl-vregs
Dump after converting virtual registers to hard registers.
-fdump-rtl-web
Dump after live range splitting.
-fdump-rtl-regclass
-fdump-rtl-subregs_of_mode_init
-fdump-rtl-subregs_of_mode_finish
-fdump-rtl-dfinit
-fdump-rtl-dfinish
These dumps are defined but always produce empty files.
-da
-fdump-rtl-all
Produce all the dumps listed above.
-dA Annotate the assembler output with miscellaneous debugging
information.
-dD Dump all macro definitions, at the end of preprocessing, in
addition to normal output.
-dH Produce a core dump whenever an error occurs.
-dp Annotate the assembler output with a comment indicating which
pattern and alternative is used. The length of each
instruction is also printed.
-dP Dump the RTL in the assembler output as a comment before each
instruction. Also turns on -dp annotation.
-dx Just generate RTL for a function instead of compiling it.
Usually used with -fdump-rtl-expand.
-fdump-noaddr
When doing debugging dumps, suppress address output. This makes
it more feasible to use diff on debugging dumps for compiler
invocations with different compiler binaries and/or different
text / bss / data / heap / stack / dso start locations.
-freport-bug
Collect and dump debug information into a temporary file if an
internal compiler error (ICE) occurs.
-fdump-unnumbered
When doing debugging dumps, suppress instruction numbers and
address output. This makes it more feasible to use diff on
debugging dumps for compiler invocations with different options,
in particular with and without -g.
-fdump-unnumbered-links
When doing debugging dumps (see -d option above), suppress
instruction numbers for the links to the previous and next
instructions in a sequence.
-fdump-translation-unit (C++ only)
-fdump-translation-unit-options (C++ only)
Dump a representation of the tree structure for the entire
translation unit to a file. The file name is made by appending
.tu to the source file name, and the file is created in the same
directory as the output file. If the -options form is used,
options controls the details of the dump as described for the
-fdump-tree options.
-fdump-class-hierarchy (C++ only)
-fdump-class-hierarchy-options (C++ only)
Dump a representation of each class's hierarchy and virtual
function table layout to a file. The file name is made by
appending .class to the source file name, and the file is created
in the same directory as the output file. If the -options form
is used, options controls the details of the dump as described
for the -fdump-tree options.
-fdump-ipa-switch
Control the dumping at various stages of inter-procedural
analysis language tree to a file. The file name is generated by
appending a switch specific suffix to the source file name, and
the file is created in the same directory as the output file.
The following dumps are possible:
all Enables all inter-procedural analysis dumps.
cgraph
Dumps information about call-graph optimization, unused
function removal, and inlining decisions.
inline
Dump after function inlining.
-fdump-passes
Print on stderr the list of optimization passes that are turned
on and off by the current command-line options.
-fdump-statistics-option
Enable and control dumping of pass statistics in a separate file.
The file name is generated by appending a suffix ending in
.statistics to the source file name, and the file is created in
the same directory as the output file. If the -option form is
used, -stats causes counters to be summed over the whole
compilation unit while -details dumps every event as the passes
generate them. The default with no option is to sum counters for
each function compiled.
-fdump-tree-all
-fdump-tree-switch
-fdump-tree-switch-options
-fdump-tree-switch-options=filename
Control the dumping at various stages of processing the
intermediate language tree to a file. The file name is generated
by appending a switch-specific suffix to the source file name,
and the file is created in the same directory as the output file.
In case of =filename option, the dump is output on the given file
instead of the auto named dump files. If the -options form is
used, options is a list of - separated options which control the
details of the dump. Not all options are applicable to all
dumps; those that are not meaningful are ignored. The following
options are available
address
Print the address of each node. Usually this is not
meaningful as it changes according to the environment and
source file. Its primary use is for tying up a dump file
with a debug environment.
asmname
If "DECL_ASSEMBLER_NAME" has been set for a given decl, use
that in the dump instead of "DECL_NAME". Its primary use is
ease of use working backward from mangled names in the
assembly file.
slim
When dumping front-end intermediate representations, inhibit
dumping of members of a scope or body of a function merely
because that scope has been reached. Only dump such items
when they are directly reachable by some other path.
When dumping pretty-printed trees, this option inhibits
dumping the bodies of control structures.
When dumping RTL, print the RTL in slim (condensed) form
instead of the default LISP-like representation.
raw Print a raw representation of the tree. By default, trees
are pretty-printed into a C-like representation.
details
Enable more detailed dumps (not honored by every dump
option). Also include information from the optimization
passes.
stats
Enable dumping various statistics about the pass (not honored
by every dump option).
blocks
Enable showing basic block boundaries (disabled in raw
dumps).
graph
For each of the other indicated dump files (-fdump-rtl-pass),
dump a representation of the control flow graph suitable for
viewing with GraphViz to file.passid.pass.dot. Each function
in the file is pretty-printed as a subgraph, so that GraphViz
can render them all in a single plot.
This option currently only works for RTL dumps, and the RTL
is always dumped in slim form.
vops
Enable showing virtual operands for every statement.
lineno
Enable showing line numbers for statements.
uid Enable showing the unique ID ("DECL_UID") for each variable.
verbose
Enable showing the tree dump for each statement.
eh Enable showing the EH region number holding each statement.
scev
Enable showing scalar evolution analysis details.
optimized
Enable showing optimization information (only available in
certain passes).
missed
Enable showing missed optimization information (only
available in certain passes).
note
Enable other detailed optimization information (only
available in certain passes).
=filename
Instead of an auto named dump file, output into the given
file name. The file names stdout and stderr are treated
specially and are considered already open standard streams.
For example,
gcc -O2 -ftree-vectorize -fdump-tree-vect-blocks=foo.dump
-fdump-tree-pre=/dev/stderr file.c
outputs vectorizer dump into foo.dump, while the PRE dump is
output on to stderr. If two conflicting dump filenames are
given for the same pass, then the latter option overrides the
earlier one.
all Turn on all options, except raw, slim, verbose and lineno.
optall
Turn on all optimization options, i.e., optimized, missed,
and note.
To determine what tree dumps are available or find the dump for a
pass of interest follow the steps below.
1. Invoke GCC with -fdump-passes and in the stderr output look
for a code that corresponds to the pass you are interested
in. For example, the codes "tree-evrp", "tree-vrp1", and
"tree-vrp2" correspond to the three Value Range Propagation
passes. The number at the end distinguishes distinct
invocations of the same pass.
2. To enable the creation of the dump file, append the pass code
to the -fdump- option prefix and invoke GCC with it. For
example, to enable the dump from the Early Value Range
Propagation pass, invoke GCC with the -fdump-tree-evrp
option. Optionally, you may specify the name of the dump
file. If you don't specify one, GCC creates as described
below.
3. Find the pass dump in a file whose name is composed of three
components separated by a period: the name of the source file
GCC was invoked to compile, a numeric suffix indicating the
pass number followed by the letter t for tree passes (and the
letter r for RTL passes), and finally the pass code. For
example, the Early VRP pass dump might be in a file named
myfile.c.038t.evrp in the current working directory. Note
that the numeric codes are not stable and may change from one
version of GCC to another.
-fopt-info
-fopt-info-options
-fopt-info-options=filename
Controls optimization dumps from various optimization passes. If
the -options form is used, options is a list of - separated
option keywords to select the dump details and optimizations.
The options can be divided into two groups: options describing
the verbosity of the dump, and options describing which
optimizations should be included. The options from both the
groups can be freely mixed as they are non-overlapping. However,
in case of any conflicts, the later options override the earlier
options on the command line.
The following options control the dump verbosity:
optimized
Print information when an optimization is successfully
applied. It is up to a pass to decide which information is
relevant. For example, the vectorizer passes print the source
location of loops which are successfully vectorized.
missed
Print information about missed optimizations. Individual
passes control which information to include in the output.
note
Print verbose information about optimizations, such as
certain transformations, more detailed messages about
decisions etc.
all Print detailed optimization information. This includes
optimized, missed, and note.
One or more of the following option keywords can be used to
describe a group of optimizations:
ipa Enable dumps from all interprocedural optimizations.
loop
Enable dumps from all loop optimizations.
inline
Enable dumps from all inlining optimizations.
omp Enable dumps from all OMP (Offloading and Multi Processing)
optimizations.
vec Enable dumps from all vectorization optimizations.
optall
Enable dumps from all optimizations. This is a superset of
the optimization groups listed above.
If options is omitted, it defaults to optimized-optall, which
means to dump all info about successful optimizations from all
the passes.
If the filename is provided, then the dumps from all the
applicable optimizations are concatenated into the filename.
Otherwise the dump is output onto stderr. Though multiple
-fopt-info options are accepted, only one of them can include a
filename. If other filenames are provided then all but the first
such option are ignored.
Note that the output filename is overwritten in case of multiple
translation units. If a combined output from multiple translation
units is desired, stderr should be used instead.
In the following example, the optimization info is output to
stderr:
gcc -O3 -fopt-info
This example:
gcc -O3 -fopt-info-missed=missed.all
outputs missed optimization report from all the passes into
missed.all, and this one:
gcc -O2 -ftree-vectorize -fopt-info-vec-missed
prints information about missed optimization opportunities from
vectorization passes on stderr. Note that -fopt-info-vec-missed
is equivalent to -fopt-info-missed-vec.
As another example,
gcc -O3 -fopt-info-inline-optimized-missed=inline.txt
outputs information about missed optimizations as well as
optimized locations from all the inlining passes into inline.txt.
Finally, consider:
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt
Here the two output filenames vec.miss and loop.opt are in
conflict since only one output file is allowed. In this case,
only the first option takes effect and the subsequent options are
ignored. Thus only vec.miss is produced which contains dumps from
the vectorizer about missed opportunities.
-fsched-verbose=n
On targets that use instruction scheduling, this option controls
the amount of debugging output the scheduler prints to the dump
files.
For n greater than zero, -fsched-verbose outputs the same
information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n
greater than one, it also output basic block probabilities,
detailed ready list information and unit/insn info. For n
greater than two, it includes RTL at abort point, control-flow
and regions info. And for n over four, -fsched-verbose also
includes dependence info.
-fenable-kind-pass
-fdisable-kind-pass=range-list
This is a set of options that are used to explicitly
disable/enable optimization passes. These options are intended
for use for debugging GCC. Compiler users should use regular
options for enabling/disabling passes instead.
-fdisable-ipa-pass
Disable IPA pass pass. pass is the pass name. If the same
pass is statically invoked in the compiler multiple times,
the pass name should be appended with a sequential number
starting from 1.
-fdisable-rtl-pass
-fdisable-rtl-pass=range-list
Disable RTL pass pass. pass is the pass name. If the same
pass is statically invoked in the compiler multiple times,
the pass name should be appended with a sequential number
starting from 1. range-list is a comma-separated list of
function ranges or assembler names. Each range is a number
pair separated by a colon. The range is inclusive in both
ends. If the range is trivial, the number pair can be
simplified as a single number. If the function's call graph
node's uid falls within one of the specified ranges, the pass
is disabled for that function. The uid is shown in the
function header of a dump file, and the pass names can be
dumped by using option -fdump-passes.
-fdisable-tree-pass
-fdisable-tree-pass=range-list
Disable tree pass pass. See -fdisable-rtl for the
description of option arguments.
-fenable-ipa-pass
Enable IPA pass pass. pass is the pass name. If the same
pass is statically invoked in the compiler multiple times,
the pass name should be appended with a sequential number
starting from 1.
-fenable-rtl-pass
-fenable-rtl-pass=range-list
Enable RTL pass pass. See -fdisable-rtl for option argument
description and examples.
-fenable-tree-pass
-fenable-tree-pass=range-list
Enable tree pass pass. See -fdisable-rtl for the description
of option arguments.
Here are some examples showing uses of these options.
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll
-fchecking
-fchecking=n
Enable internal consistency checking. The default depends on the
compiler configuration. -fchecking=2 enables further internal
consistency checking that might affect code generation.
-frandom-seed=string
This option provides a seed that GCC uses in place of random
numbers in generating certain symbol names that have to be
different in every compiled file. It is also used to place
unique stamps in coverage data files and the object files that
produce them. You can use the -frandom-seed option to produce
reproducibly identical object files.
The string can either be a number (decimal, octal or hex) or an
arbitrary string (in which case it's converted to a number by
computing CRC32).
The string should be different for every file you compile.
-save-temps
-save-temps=cwd
Store the usual "temporary" intermediate files permanently; place
them in the current directory and name them based on the source
file. Thus, compiling foo.c with -c -save-temps produces files
foo.i and foo.s, as well as foo.o. This creates a preprocessed
foo.i output file even though the compiler now normally uses an
integrated preprocessor.
When used in combination with the -x command-line option,
-save-temps is sensible enough to avoid over writing an input
source file with the same extension as an intermediate file. The
corresponding intermediate file may be obtained by renaming the
source file before using -save-temps.
If you invoke GCC in parallel, compiling several different source
files that share a common base name in different subdirectories
or the same source file compiled for multiple output
destinations, it is likely that the different parallel compilers
will interfere with each other, and overwrite the temporary
files. For instance:
gcc -save-temps -o outdir1/foo.o indir1/foo.c&
gcc -save-temps -o outdir2/foo.o indir2/foo.c&
may result in foo.i and foo.o being written to simultaneously by
both compilers.
-save-temps=obj
Store the usual "temporary" intermediate files permanently. If
the -o option is used, the temporary files are based on the
object file. If the -o option is not used, the -save-temps=obj
switch behaves like -save-temps.
For example:
gcc -save-temps=obj -c foo.c
gcc -save-temps=obj -c bar.c -o dir/xbar.o
gcc -save-temps=obj foobar.c -o dir2/yfoobar
creates foo.i, foo.s, dir/xbar.i, dir/xbar.s, dir2/yfoobar.i,
dir2/yfoobar.s, and dir2/yfoobar.o.
-time[=file]
Report the CPU time taken by each subprocess in the compilation
sequence. For C source files, this is the compiler proper and
assembler (plus the linker if linking is done).
Without the specification of an output file, the output looks
like this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the "user time", that is time
spent executing the program itself. The second number is "system
time", time spent executing operating system routines on behalf
of the program. Both numbers are in seconds.
With the specification of an output file, the output is appended
to the named file, and it looks like this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>
The "user time" and the "system time" are moved before the
program name, and the options passed to the program are
displayed, so that one can later tell what file was being
compiled, and with which options.
-fdump-final-insns[=file]
Dump the final internal representation (RTL) to file. If the
optional argument is omitted (or if file is "."), the name of the
dump file is determined by appending ".gkd" to the compilation
output file name.
-fcompare-debug[=opts]
If no error occurs during compilation, run the compiler a second
time, adding opts and -fcompare-debug-second to the arguments
passed to the second compilation. Dump the final internal
representation in both compilations, and print an error if they
differ.
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if defined, non-empty
and nonzero, implicitly enables -fcompare-debug. If
GCC_COMPARE_DEBUG is defined to a string starting with a dash,
then it is used for opts, otherwise the default -gtoggle is used.
-fcompare-debug=, with the equal sign but without opts, is
equivalent to -fno-compare-debug, which disables the dumping of
the final representation and the second compilation, preventing
even GCC_COMPARE_DEBUG from taking effect.
To verify full coverage during -fcompare-debug testing, set
GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which
GCC rejects as an invalid option in any actual compilation
(rather than preprocessing, assembly or linking). To get just a
warning, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not
overridden will do.
-fcompare-debug-second
This option is implicitly passed to the compiler for the second
compilation requested by -fcompare-debug, along with options to
silence warnings, and omitting other options that would cause
side-effect compiler outputs to files or to the standard output.
Dump files and preserved temporary files are renamed so as to
contain the ".gk" additional extension during the second
compilation, to avoid overwriting those generated by the first.
When this option is passed to the compiler driver, it causes the
first compilation to be skipped, which makes it useful for little
other than debugging the compiler proper.
-gtoggle
Turn off generation of debug info, if leaving out this option
generates it, or turn it on at level 2 otherwise. The position
of this argument in the command line does not matter; it takes
effect after all other options are processed, and it does so only
once, no matter how many times it is given. This is mainly
intended to be used with -fcompare-debug.
-fvar-tracking-assignments-toggle
Toggle -fvar-tracking-assignments, in the same way that -gtoggle
toggles -g.
-Q Makes the compiler print out each function name as it is
compiled, and print some statistics about each pass when it
finishes.
-ftime-report
Makes the compiler print some statistics about the time consumed
by each pass when it finishes.
-ftime-report-details
Record the time consumed by infrastructure parts separately for
each pass.
-fira-verbose=n
Control the verbosity of the dump file for the integrated
register allocator. The default value is 5. If the value n is
greater or equal to 10, the dump output is sent to stderr using
the same format as n minus 10.
-flto-report
Prints a report with internal details on the workings of the
link-time optimizer. The contents of this report vary from
version to version. It is meant to be useful to GCC developers
when processing object files in LTO mode (via -flto).
Disabled by default.
-flto-report-wpa
Like -flto-report, but only print for the WPA phase of Link Time
Optimization.
-fmem-report
Makes the compiler print some statistics about permanent memory
allocation when it finishes.
-fmem-report-wpa
Makes the compiler print some statistics about permanent memory
allocation for the WPA phase only.
-fpre-ipa-mem-report
-fpost-ipa-mem-report
Makes the compiler print some statistics about permanent memory
allocation before or after interprocedural optimization.
-fprofile-report
Makes the compiler print some statistics about consistency of the
(estimated) profile and effect of individual passes.
-fstack-usage
Makes the compiler output stack usage information for the
program, on a per-function basis. The filename for the dump is
made by appending .su to the auxname. auxname is generated from
the name of the output file, if explicitly specified and it is
not an executable, otherwise it is the basename of the source
file. An entry is made up of three fields:
* The name of the function.
* A number of bytes.
* One or more qualifiers: "static", "dynamic", "bounded".
The qualifier "static" means that the function manipulates the
stack statically: a fixed number of bytes are allocated for the
frame on function entry and released on function exit; no stack
adjustments are otherwise made in the function. The second field
is this fixed number of bytes.
The qualifier "dynamic" means that the function manipulates the
stack dynamically: in addition to the static allocation described
above, stack adjustments are made in the body of the function,
for example to push/pop arguments around function calls. If the
qualifier "bounded" is also present, the amount of these
adjustments is bounded at compile time and the second field is an
upper bound of the total amount of stack used by the function.
If it is not present, the amount of these adjustments is not
bounded at compile time and the second field only represents the
bounded part.
-fstats
Emit statistics about front-end processing at the end of the
compilation. This option is supported only by the C++ front end,
and the information is generally only useful to the G++
development team.
-fdbg-cnt-list
Print the name and the counter upper bound for all debug
counters.
-fdbg-cnt=counter-value-list
Set the internal debug counter upper bound. counter-value-list
is a comma-separated list of name:value pairs which sets the
upper bound of each debug counter name to value. All debug
counters have the initial upper bound of "UINT_MAX"; thus
"dbg_cnt" returns true always unless the upper bound is set by
this option. For example, with -fdbg-cnt=dce:10,tail_call:0,
"dbg_cnt(dce)" returns true only for first 10 invocations.
-print-file-name=library
Print the full absolute name of the library file library that
would be used when linking---and don't do anything else. With
this option, GCC does not compile or link anything; it just
prints the file name.
-print-multi-directory
Print the directory name corresponding to the multilib selected
by any other switches present in the command line. This
directory is supposed to exist in GCC_EXEC_PREFIX.
-print-multi-lib
Print the mapping from multilib directory names to compiler
switches that enable them. The directory name is separated from
the switches by ;, and each switch starts with an @ instead of
the -, without spaces between multiple switches. This is
supposed to ease shell processing.
-print-multi-os-directory
Print the path to OS libraries for the selected multilib,
relative to some lib subdirectory. If OS libraries are present
in the lib subdirectory and no multilibs are used, this is
usually just ., if OS libraries are present in libsuffix sibling
directories this prints e.g. ../lib64, ../lib or ../lib32, or if
OS libraries are present in lib/subdir subdirectories it prints
e.g. amd64, sparcv9 or ev6.
-print-multiarch
Print the path to OS libraries for the selected multiarch,
relative to some lib subdirectory.
-print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you
do want to link with libgcc.a. You can do:
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
-print-search-dirs
Print the name of the configured installation directory and a
list of program and library directories gcc searches---and don't
do anything else.
This is useful when gcc prints the error message installation
problem, cannot exec cpp0: No such file or directory. To resolve
this you either need to put cpp0 and the other compiler
components where gcc expects to find them, or you can set the
environment variable GCC_EXEC_PREFIX to the directory where you
installed them. Don't forget the trailing /.
-print-sysroot
Print the target sysroot directory that is used during
compilation. This is the target sysroot specified either at
configure time or using the --sysroot option, possibly with an
extra suffix that depends on compilation options. If no target
sysroot is specified, the option prints nothing.
-print-sysroot-headers-suffix
Print the suffix added to the target sysroot when searching for
headers, or give an error if the compiler is not configured with
such a suffix---and don't do anything else.
-dumpmachine
Print the compiler's target machine (for example,
i686-pc-linux-gnu)---and don't do anything else.
-dumpversion
Print the compiler version (for example, 3.0, 6.3.0 or 7)---and
don't do anything else. This is the compiler version used in
filesystem paths, specs, can be depending on how the compiler has
been configured just a single number (major version), two numbers
separated by dot (major and minor version) or three numbers
separated by dots (major, minor and patchlevel version).
-dumpfullversion
Print the full compiler version, always 3 numbers separated by
dots, major, minor and patchlevel version.
-dumpspecs
Print the compiler's built-in specs---and don't do anything else.
(This is used when GCC itself is being built.)
Machine-Dependent Options
Each target machine supported by GCC can have its own options---for
example, to allow you to compile for a particular processor variant
or ABI, or to control optimizations specific to that machine. By
convention, the names of machine-specific options start with -m.
Some configurations of the compiler also support additional target-
specific options, usually for compatibility with other compilers on
the same platform.
AArch64 Options
These options are defined for AArch64 implementations:
-mabi=name
Generate code for the specified data model. Permissible values
are ilp32 for SysV-like data model where int, long int and
pointers are 32 bits, and lp64 for SysV-like data model where int
is 32 bits, but long int and pointers are 64 bits.
The default depends on the specific target configuration. Note
that the LP64 and ILP32 ABIs are not link-compatible; you must
compile your entire program with the same ABI, and link with a
compatible set of libraries.
-mbig-endian
Generate big-endian code. This is the default when GCC is
configured for an aarch64_be-*-* target.
-mgeneral-regs-only
Generate code which uses only the general-purpose registers.
This will prevent the compiler from using floating-point and
Advanced SIMD registers but will not impose any restrictions on
the assembler.
-mlittle-endian
Generate little-endian code. This is the default when GCC is
configured for an aarch64-*-* but not an aarch64_be-*-* target.
-mcmodel=tiny
Generate code for the tiny code model. The program and its
statically defined symbols must be within 1MB of each other.
Programs can be statically or dynamically linked.
-mcmodel=small
Generate code for the small code model. The program and its
statically defined symbols must be within 4GB of each other.
Programs can be statically or dynamically linked. This is the
default code model.
-mcmodel=large
Generate code for the large code model. This makes no
assumptions about addresses and sizes of sections. Programs can
be statically linked only.
-mstrict-align
Avoid generating memory accesses that may not be aligned on a
natural object boundary as described in the architecture
specification.
-momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer
Omit or keep the frame pointer in leaf functions. The former
behavior is the default.
-mtls-dialect=desc
Use TLS descriptors as the thread-local storage mechanism for
dynamic accesses of TLS variables. This is the default.
-mtls-dialect=traditional
Use traditional TLS as the thread-local storage mechanism for
dynamic accesses of TLS variables.
-mtls-size=size
Specify bit size of immediate TLS offsets. Valid values are 12,
24, 32, 48. This option requires binutils 2.26 or newer.
-mfix-cortex-a53-835769
-mno-fix-cortex-a53-835769
Enable or disable the workaround for the ARM Cortex-A53 erratum
number 835769. This involves inserting a NOP instruction between
memory instructions and 64-bit integer multiply-accumulate
instructions.
-mfix-cortex-a53-843419
-mno-fix-cortex-a53-843419
Enable or disable the workaround for the ARM Cortex-A53 erratum
number 843419. This erratum workaround is made at link time and
this will only pass the corresponding flag to the linker.
-mlow-precision-recip-sqrt
-mno-low-precision-recip-sqrt
Enable or disable the reciprocal square root approximation. This
option only has an effect if -ffast-math or
-funsafe-math-optimizations is used as well. Enabling this
reduces precision of reciprocal square root results to about 16
bits for single precision and to 32 bits for double precision.
-mlow-precision-sqrt
-mno-low-precision-sqrt
Enable or disable the square root approximation. This option
only has an effect if -ffast-math or -funsafe-math-optimizations
is used as well. Enabling this reduces precision of square root
results to about 16 bits for single precision and to 32 bits for
double precision. If enabled, it implies
-mlow-precision-recip-sqrt.
-mlow-precision-div
-mno-low-precision-div
Enable or disable the division approximation. This option only
has an effect if -ffast-math or -funsafe-math-optimizations is
used as well. Enabling this reduces precision of division
results to about 16 bits for single precision and to 32 bits for
double precision.
-march=name
Specify the name of the target architecture and, optionally, one
or more feature modifiers. This option has the form
-march=arch{+[no]feature}*.
The permissible values for arch are armv8-a, armv8.1-a,
armv8.2-a, armv8.3-a or native.
The value armv8.3-a implies armv8.2-a and enables compiler
support for the ARMv8.3-A architecture extensions.
The value armv8.2-a implies armv8.1-a and enables compiler
support for the ARMv8.2-A architecture extensions.
The value armv8.1-a implies armv8-a and enables compiler support
for the ARMv8.1-A architecture extension. In particular, it
enables the +crc and +lse features.
The value native is available on native AArch64 GNU/Linux and
causes the compiler to pick the architecture of the host system.
This option has no effect if the compiler is unable to recognize
the architecture of the host system,
The permissible values for feature are listed in the sub-section
on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers.
Where conflicting feature modifiers are specified, the right-most
feature is used.
GCC uses name to determine what kind of instructions it can emit
when generating assembly code. If -march is specified without
either of -mtune or -mcpu also being specified, the code is tuned
to perform well across a range of target processors implementing
the target architecture.
-mtune=name
Specify the name of the target processor for which GCC should
tune the performance of the code. Permissible values for this
option are: generic, cortex-a35, cortex-a53, cortex-a57,
cortex-a72, cortex-a73, exynos-m1, falkor, qdf24xx, xgene1,
vulcan, thunderx, thunderxt88, thunderxt88p1, thunderxt81,
thunderxt83, thunderx2t99, cortex-a57.cortex-a53,
cortex-a72.cortex-a53, cortex-a73.cortex-a35,
cortex-a73.cortex-a53, native.
The values cortex-a57.cortex-a53, cortex-a72.cortex-a53,
cortex-a73.cortex-a35, cortex-a73.cortex-a53 specify that GCC
should tune for a big.LITTLE system.
Additionally on native AArch64 GNU/Linux systems the value native
tunes performance to the host system. This option has no effect
if the compiler is unable to recognize the processor of the host
system.
Where none of -mtune=, -mcpu= or -march= are specified, the code
is tuned to perform well across a range of target processors.
This option cannot be suffixed by feature modifiers.
-mcpu=name
Specify the name of the target processor, optionally suffixed by
one or more feature modifiers. This option has the form
-mcpu=cpu{+[no]feature}*, where the permissible values for cpu
are the same as those available for -mtune. The permissible
values for feature are documented in the sub-section on
aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers.
Where conflicting feature modifiers are specified, the right-most
feature is used.
GCC uses name to determine what kind of instructions it can emit
when generating assembly code (as if by -march) and to determine
the target processor for which to tune for performance (as if by
-mtune). Where this option is used in conjunction with -march or
-mtune, those options take precedence over the appropriate part
of this option.
-moverride=string
Override tuning decisions made by the back-end in response to a
-mtune= switch. The syntax, semantics, and accepted values for
string in this option are not guaranteed to be consistent across
releases.
This option is only intended to be useful when developing GCC.
-mpc-relative-literal-loads
Enable PC-relative literal loads. With this option literal pools
are accessed using a single instruction and emitted after each
function. This limits the maximum size of functions to 1MB.
This is enabled by default for -mcmodel=tiny.
-msign-return-address=scope
Select the function scope on which return address signing will be
applied. Permissible values are none, which disables return
address signing, non-leaf, which enables pointer signing for
functions which are not leaf functions, and all, which enables
pointer signing for all functions. The default value is none.
-march and -mcpu Feature Modifiers
Feature modifiers used with -march and -mcpu can be any of the
following and their inverses nofeature:
crc Enable CRC extension. This is on by default for
-march=armv8.1-a.
crypto
Enable Crypto extension. This also enables Advanced SIMD and
floating-point instructions.
fp Enable floating-point instructions. This is on by default for
all possible values for options -march and -mcpu.
simd
Enable Advanced SIMD instructions. This also enables floating-
point instructions. This is on by default for all possible
values for options -march and -mcpu.
lse Enable Large System Extension instructions. This is on by
default for -march=armv8.1-a.
fp16
Enable FP16 extension. This also enables floating-point
instructions.
Feature crypto implies simd, which implies fp. Conversely, nofp
implies nosimd, which implies nocrypto.
Adapteva Epiphany Options
These -m options are defined for Adapteva Epiphany:
-mhalf-reg-file
Don't allocate any register in the range "r32"..."r63". That
allows code to run on hardware variants that lack these
registers.
-mprefer-short-insn-regs
Preferentially allocate registers that allow short instruction
generation. This can result in increased instruction count, so
this may either reduce or increase overall code size.
-mbranch-cost=num
Set the cost of branches to roughly num "simple" instructions.
This cost is only a heuristic and is not guaranteed to produce
consistent results across releases.
-mcmove
Enable the generation of conditional moves.
-mnops=num
Emit num NOPs before every other generated instruction.
-mno-soft-cmpsf
For single-precision floating-point comparisons, emit an "fsub"
instruction and test the flags. This is faster than a software
comparison, but can get incorrect results in the presence of
NaNs, or when two different small numbers are compared such that
their difference is calculated as zero. The default is
-msoft-cmpsf, which uses slower, but IEEE-compliant, software
comparisons.
-mstack-offset=num
Set the offset between the top of the stack and the stack
pointer. E.g., a value of 8 means that the eight bytes in the
range "sp+0...sp+7" can be used by leaf functions without stack
allocation. Values other than 8 or 16 are untested and unlikely
to work. Note also that this option changes the ABI; compiling a
program with a different stack offset than the libraries have
been compiled with generally does not work. This option can be
useful if you want to evaluate if a different stack offset would
give you better code, but to actually use a different stack
offset to build working programs, it is recommended to configure
the toolchain with the appropriate --with-stack-offset=num
option.
-mno-round-nearest
Make the scheduler assume that the rounding mode has been set to
truncating. The default is -mround-nearest.
-mlong-calls
If not otherwise specified by an attribute, assume all calls
might be beyond the offset range of the "b" / "bl" instructions,
and therefore load the function address into a register before
performing a (otherwise direct) call. This is the default.
-mshort-calls
If not otherwise specified by an attribute, assume all direct
calls are in the range of the "b" / "bl" instructions, so use
these instructions for direct calls. The default is
-mlong-calls.
-msmall16
Assume addresses can be loaded as 16-bit unsigned values. This
does not apply to function addresses for which -mlong-calls
semantics are in effect.
-mfp-mode=mode
Set the prevailing mode of the floating-point unit. This
determines the floating-point mode that is provided and expected
at function call and return time. Making this mode match the
mode you predominantly need at function start can make your
programs smaller and faster by avoiding unnecessary mode
switches.
mode can be set to one the following values:
caller
Any mode at function entry is valid, and retained or restored
when the function returns, and when it calls other functions.
This mode is useful for compiling libraries or other
compilation units you might want to incorporate into
different programs with different prevailing FPU modes, and
the convenience of being able to use a single object file
outweighs the size and speed overhead for any extra mode
switching that might be needed, compared with what would be
needed with a more specific choice of prevailing FPU mode.
truncate
This is the mode used for floating-point calculations with
truncating (i.e. round towards zero) rounding mode. That
includes conversion from floating point to integer.
round-nearest
This is the mode used for floating-point calculations with
round-to-nearest-or-even rounding mode.
int This is the mode used to perform integer calculations in the
FPU, e.g. integer multiply, or integer multiply-and-
accumulate.
The default is -mfp-mode=caller
-mnosplit-lohi
-mno-postinc
-mno-postmodify
Code generation tweaks that disable, respectively, splitting of
32-bit loads, generation of post-increment addresses, and
generation of post-modify addresses. The defaults are msplit-
lohi, -mpost-inc, and -mpost-modify.
-mnovect-double
Change the preferred SIMD mode to SImode. The default is
-mvect-double, which uses DImode as preferred SIMD mode.
-max-vect-align=num
The maximum alignment for SIMD vector mode types. num may be 4
or 8. The default is 8. Note that this is an ABI change, even
though many library function interfaces are unaffected if they
don't use SIMD vector modes in places that affect size and/or
alignment of relevant types.
-msplit-vecmove-early
Split vector moves into single word moves before reload. In
theory this can give better register allocation, but so far the
reverse seems to be generally the case.
-m1reg-reg
Specify a register to hold the constant -1, which makes loading
small negative constants and certain bitmasks faster. Allowable
values for reg are r43 and r63, which specify use of that
register as a fixed register, and none, which means that no
register is used for this purpose. The default is -m1reg-none.
ARC Options
The following options control the architecture variant for which code
is being compiled:
-mbarrel-shifter
Generate instructions supported by barrel shifter. This is the
default unless -mcpu=ARC601 or -mcpu=ARCEM is in effect.
-mcpu=cpu
Set architecture type, register usage, and instruction scheduling
parameters for cpu. There are also shortcut alias options
available for backward compatibility and convenience. Supported
values for cpu are
arc600
Compile for ARC600. Aliases: -mA6, -mARC600.
arc601
Compile for ARC601. Alias: -mARC601.
arc700
Compile for ARC700. Aliases: -mA7, -mARC700. This is the
default when configured with --with-cpu=arc700.
arcem
Compile for ARC EM.
archs
Compile for ARC HS.
em Compile for ARC EM CPU with no hardware extensions.
em4 Compile for ARC EM4 CPU.
em4_dmips
Compile for ARC EM4 DMIPS CPU.
em4_fpus
Compile for ARC EM4 DMIPS CPU with the single-precision
floating-point extension.
em4_fpuda
Compile for ARC EM4 DMIPS CPU with single-precision floating-
point and double assist instructions.
hs Compile for ARC HS CPU with no hardware extensions except the
atomic instructions.
hs34
Compile for ARC HS34 CPU.
hs38
Compile for ARC HS38 CPU.
hs38_linux
Compile for ARC HS38 CPU with all hardware extensions on.
arc600_norm
Compile for ARC 600 CPU with "norm" instructions enabled.
arc600_mul32x16
Compile for ARC 600 CPU with "norm" and 32x16-bit multiply
instructions enabled.
arc600_mul64
Compile for ARC 600 CPU with "norm" and "mul64"-family
instructions enabled.
arc601_norm
Compile for ARC 601 CPU with "norm" instructions enabled.
arc601_mul32x16
Compile for ARC 601 CPU with "norm" and 32x16-bit multiply
instructions enabled.
arc601_mul64
Compile for ARC 601 CPU with "norm" and "mul64"-family
instructions enabled.
nps400
Compile for ARC 700 on NPS400 chip.
-mdpfp
-mdpfp-compact
Generate double-precision FPX instructions, tuned for the compact
implementation.
-mdpfp-fast
Generate double-precision FPX instructions, tuned for the fast
implementation.
-mno-dpfp-lrsr
Disable "lr" and "sr" instructions from using FPX extension aux
registers.
-mea
Generate extended arithmetic instructions. Currently only
"divaw", "adds", "subs", and "sat16" are supported. This is
always enabled for -mcpu=ARC700.
-mno-mpy
Do not generate "mpy"-family instructions for ARC700. This
option is deprecated.
-mmul32x16
Generate 32x16-bit multiply and multiply-accumulate instructions.
-mmul64
Generate "mul64" and "mulu64" instructions. Only valid for
-mcpu=ARC600.
-mnorm
Generate "norm" instructions. This is the default if
-mcpu=ARC700 is in effect.
-mspfp
-mspfp-compact
Generate single-precision FPX instructions, tuned for the compact
implementation.
-mspfp-fast
Generate single-precision FPX instructions, tuned for the fast
implementation.
-msimd
Enable generation of ARC SIMD instructions via target-specific
builtins. Only valid for -mcpu=ARC700.
-msoft-float
This option ignored; it is provided for compatibility purposes
only. Software floating-point code is emitted by default, and
this default can overridden by FPX options; -mspfp,
-mspfp-compact, or -mspfp-fast for single precision, and -mdpfp,
-mdpfp-compact, or -mdpfp-fast for double precision.
-mswap
Generate "swap" instructions.
-matomic
This enables use of the locked load/store conditional extension
to implement atomic memory built-in functions. Not available for
ARC 6xx or ARC EM cores.
-mdiv-rem
Enable "div" and "rem" instructions for ARCv2 cores.
-mcode-density
Enable code density instructions for ARC EM. This option is on
by default for ARC HS.
-mll64
Enable double load/store operations for ARC HS cores.
-mtp-regno=regno
Specify thread pointer register number.
-mmpy-option=multo
Compile ARCv2 code with a multiplier design option. You can
specify the option using either a string or numeric value for
multo. wlh1 is the default value. The recognized values are:
0
none
No multiplier available.
1
w 16x16 multiplier, fully pipelined. The following
instructions are enabled: "mpyw" and "mpyuw".
2
wlh1
32x32 multiplier, fully pipelined (1 stage). The following
instructions are additionally enabled: "mpy", "mpyu", "mpym",
"mpymu", and "mpy_s".
3
wlh2
32x32 multiplier, fully pipelined (2 stages). The following
instructions are additionally enabled: "mpy", "mpyu", "mpym",
"mpymu", and "mpy_s".
4
wlh3
Two 16x16 multipliers, blocking, sequential. The following
instructions are additionally enabled: "mpy", "mpyu", "mpym",
"mpymu", and "mpy_s".
5
wlh4
One 16x16 multiplier, blocking, sequential. The following
instructions are additionally enabled: "mpy", "mpyu", "mpym",
"mpymu", and "mpy_s".
6
wlh5
One 32x4 multiplier, blocking, sequential. The following
instructions are additionally enabled: "mpy", "mpyu", "mpym",
"mpymu", and "mpy_s".
7
plus_dmpy
ARC HS SIMD support.
8
plus_macd
ARC HS SIMD support.
9
plus_qmacw
ARC HS SIMD support.
This option is only available for ARCv2 cores.
-mfpu=fpu
Enables support for specific floating-point hardware extensions
for ARCv2 cores. Supported values for fpu are:
fpus
Enables support for single-precision floating-point hardware
extensions.
fpud
Enables support for double-precision floating-point hardware
extensions. The single-precision floating-point extension is
also enabled. Not available for ARC EM.
fpuda
Enables support for double-precision floating-point hardware
extensions using double-precision assist instructions. The
single-precision floating-point extension is also enabled.
This option is only available for ARC EM.
fpuda_div
Enables support for double-precision floating-point hardware
extensions using double-precision assist instructions. The
single-precision floating-point, square-root, and divide
extensions are also enabled. This option is only available
for ARC EM.
fpuda_fma
Enables support for double-precision floating-point hardware
extensions using double-precision assist instructions. The
single-precision floating-point and fused multiply and add
hardware extensions are also enabled. This option is only
available for ARC EM.
fpuda_all
Enables support for double-precision floating-point hardware
extensions using double-precision assist instructions. All
single-precision floating-point hardware extensions are also
enabled. This option is only available for ARC EM.
fpus_div
Enables support for single-precision floating-point, square-
root and divide hardware extensions.
fpud_div
Enables support for double-precision floating-point, square-
root and divide hardware extensions. This option includes
option fpus_div. Not available for ARC EM.
fpus_fma
Enables support for single-precision floating-point and fused
multiply and add hardware extensions.
fpud_fma
Enables support for double-precision floating-point and fused
multiply and add hardware extensions. This option includes
option fpus_fma. Not available for ARC EM.
fpus_all
Enables support for all single-precision floating-point
hardware extensions.
fpud_all
Enables support for all single- and double-precision
floating-point hardware extensions. Not available for ARC
EM.
The following options are passed through to the assembler, and also
define preprocessor macro symbols.
-mdsp-packa
Passed down to the assembler to enable the DSP Pack A extensions.
Also sets the preprocessor symbol "__Xdsp_packa". This option is
deprecated.
-mdvbf
Passed down to the assembler to enable the dual Viterbi butterfly
extension. Also sets the preprocessor symbol "__Xdvbf". This
option is deprecated.
-mlock
Passed down to the assembler to enable the locked load/store
conditional extension. Also sets the preprocessor symbol
"__Xlock".
-mmac-d16
Passed down to the assembler. Also sets the preprocessor symbol
"__Xxmac_d16". This option is deprecated.
-mmac-24
Passed down to the assembler. Also sets the preprocessor symbol
"__Xxmac_24". This option is deprecated.
-mrtsc
Passed down to the assembler to enable the 64-bit time-stamp
counter extension instruction. Also sets the preprocessor symbol
"__Xrtsc". This option is deprecated.
-mswape
Passed down to the assembler to enable the swap byte ordering
extension instruction. Also sets the preprocessor symbol
"__Xswape".
-mtelephony
Passed down to the assembler to enable dual- and single-operand
instructions for telephony. Also sets the preprocessor symbol
"__Xtelephony". This option is deprecated.
-mxy
Passed down to the assembler to enable the XY memory extension.
Also sets the preprocessor symbol "__Xxy".
The following options control how the assembly code is annotated:
-misize
Annotate assembler instructions with estimated addresses.
-mannotate-align
Explain what alignment considerations lead to the decision to
make an instruction short or long.
The following options are passed through to the linker:
-marclinux
Passed through to the linker, to specify use of the "arclinux"
emulation. This option is enabled by default in tool chains
built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets
when profiling is not requested.
-marclinux_prof
Passed through to the linker, to specify use of the
"arclinux_prof" emulation. This option is enabled by default in
tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc"
targets when profiling is requested.
The following options control the semantics of generated code:
-mlong-calls
Generate calls as register indirect calls, thus providing access
to the full 32-bit address range.
-mmedium-calls
Don't use less than 25-bit addressing range for calls, which is
the offset available for an unconditional branch-and-link
instruction. Conditional execution of function calls is
suppressed, to allow use of the 25-bit range, rather than the
21-bit range with conditional branch-and-link. This is the
default for tool chains built for "arc-linux-uclibc" and
"arceb-linux-uclibc" targets.
-mno-sdata
Do not generate sdata references. This is the default for tool
chains built for "arc-linux-uclibc" and "arceb-linux-uclibc"
targets.
-mvolatile-cache
Use ordinarily cached memory accesses for volatile references.
This is the default.
-mno-volatile-cache
Enable cache bypass for volatile references.
The following options fine tune code generation:
-malign-call
Do alignment optimizations for call instructions.
-mauto-modify-reg
Enable the use of pre/post modify with register displacement.
-mbbit-peephole
Enable bbit peephole2.
-mno-brcc
This option disables a target-specific pass in arc_reorg to
generate compare-and-branch ("brcc") instructions. It has no
effect on generation of these instructions driven by the combiner
pass.
-mcase-vector-pcrel
Use PC-relative switch case tables to enable case table
shortening. This is the default for -Os.
-mcompact-casesi
Enable compact "casesi" pattern. This is the default for -Os,
and only available for ARCv1 cores.
-mno-cond-exec
Disable the ARCompact-specific pass to generate conditional
execution instructions.
Due to delay slot scheduling and interactions between operand
numbers, literal sizes, instruction lengths, and the support for
conditional execution, the target-independent pass to generate
conditional execution is often lacking, so the ARC port has kept
a special pass around that tries to find more conditional
execution generation opportunities after register allocation,
branch shortening, and delay slot scheduling have been done.
This pass generally, but not always, improves performance and
code size, at the cost of extra compilation time, which is why
there is an option to switch it off. If you have a problem with
call instructions exceeding their allowable offset range because
they are conditionalized, you should consider using
-mmedium-calls instead.
-mearly-cbranchsi
Enable pre-reload use of the "cbranchsi" pattern.
-mexpand-adddi
Expand "adddi3" and "subdi3" at RTL generation time into "add.f",
"adc" etc.
-mindexed-loads
Enable the use of indexed loads. This can be problematic because
some optimizers then assume that indexed stores exist, which is
not the case.
Enable Local Register Allocation. This is still experimental for
ARC, so by default the compiler uses standard reload (i.e.
-mno-lra).
-mlra-priority-none
Don't indicate any priority for target registers.
-mlra-priority-compact
Indicate target register priority for r0..r3 / r12..r15.
-mlra-priority-noncompact
Reduce target register priority for r0..r3 / r12..r15.
-mno-millicode
When optimizing for size (using -Os), prologues and epilogues
that have to save or restore a large number of registers are
often shortened by using call to a special function in libgcc;
this is referred to as a millicode call. As these calls can pose
performance issues, and/or cause linking issues when linking in a
nonstandard way, this option is provided to turn off millicode
call generation.
-mmixed-code
Tweak register allocation to help 16-bit instruction generation.
This generally has the effect of decreasing the average
instruction size while increasing the instruction count.
-mq-class
Enable q instruction alternatives. This is the default for -Os.
-mRcq
Enable Rcq constraint handling. Most short code generation
depends on this. This is the default.
-mRcw
Enable Rcw constraint handling. Most ccfsm condexec mostly
depends on this. This is the default.
-msize-level=level
Fine-tune size optimization with regards to instruction lengths
and alignment. The recognized values for level are:
0 No size optimization. This level is deprecated and treated
like 1.
1 Short instructions are used opportunistically.
2 In addition, alignment of loops and of code after barriers
are dropped.
3 In addition, optional data alignment is dropped, and the
option Os is enabled.
This defaults to 3 when -Os is in effect. Otherwise, the
behavior when this is not set is equivalent to level 1.
-mtune=cpu
Set instruction scheduling parameters for cpu, overriding any
implied by -mcpu=.
Supported values for cpu are
ARC600
Tune for ARC600 CPU.
ARC601
Tune for ARC601 CPU.
ARC700
Tune for ARC700 CPU with standard multiplier block.
ARC700-xmac
Tune for ARC700 CPU with XMAC block.
ARC725D
Tune for ARC725D CPU.
ARC750D
Tune for ARC750D CPU.
-mmultcost=num
Cost to assume for a multiply instruction, with 4 being equal to
a normal instruction.
-munalign-prob-threshold=probability
Set probability threshold for unaligning branches. When tuning
for ARC700 and optimizing for speed, branches without filled
delay slot are preferably emitted unaligned and long, unless
profiling indicates that the probability for the branch to be
taken is below probability. The default is (REG_BR_PROB_BASE/2),
i.e. 5000.
The following options are maintained for backward compatibility, but
are now deprecated and will be removed in a future release:
-margonaut
Obsolete FPX.
-mbig-endian
-EB Compile code for big-endian targets. Use of these options is now
deprecated. Big-endian code is supported by configuring GCC to
build "arceb-elf32" and "arceb-linux-uclibc" targets, for which
big endian is the default.
-mlittle-endian
-EL Compile code for little-endian targets. Use of these options is
now deprecated. Little-endian code is supported by configuring
GCC to build "arc-elf32" and "arc-linux-uclibc" targets, for
which little endian is the default.
-mbarrel_shifter
Replaced by -mbarrel-shifter.
-mdpfp_compact
Replaced by -mdpfp-compact.
-mdpfp_fast
Replaced by -mdpfp-fast.
-mdsp_packa
Replaced by -mdsp-packa.
-mEA
Replaced by -mea.
-mmac_24
Replaced by -mmac-24.
-mmac_d16
Replaced by -mmac-d16.
-mspfp_compact
Replaced by -mspfp-compact.
-mspfp_fast
Replaced by -mspfp-fast.
-mtune=cpu
Values arc600, arc601, arc700 and arc700-xmac for cpu are
replaced by ARC600, ARC601, ARC700 and ARC700-xmac respectively.
-multcost=num
Replaced by -mmultcost.
ARM Options
These -m options are defined for the ARM port:
-mabi=name
Generate code for the specified ABI. Permissible values are:
apcs-gnu, atpcs, aapcs, aapcs-linux and iwmmxt.
-mapcs-frame
Generate a stack frame that is compliant with the ARM Procedure
Call Standard for all functions, even if this is not strictly
necessary for correct execution of the code. Specifying
-fomit-frame-pointer with this option causes the stack frames not
to be generated for leaf functions. The default is
-mno-apcs-frame. This option is deprecated.
-mapcs
This is a synonym for -mapcs-frame and is deprecated.
-mthumb-interwork
Generate code that supports calling between the ARM and Thumb
instruction sets. Without this option, on pre-v5 architectures,
the two instruction sets cannot be reliably used inside one
program. The default is -mno-thumb-interwork, since slightly
larger code is generated when -mthumb-interwork is specified. In
AAPCS configurations this option is meaningless.
-mno-sched-prolog
Prevent the reordering of instructions in the function prologue,
or the merging of those instruction with the instructions in the
function's body. This means that all functions start with a
recognizable set of instructions (or in fact one of a choice from
a small set of different function prologues), and this
information can be used to locate the start of functions inside
an executable piece of code. The default is -msched-prolog.
-mfloat-abi=name
Specifies which floating-point ABI to use. Permissible values
are: soft, softfp and hard.
Specifying soft causes GCC to generate output containing library
calls for floating-point operations. softfp allows the
generation of code using hardware floating-point instructions,
but still uses the soft-float calling conventions. hard allows
generation of floating-point instructions and uses FPU-specific
calling conventions.
The default depends on the specific target configuration. Note
that the hard-float and soft-float ABIs are not link-compatible;
you must compile your entire program with the same ABI, and link
with a compatible set of libraries.
-mlittle-endian
Generate code for a processor running in little-endian mode.
This is the default for all standard configurations.
-mbig-endian
Generate code for a processor running in big-endian mode; the
default is to compile code for a little-endian processor.
-march=name
This specifies the name of the target ARM architecture. GCC uses
this name to determine what kind of instructions it can emit when
generating assembly code. This option can be used in conjunction
with or instead of the -mcpu= option. Permissible names are:
armv2, armv2a, armv3, armv3m, armv4, armv4t, armv5, armv5e,
armv5t, armv5te, armv6, armv6-m, armv6j, armv6k, armv6kz,
armv6s-m, armv6t2, armv6z, armv6zk, armv7, armv7-a, armv7-m,
armv7-r, armv7e-m, armv7ve, armv8-a, armv8-a+crc, armv8.1-a,
armv8.1-a+crc, armv8-m.base, armv8-m.main, armv8-m.main+dsp,
iwmmxt, iwmmxt2.
Architecture revisions older than armv4t are deprecated.
-march=armv6s-m is the armv6-m architecture with support for the
(now mandatory) SVC instruction.
-march=armv6zk is an alias for armv6kz, existing for backwards
compatibility.
-march=armv7ve is the armv7-a architecture with virtualization
extensions.
-march=armv8-a+crc enables code generation for the ARMv8-A
architecture together with the optional CRC32 extensions.
-march=armv8.1-a enables compiler support for the ARMv8.1-A
architecture. This also enables the features provided by
-march=armv8-a+crc.
-march=armv8.2-a enables compiler support for the ARMv8.2-A
architecture. This also enables the features provided by
-march=armv8.1-a.
-march=armv8.2-a+fp16 enables compiler support for the ARMv8.2-A
architecture with the optional FP16 instructions extension. This
also enables the features provided by -march=armv8.1-a and
implies -mfp16-format=ieee.
-march=native causes the compiler to auto-detect the architecture
of the build computer. At present, this feature is only
supported on GNU/Linux, and not all architectures are recognized.
If the auto-detect is unsuccessful the option has no effect.
-mtune=name
This option specifies the name of the target ARM processor for
which GCC should tune the performance of the code. For some ARM
implementations better performance can be obtained by using this
option. Permissible names are: arm2, arm250, arm3, arm6, arm60,
arm600, arm610, arm620, arm7, arm7m, arm7d, arm7dm, arm7di,
arm7dmi, arm70, arm700, arm700i, arm710, arm710c, arm7100,
arm720, arm7500, arm7500fe, arm7tdmi, arm7tdmi-s, arm710t,
arm720t, arm740t, strongarm, strongarm110, strongarm1100,
strongarm1110, arm8, arm810, arm9, arm9e, arm920, arm920t,
arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t,
arm9tdmi, arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e,
arm1022e, arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp,
arm1156t2-s, arm1156t2f-s, arm1176jz-s, arm1176jzf-s,
generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9,
cortex-a12, cortex-a15, cortex-a17, cortex-a32, cortex-a35,
cortex-a53, cortex-a57, cortex-a72, cortex-a73, cortex-r4,
cortex-r4f, cortex-r5, cortex-r7, cortex-r8, cortex-m33,
cortex-m23, cortex-m7, cortex-m4, cortex-m3, cortex-m1,
cortex-m0, cortex-m0plus, cortex-m1.small-multiply,
cortex-m0.small-multiply, cortex-m0plus.small-multiply,
exynos-m1, falkor, qdf24xx, marvell-pj4, xscale, iwmmxt, iwmmxt2,
ep9312, fa526, fa626, fa606te, fa626te, fmp626, fa726te, xgene1.
Additionally, this option can specify that GCC should tune the
performance of the code for a big.LITTLE system. Permissible
names are: cortex-a15.cortex-a7, cortex-a17.cortex-a7,
cortex-a57.cortex-a53, cortex-a72.cortex-a53,
cortex-a72.cortex-a35, cortex-a73.cortex-a53.
-mtune=generic-arch specifies that GCC should tune the
performance for a blend of processors within architecture arch.
The aim is to generate code that run well on the current most
popular processors, balancing between optimizations that benefit
some CPUs in the range, and avoiding performance pitfalls of
other CPUs. The effects of this option may change in future GCC
versions as CPU models come and go.
-mtune=native causes the compiler to auto-detect the CPU of the
build computer. At present, this feature is only supported on
GNU/Linux, and not all architectures are recognized. If the
auto-detect is unsuccessful the option has no effect.
-mcpu=name
This specifies the name of the target ARM processor. GCC uses
this name to derive the name of the target ARM architecture (as
if specified by -march) and the ARM processor type for which to
tune for performance (as if specified by -mtune). Where this
option is used in conjunction with -march or -mtune, those
options take precedence over the appropriate part of this option.
Permissible names for this option are the same as those for
-mtune.
-mcpu=generic-arch is also permissible, and is equivalent to
-march=arch -mtune=generic-arch. See -mtune for more
information.
-mcpu=native causes the compiler to auto-detect the CPU of the
build computer. At present, this feature is only supported on
GNU/Linux, and not all architectures are recognized. If the
auto-detect is unsuccessful the option has no effect.
-mfpu=name
This specifies what floating-point hardware (or hardware
emulation) is available on the target. Permissible names are:
vfpv2, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd,
vfpv3xd-fp16, neon-vfpv3, neon-fp16, vfpv4, vfpv4-d16,
fpv4-sp-d16, neon-vfpv4, fpv5-d16, fpv5-sp-d16, fp-armv8,
neon-fp-armv8 and crypto-neon-fp-armv8. Note that neon is an
alias for neon-vfpv3 and vfp is an alias for vfpv2.
If -msoft-float is specified this specifies the format of
floating-point values.
If the selected floating-point hardware includes the NEON
extension (e.g. -mfpu=neon), note that floating-point operations
are not generated by GCC's auto-vectorization pass unless
-funsafe-math-optimizations is also specified. This is because
NEON hardware does not fully implement the IEEE 754 standard for
floating-point arithmetic (in particular denormal values are
treated as zero), so the use of NEON instructions may lead to a
loss of precision.
You can also set the fpu name at function level by using the
"target("fpu=")" function attributes or pragmas.
-mfp16-format=name
Specify the format of the "__fp16" half-precision floating-point
type. Permissible names are none, ieee, and alternative; the
default is none, in which case the "__fp16" type is not defined.
-mstructure-size-boundary=n
The sizes of all structures and unions are rounded up to a
multiple of the number of bits set by this option. Permissible
values are 8, 32 and 64. The default value varies for different
toolchains. For the COFF targeted toolchain the default value is
8. A value of 64 is only allowed if the underlying ABI supports
it.
Specifying a larger number can produce faster, more efficient
code, but can also increase the size of the program. Different
values are potentially incompatible. Code compiled with one
value cannot necessarily expect to work with code or libraries
compiled with another value, if they exchange information using
structures or unions.
-mabort-on-noreturn
Generate a call to the function "abort" at the end of a
"noreturn" function. It is executed if the function tries to
return.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the
address of the function into a register and then performing a
subroutine call on this register. This switch is needed if the
target function lies outside of the 64-megabyte addressing range
of the offset-based version of subroutine call instruction.
Even if this switch is enabled, not all function calls are turned
into long calls. The heuristic is that static functions,
functions that have the "short_call" attribute, functions that
are inside the scope of a "#pragma no_long_calls" directive, and
functions whose definitions have already been compiled within the
current compilation unit are not turned into long calls. The
exceptions to this rule are that weak function definitions,
functions with the "long_call" attribute or the "section"
attribute, and functions that are within the scope of a "#pragma
long_calls" directive are always turned into long calls.
This feature is not enabled by default. Specifying
-mno-long-calls restores the default behavior, as does placing
the function calls within the scope of a "#pragma long_calls_off"
directive. Note these switches have no effect on how the
compiler generates code to handle function calls via function
pointers.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather
than loading it in the prologue for each function. The runtime
system is responsible for initializing this register with an
appropriate value before execution begins.
-mpic-register=reg
Specify the register to be used for PIC addressing. For standard
PIC base case, the default is any suitable register determined by
compiler. For single PIC base case, the default is R9 if target
is EABI based or stack-checking is enabled, otherwise the default
is R10.
-mpic-data-is-text-relative
Assume that the displacement between the text and data segments
is fixed at static link time. This permits using PC-relative
addressing operations to access data known to be in the data
segment. For non-VxWorks RTP targets, this option is enabled by
default. When disabled on such targets, it will enable
-msingle-pic-base by default.
-mpoke-function-name
Write the name of each function into the text section, directly
preceding the function prologue. The generated code is similar
to this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
When performing a stack backtrace, code can inspect the value of
"pc" stored at "fp + 0". If the trace function then looks at
location "pc - 12" and the top 8 bits are set, then we know that
there is a function name embedded immediately preceding this
location and has length "((pc[-3]) & 0xff000000)".
-mthumb
-marm
Select between generating code that executes in ARM and Thumb
states. The default for most configurations is to generate code
that executes in ARM state, but the default can be changed by
configuring GCC with the --with-mode=state configure option.
You can also override the ARM and Thumb mode for each function by
using the "target("thumb")" and "target("arm")" function
attributes or pragmas.
-mtpcs-frame
Generate a stack frame that is compliant with the Thumb Procedure
Call Standard for all non-leaf functions. (A leaf function is
one that does not call any other functions.) The default is
-mno-tpcs-frame.
-mtpcs-leaf-frame
Generate a stack frame that is compliant with the Thumb Procedure
Call Standard for all leaf functions. (A leaf function is one
that does not call any other functions.) The default is
-mno-apcs-leaf-frame.
-mcallee-super-interworking
Gives all externally visible functions in the file being compiled
an ARM instruction set header which switches to Thumb mode before
executing the rest of the function. This allows these functions
to be called from non-interworking code. This option is not
valid in AAPCS configurations because interworking is enabled by
default.
-mcaller-super-interworking
Allows calls via function pointers (including virtual functions)
to execute correctly regardless of whether the target code has
been compiled for interworking or not. There is a small overhead
in the cost of executing a function pointer if this option is
enabled. This option is not valid in AAPCS configurations
because interworking is enabled by default.
-mtp=name
Specify the access model for the thread local storage pointer.
The valid models are soft, which generates calls to
"__aeabi_read_tp", cp15, which fetches the thread pointer from
"cp15" directly (supported in the arm6k architecture), and auto,
which uses the best available method for the selected processor.
The default setting is auto.
-mtls-dialect=dialect
Specify the dialect to use for accessing thread local storage.
Two dialects are supported---gnu and gnu2. The gnu dialect
selects the original GNU scheme for supporting local and global
dynamic TLS models. The gnu2 dialect selects the GNU descriptor
scheme, which provides better performance for shared libraries.
The GNU descriptor scheme is compatible with the original scheme,
but does require new assembler, linker and library support.
Initial and local exec TLS models are unaffected by this option
and always use the original scheme.
-mword-relocations
Only generate absolute relocations on word-sized values (i.e.
R_ARM_ABS32). This is enabled by default on targets (uClinux,
SymbianOS) where the runtime loader imposes this restriction, and
when -fpic or -fPIC is specified.
-mfix-cortex-m3-ldrd
Some Cortex-M3 cores can cause data corruption when "ldrd"
instructions with overlapping destination and base registers are
used. This option avoids generating these instructions. This
option is enabled by default when -mcpu=cortex-m3 is specified.
-munaligned-access
-mno-unaligned-access
Enables (or disables) reading and writing of 16- and 32- bit
values from addresses that are not 16- or 32- bit aligned. By
default unaligned access is disabled for all pre-ARMv6, all
ARMv6-M and for ARMv8-M Baseline architectures, and enabled for
all other architectures. If unaligned access is not enabled then
words in packed data structures are accessed a byte at a time.
The ARM attribute "Tag_CPU_unaligned_access" is set in the
generated object file to either true or false, depending upon the
setting of this option. If unaligned access is enabled then the
preprocessor symbol "__ARM_FEATURE_UNALIGNED" is also defined.
-mneon-for-64bits
Enables using Neon to handle scalar 64-bits operations. This is
disabled by default since the cost of moving data from core
registers to Neon is high.
-mslow-flash-data
Assume loading data from flash is slower than fetching
instruction. Therefore literal load is minimized for better
performance. This option is only supported when compiling for
ARMv7 M-profile and off by default.
-masm-syntax-unified
Assume inline assembler is using unified asm syntax. The default
is currently off which implies divided syntax. This option has
no impact on Thumb2. However, this may change in future releases
of GCC. Divided syntax should be considered deprecated.
-mrestrict-it
Restricts generation of IT blocks to conform to the rules of
ARMv8. IT blocks can only contain a single 16-bit instruction
from a select set of instructions. This option is on by default
for ARMv8 Thumb mode.
-mprint-tune-info
Print CPU tuning information as comment in assembler file. This
is an option used only for regression testing of the compiler and
not intended for ordinary use in compiling code. This option is
disabled by default.
-mpure-code
Do not allow constant data to be placed in code sections.
Additionally, when compiling for ELF object format give all text
sections the ELF processor-specific section attribute
"SHF_ARM_PURECODE". This option is only available when
generating non-pic code for ARMv7-M targets.
-mcmse
Generate secure code as per the "ARMv8-M Security Extensions:
Requirements on Development Tools Engineering Specification",
which can be found on
<http://infocenter.arm.com/help/topic/com.arm.doc.ecm0359818/ECM0359818_armv8m_security_extensions_reqs_on_dev_tools_1_0.pdf >.
AVR Options
These options are defined for AVR implementations:
-mmcu=mcu
Specify Atmel AVR instruction set architectures (ISA) or MCU
type.
The default for this option is@tie{}avr2.
GCC supports the following AVR devices and ISAs:
"avr2"
"Classic" devices with up to 8@tie{}KiB of program memory.
mcu@tie{}= "attiny22", "attiny26", "at90c8534", "at90s2313",
"at90s2323", "at90s2333", "at90s2343", "at90s4414",
"at90s4433", "at90s4434", "at90s8515", "at90s8535".
"avr25"
"Classic" devices with up to 8@tie{}KiB of program memory and
with the "MOVW" instruction. mcu@tie{}= "ata5272",
"ata6616c", "attiny13", "attiny13a", "attiny2313",
"attiny2313a", "attiny24", "attiny24a", "attiny25",
"attiny261", "attiny261a", "attiny43u", "attiny4313",
"attiny44", "attiny44a", "attiny441", "attiny45",
"attiny461", "attiny461a", "attiny48", "attiny828",
"attiny84", "attiny84a", "attiny841", "attiny85",
"attiny861", "attiny861a", "attiny87", "attiny88",
"at86rf401".
"avr3"
"Classic" devices with 16@tie{}KiB up to 64@tie{}KiB of
program memory. mcu@tie{}= "at43usb355", "at76c711".
"avr31"
"Classic" devices with 128@tie{}KiB of program memory.
mcu@tie{}= "atmega103", "at43usb320".
"avr35"
"Classic" devices with 16@tie{}KiB up to 64@tie{}KiB of
program memory and with the "MOVW" instruction. mcu@tie{}=
"ata5505", "ata6617c", "ata664251", "atmega16u2",
"atmega32u2", "atmega8u2", "attiny1634", "attiny167",
"at90usb162", "at90usb82".
"avr4"
"Enhanced" devices with up to 8@tie{}KiB of program memory.
mcu@tie{}= "ata6285", "ata6286", "ata6289", "ata6612c",
"atmega48", "atmega48a", "atmega48p", "atmega48pa",
"atmega48pb", "atmega8", "atmega8a", "atmega8hva",
"atmega8515", "atmega8535", "atmega88", "atmega88a",
"atmega88p", "atmega88pa", "atmega88pb", "at90pwm1",
"at90pwm2", "at90pwm2b", "at90pwm3", "at90pwm3b",
"at90pwm81".
"avr5"
"Enhanced" devices with 16@tie{}KiB up to 64@tie{}KiB of
program memory. mcu@tie{}= "ata5702m322", "ata5782",
"ata5790", "ata5790n", "ata5791", "ata5795", "ata5831",
"ata6613c", "ata6614q", "ata8210", "ata8510", "atmega16",
"atmega16a", "atmega16hva", "atmega16hva2", "atmega16hvb",
"atmega16hvbrevb", "atmega16m1", "atmega16u4", "atmega161",
"atmega162", "atmega163", "atmega164a", "atmega164p",
"atmega164pa", "atmega165", "atmega165a", "atmega165p",
"atmega165pa", "atmega168", "atmega168a", "atmega168p",
"atmega168pa", "atmega168pb", "atmega169", "atmega169a",
"atmega169p", "atmega169pa", "atmega32", "atmega32a",
"atmega32c1", "atmega32hvb", "atmega32hvbrevb", "atmega32m1",
"atmega32u4", "atmega32u6", "atmega323", "atmega324a",
"atmega324p", "atmega324pa", "atmega325", "atmega325a",
"atmega325p", "atmega325pa", "atmega3250", "atmega3250a",
"atmega3250p", "atmega3250pa", "atmega328", "atmega328p",
"atmega328pb", "atmega329", "atmega329a", "atmega329p",
"atmega329pa", "atmega3290", "atmega3290a", "atmega3290p",
"atmega3290pa", "atmega406", "atmega64", "atmega64a",
"atmega64c1", "atmega64hve", "atmega64hve2", "atmega64m1",
"atmega64rfr2", "atmega640", "atmega644", "atmega644a",
"atmega644p", "atmega644pa", "atmega644rfr2", "atmega645",
"atmega645a", "atmega645p", "atmega6450", "atmega6450a",
"atmega6450p", "atmega649", "atmega649a", "atmega649p",
"atmega6490", "atmega6490a", "atmega6490p", "at90can32",
"at90can64", "at90pwm161", "at90pwm216", "at90pwm316",
"at90scr100", "at90usb646", "at90usb647", "at94k", "m3000".
"avr51"
"Enhanced" devices with 128@tie{}KiB of program memory.
mcu@tie{}= "atmega128", "atmega128a", "atmega128rfa1",
"atmega128rfr2", "atmega1280", "atmega1281", "atmega1284",
"atmega1284p", "atmega1284rfr2", "at90can128", "at90usb1286",
"at90usb1287".
"avr6"
"Enhanced" devices with 3-byte PC, i.e. with more than
128@tie{}KiB of program memory. mcu@tie{}= "atmega256rfr2",
"atmega2560", "atmega2561", "atmega2564rfr2".
"avrxmega2"
"XMEGA" devices with more than 8@tie{}KiB and up to
64@tie{}KiB of program memory. mcu@tie{}= "atxmega16a4",
"atxmega16a4u", "atxmega16c4", "atxmega16d4", "atxmega16e5",
"atxmega32a4", "atxmega32a4u", "atxmega32c3", "atxmega32c4",
"atxmega32d3", "atxmega32d4", "atxmega32e5", "atxmega8e5".
"avrxmega4"
"XMEGA" devices with more than 64@tie{}KiB and up to
128@tie{}KiB of program memory. mcu@tie{}= "atxmega64a3",
"atxmega64a3u", "atxmega64a4u", "atxmega64b1", "atxmega64b3",
"atxmega64c3", "atxmega64d3", "atxmega64d4".
"avrxmega5"
"XMEGA" devices with more than 64@tie{}KiB and up to
128@tie{}KiB of program memory and more than 64@tie{}KiB of
RAM. mcu@tie{}= "atxmega64a1", "atxmega64a1u".
"avrxmega6"
"XMEGA" devices with more than 128@tie{}KiB of program
memory. mcu@tie{}= "atxmega128a3", "atxmega128a3u",
"atxmega128b1", "atxmega128b3", "atxmega128c3",
"atxmega128d3", "atxmega128d4", "atxmega192a3",
"atxmega192a3u", "atxmega192c3", "atxmega192d3",
"atxmega256a3", "atxmega256a3b", "atxmega256a3bu",
"atxmega256a3u", "atxmega256c3", "atxmega256d3",
"atxmega384c3", "atxmega384d3".
"avrxmega7"
"XMEGA" devices with more than 128@tie{}KiB of program memory
and more than 64@tie{}KiB of RAM. mcu@tie{}= "atxmega128a1",
"atxmega128a1u", "atxmega128a4u".
"avrtiny"
"TINY" Tiny core devices with 512@tie{}B up to 4@tie{}KiB of
program memory. mcu@tie{}= "attiny10", "attiny20",
"attiny4", "attiny40", "attiny5", "attiny9".
"avr1"
This ISA is implemented by the minimal AVR core and supported
for assembler only. mcu@tie{}= "attiny11", "attiny12",
"attiny15", "attiny28", "at90s1200".
-mabsdata
Assume that all data in static storage can be accessed by LDS /
STS instructions. This option has only an effect on reduced Tiny
devices like ATtiny40. See also the "absdata" AVR Variable
Attributes,variable attribute.
-maccumulate-args
Accumulate outgoing function arguments and acquire/release the
needed stack space for outgoing function arguments once in
function prologue/epilogue. Without this option, outgoing
arguments are pushed before calling a function and popped
afterwards.
Popping the arguments after the function call can be expensive on
AVR so that accumulating the stack space might lead to smaller
executables because arguments need not be removed from the stack
after such a function call.
This option can lead to reduced code size for functions that
perform several calls to functions that get their arguments on
the stack like calls to printf-like functions.
-mbranch-cost=cost
Set the branch costs for conditional branch instructions to cost.
Reasonable values for cost are small, non-negative integers. The
default branch cost is 0.
-mcall-prologues
Functions prologues/epilogues are expanded as calls to
appropriate subroutines. Code size is smaller.
-mint8
Assume "int" to be 8-bit integer. This affects the sizes of all
types: a "char" is 1 byte, an "int" is 1 byte, a "long" is 2
bytes, and "long long" is 4 bytes. Please note that this option
does not conform to the C standards, but it results in smaller
code size.
-mn-flash=num
Assume that the flash memory has a size of num times 64@tie{}KiB.
-mno-interrupts
Generated code is not compatible with hardware interrupts. Code
size is smaller.
-mrelax
Try to replace "CALL" resp. "JMP" instruction by the shorter
"RCALL" resp. "RJMP" instruction if applicable. Setting -mrelax
just adds the --mlink-relax option to the assembler's command
line and the --relax option to the linker's command line.
Jump relaxing is performed by the linker because jump offsets are
not known before code is located. Therefore, the assembler code
generated by the compiler is the same, but the instructions in
the executable may differ from instructions in the assembler
code.
Relaxing must be turned on if linker stubs are needed, see the
section on "EIND" and linker stubs below.
-mrmw
Assume that the device supports the Read-Modify-Write
instructions "XCH", "LAC", "LAS" and "LAT".
-msp8
Treat the stack pointer register as an 8-bit register, i.e.
assume the high byte of the stack pointer is zero. In general,
you don't need to set this option by hand.
This option is used internally by the compiler to select and
build multilibs for architectures "avr2" and "avr25". These
architectures mix devices with and without "SPH". For any
setting other than -mmcu=avr2 or -mmcu=avr25 the compiler driver
adds or removes this option from the compiler proper's command
line, because the compiler then knows if the device or
architecture has an 8-bit stack pointer and thus no "SPH"
register or not.
-mstrict-X
Use address register "X" in a way proposed by the hardware. This
means that "X" is only used in indirect, post-increment or pre-
decrement addressing.
Without this option, the "X" register may be used in the same way
as "Y" or "Z" which then is emulated by additional instructions.
For example, loading a value with "X+const" addressing with a
small non-negative "const < 64" to a register Rn is performed as
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const
-mtiny-stack
Only change the lower 8@tie{}bits of the stack pointer.
-mfract-convert-truncate
Allow to use truncation instead of rounding towards zero for
fractional fixed-point types.
-nodevicelib
Don't link against AVR-LibC's device specific library
"lib<mcu>.a".
-Waddr-space-convert
Warn about conversions between address spaces in the case where
the resulting address space is not contained in the incoming
address space.
-Wmisspelled-isr
Warn if the ISR is misspelled, i.e. without __vector prefix.
Enabled by default.
"EIND" and Devices with More Than 128 Ki Bytes of Flash
Pointers in the implementation are 16@tie{}bits wide. The address of
a function or label is represented as word address so that indirect
jumps and calls can target any code address in the range of
64@tie{}Ki words.
In order to facilitate indirect jump on devices with more than
128@tie{}Ki bytes of program memory space, there is a special
function register called "EIND" that serves as most significant part
of the target address when "EICALL" or "EIJMP" instructions are used.
Indirect jumps and calls on these devices are handled as follows by
the compiler and are subject to some limitations:
* The compiler never sets "EIND".
* The compiler uses "EIND" implicitly in "EICALL"/"EIJMP"
instructions or might read "EIND" directly in order to emulate an
indirect call/jump by means of a "RET" instruction.
* The compiler assumes that "EIND" never changes during the startup
code or during the application. In particular, "EIND" is not
saved/restored in function or interrupt service routine
prologue/epilogue.
* For indirect calls to functions and computed goto, the linker
generates stubs. Stubs are jump pads sometimes also called
trampolines. Thus, the indirect call/jump jumps to such a stub.
The stub contains a direct jump to the desired address.
* Linker relaxation must be turned on so that the linker generates
the stubs correctly in all situations. See the compiler option
-mrelax and the linker option --relax. There are corner cases
where the linker is supposed to generate stubs but aborts without
relaxation and without a helpful error message.
* The default linker script is arranged for code with "EIND = 0".
If code is supposed to work for a setup with "EIND != 0", a
custom linker script has to be used in order to place the
sections whose name start with ".trampolines" into the segment
where "EIND" points to.
* The startup code from libgcc never sets "EIND". Notice that
startup code is a blend of code from libgcc and AVR-LibC. For
the impact of AVR-LibC on "EIND", see the AVR-LibC user manual
("http://nongnu.org/avr-libc/user-manual/").
* It is legitimate for user-specific startup code to set up "EIND"
early, for example by means of initialization code located in
section ".init3". Such code runs prior to general startup code
that initializes RAM and calls constructors, but after the bit of
startup code from AVR-LibC that sets "EIND" to the segment where
the vector table is located.
#include <avr/io.h>
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}
The "__trampolines_start" symbol is defined in the linker script.
* Stubs are generated automatically by the linker if the following
two conditions are met:
-<The address of a label is taken by means of the "gs" modifier>
(short for generate stubs) like so:
LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))
-<The final location of that label is in a code segment>
outside the segment where the stubs are located.
* The compiler emits such "gs" modifiers for code labels in the
following situations:
-<Taking address of a function or code label.>
-<Computed goto.>
-<If prologue-save function is used, see -mcall-prologues>
command-line option.
-<Switch/case dispatch tables. If you do not want such dispatch>
tables you can specify the -fno-jump-tables command-line
option.
-<C and C++ constructors/destructors called during
startup/shutdown.>
-<If the tools hit a "gs()" modifier explained above.>
* Jumping to non-symbolic addresses like so is not supported:
int main (void)
{
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
}
Instead, a stub has to be set up, i.e. the function has to be
called through a symbol ("func_4" in the example):
int main (void)
{
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
}
and the application be linked with -Wl,--defsym,func_4=0x4.
Alternatively, "func_4" can be defined in the linker script.
Handling of the "RAMPD", "RAMPX", "RAMPY" and "RAMPZ" Special
Function Registers
Some AVR devices support memories larger than the 64@tie{}KiB range
that can be accessed with 16-bit pointers. To access memory
locations outside this 64@tie{}KiB range, the content of a "RAMP"
register is used as high part of the address: The "X", "Y", "Z"
address register is concatenated with the "RAMPX", "RAMPY", "RAMPZ"
special function register, respectively, to get a wide address.
Similarly, "RAMPD" is used together with direct addressing.
* The startup code initializes the "RAMP" special function
registers with zero.
* If a AVR Named Address Spaces,named address space other than
generic or "__flash" is used, then "RAMPZ" is set as needed
before the operation.
* If the device supports RAM larger than 64@tie{}KiB and the
compiler needs to change "RAMPZ" to accomplish an operation,
"RAMPZ" is reset to zero after the operation.
* If the device comes with a specific "RAMP" register, the ISR
prologue/epilogue saves/restores that SFR and initializes it with
zero in case the ISR code might (implicitly) use it.
* RAM larger than 64@tie{}KiB is not supported by GCC for AVR
targets. If you use inline assembler to read from locations
outside the 16-bit address range and change one of the "RAMP"
registers, you must reset it to zero after the access.
AVR Built-in Macros
GCC defines several built-in macros so that the user code can test
for the presence or absence of features. Almost any of the following
built-in macros are deduced from device capabilities and thus
triggered by the -mmcu= command-line option.
For even more AVR-specific built-in macros see AVR Named Address
Spaces and AVR Built-in Functions.
"__AVR_ARCH__"
Build-in macro that resolves to a decimal number that identifies
the architecture and depends on the -mmcu=mcu option. Possible
values are:
2, 25, 3, 31, 35, 4, 5, 51, 6
for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4",
"avr5", "avr51", "avr6",
respectively and
100, 102, 104, 105, 106, 107
for mcu="avrtiny", "avrxmega2", "avrxmega4", "avrxmega5",
"avrxmega6", "avrxmega7", respectively. If mcu specifies a
device, this built-in macro is set accordingly. For example, with
-mmcu=atmega8 the macro is defined to 4.
"__AVR_Device__"
Setting -mmcu=device defines this built-in macro which reflects
the device's name. For example, -mmcu=atmega8 defines the built-
in macro "__AVR_ATmega8__", -mmcu=attiny261a defines
"__AVR_ATtiny261A__", etc.
The built-in macros' names follow the scheme "__AVR_Device__"
where Device is the device name as from the AVR user manual. The
difference between Device in the built-in macro and device in
-mmcu=device is that the latter is always lowercase.
If device is not a device but only a core architecture like
avr51, this macro is not defined.
"__AVR_DEVICE_NAME__"
Setting -mmcu=device defines this built-in macro to the device's
name. For example, with -mmcu=atmega8 the macro is defined to
"atmega8".
If device is not a device but only a core architecture like
avr51, this macro is not defined.
"__AVR_XMEGA__"
The device / architecture belongs to the XMEGA family of devices.
"__AVR_HAVE_ELPM__"
The device has the "ELPM" instruction.
"__AVR_HAVE_ELPMX__"
The device has the "ELPM Rn,Z" and "ELPM Rn,Z+" instructions.
"__AVR_HAVE_MOVW__"
The device has the "MOVW" instruction to perform 16-bit register-
register moves.
"__AVR_HAVE_LPMX__"
The device has the "LPM Rn,Z" and "LPM Rn,Z+" instructions.
"__AVR_HAVE_MUL__"
The device has a hardware multiplier.
"__AVR_HAVE_JMP_CALL__"
The device has the "JMP" and "CALL" instructions. This is the
case for devices with at least 16@tie{}KiB of program memory.
"__AVR_HAVE_EIJMP_EICALL__"
"__AVR_3_BYTE_PC__"
The device has the "EIJMP" and "EICALL" instructions. This is
the case for devices with more than 128@tie{}KiB of program
memory. This also means that the program counter (PC) is
3@tie{}bytes wide.
"__AVR_2_BYTE_PC__"
The program counter (PC) is 2@tie{}bytes wide. This is the case
for devices with up to 128@tie{}KiB of program memory.
"__AVR_HAVE_8BIT_SP__"
"__AVR_HAVE_16BIT_SP__"
The stack pointer (SP) register is treated as 8-bit respectively
16-bit register by the compiler. The definition of these macros
is affected by -mtiny-stack.
"__AVR_HAVE_SPH__"
"__AVR_SP8__"
The device has the SPH (high part of stack pointer) special
function register or has an 8-bit stack pointer, respectively.
The definition of these macros is affected by -mmcu= and in the
cases of -mmcu=avr2 and -mmcu=avr25 also by -msp8.
"__AVR_HAVE_RAMPD__"
"__AVR_HAVE_RAMPX__"
"__AVR_HAVE_RAMPY__"
"__AVR_HAVE_RAMPZ__"
The device has the "RAMPD", "RAMPX", "RAMPY", "RAMPZ" special
function register, respectively.
"__NO_INTERRUPTS__"
This macro reflects the -mno-interrupts command-line option.
"__AVR_ERRATA_SKIP__"
"__AVR_ERRATA_SKIP_JMP_CALL__"
Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit
instructions because of a hardware erratum. Skip instructions
are "SBRS", "SBRC", "SBIS", "SBIC" and "CPSE". The second macro
is only defined if "__AVR_HAVE_JMP_CALL__" is also set.
"__AVR_ISA_RMW__"
The device has Read-Modify-Write instructions (XCH, LAC, LAS and
LAT).
"__AVR_SFR_OFFSET__=offset"
Instructions that can address I/O special function registers
directly like "IN", "OUT", "SBI", etc. may use a different
address as if addressed by an instruction to access RAM like "LD"
or "STS". This offset depends on the device architecture and has
to be subtracted from the RAM address in order to get the
respective I/O@tie{}address.
"__WITH_AVRLIBC__"
The compiler is configured to be used together with AVR-Libc.
See the --with-avrlibc configure option.
Blackfin Options
-mcpu=cpu[-sirevision]
Specifies the name of the target Blackfin processor. Currently,
cpu can be one of bf512, bf514, bf516, bf518, bf522, bf523,
bf524, bf525, bf526, bf527, bf531, bf532, bf533, bf534, bf536,
bf537, bf538, bf539, bf542, bf544, bf547, bf548, bf549, bf542m,
bf544m, bf547m, bf548m, bf549m, bf561, bf592.
The optional sirevision specifies the silicon revision of the
target Blackfin processor. Any workarounds available for the
targeted silicon revision are enabled. If sirevision is none, no
workarounds are enabled. If sirevision is any, all workarounds
for the targeted processor are enabled. The
"__SILICON_REVISION__" macro is defined to two hexadecimal digits
representing the major and minor numbers in the silicon revision.
If sirevision is none, the "__SILICON_REVISION__" is not defined.
If sirevision is any, the "__SILICON_REVISION__" is defined to be
0xffff. If this optional sirevision is not used, GCC assumes the
latest known silicon revision of the targeted Blackfin processor.
GCC defines a preprocessor macro for the specified cpu. For the
bfin-elf toolchain, this option causes the hardware BSP provided
by libgloss to be linked in if -msim is not given.
Without this option, bf532 is used as the processor by default.
Note that support for bf561 is incomplete. For bf561, only the
preprocessor macro is defined.
-msim
Specifies that the program will be run on the simulator. This
causes the simulator BSP provided by libgloss to be linked in.
This option has effect only for bfin-elf toolchain. Certain
other options, such as -mid-shared-library and -mfdpic, imply
-msim.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions.
This avoids the instructions to save, set up and restore frame
pointers and makes an extra register available in leaf functions.
The option -fomit-frame-pointer removes the frame pointer for all
functions, which might make debugging harder.
-mspecld-anomaly
When enabled, the compiler ensures that the generated code does
not contain speculative loads after jump instructions. If this
option is used, "__WORKAROUND_SPECULATIVE_LOADS" is defined.
-mno-specld-anomaly
Don't generate extra code to prevent speculative loads from
occurring.
-mcsync-anomaly
When enabled, the compiler ensures that the generated code does
not contain CSYNC or SSYNC instructions too soon after
conditional branches. If this option is used,
"__WORKAROUND_SPECULATIVE_SYNCS" is defined.
-mno-csync-anomaly
Don't generate extra code to prevent CSYNC or SSYNC instructions
from occurring too soon after a conditional branch.
-mlow-64k
When enabled, the compiler is free to take advantage of the
knowledge that the entire program fits into the low 64k of
memory.
-mno-low-64k
Assume that the program is arbitrarily large. This is the
default.
-mstack-check-l1
Do stack checking using information placed into L1 scratchpad
memory by the uClinux kernel.
-mid-shared-library
Generate code that supports shared libraries via the library ID
method. This allows for execute in place and shared libraries in
an environment without virtual memory management. This option
implies -fPIC. With a bfin-elf target, this option implies
-msim.
-mno-id-shared-library
Generate code that doesn't assume ID-based shared libraries are
being used. This is the default.
-mleaf-id-shared-library
Generate code that supports shared libraries via the library ID
method, but assumes that this library or executable won't link
against any other ID shared libraries. That allows the compiler
to use faster code for jumps and calls.
-mno-leaf-id-shared-library
Do not assume that the code being compiled won't link against any
ID shared libraries. Slower code is generated for jump and call
insns.
-mshared-library-id=n
Specifies the identification number of the ID-based shared
library being compiled. Specifying a value of 0 generates more
compact code; specifying other values forces the allocation of
that number to the current library but is no more space- or time-
efficient than omitting this option.
-msep-data
Generate code that allows the data segment to be located in a
different area of memory from the text segment. This allows for
execute in place in an environment without virtual memory
management by eliminating relocations against the text section.
-mno-sep-data
Generate code that assumes that the data segment follows the text
segment. This is the default.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the
address of the function into a register and then performing a
subroutine call on this register. This switch is needed if the
target function lies outside of the 24-bit addressing range of
the offset-based version of subroutine call instruction.
This feature is not enabled by default. Specifying
-mno-long-calls restores the default behavior. Note these
switches have no effect on how the compiler generates code to
handle function calls via function pointers.
-mfast-fp
Link with the fast floating-point library. This library relaxes
some of the IEEE floating-point standard's rules for checking
inputs against Not-a-Number (NAN), in the interest of
performance.
-minline-plt
Enable inlining of PLT entries in function calls to functions
that are not known to bind locally. It has no effect without
-mfdpic.
-mmulticore
Build a standalone application for multicore Blackfin processors.
This option causes proper start files and link scripts supporting
multicore to be used, and defines the macro "__BFIN_MULTICORE".
It can only be used with -mcpu=bf561[-sirevision].
This option can be used with -mcorea or -mcoreb, which selects
the one-application-per-core programming model. Without -mcorea
or -mcoreb, the single-application/dual-core programming model is
used. In this model, the main function of Core B should be named
as "coreb_main".
If this option is not used, the single-core application
programming model is used.
-mcorea
Build a standalone application for Core A of BF561 when using the
one-application-per-core programming model. Proper start files
and link scripts are used to support Core A, and the macro
"__BFIN_COREA" is defined. This option can only be used in
conjunction with -mmulticore.
-mcoreb
Build a standalone application for Core B of BF561 when using the
one-application-per-core programming model. Proper start files
and link scripts are used to support Core B, and the macro
"__BFIN_COREB" is defined. When this option is used, "coreb_main"
should be used instead of "main". This option can only be used
in conjunction with -mmulticore.
-msdram
Build a standalone application for SDRAM. Proper start files and
link scripts are used to put the application into SDRAM, and the
macro "__BFIN_SDRAM" is defined. The loader should initialize
SDRAM before loading the application.
-micplb
Assume that ICPLBs are enabled at run time. This has an effect
on certain anomaly workarounds. For Linux targets, the default
is to assume ICPLBs are enabled; for standalone applications the
default is off.
C6X Options
-march=name
This specifies the name of the target architecture. GCC uses
this name to determine what kind of instructions it can emit when
generating assembly code. Permissible names are: c62x, c64x,
c64x+, c67x, c67x+, c674x.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target. This is the default.
-msim
Choose startup files and linker script suitable for the
simulator.
-msdata=default
Put small global and static data in the ".neardata" section,
which is pointed to by register "B14". Put small uninitialized
global and static data in the ".bss" section, which is adjacent
to the ".neardata" section. Put small read-only data into the
".rodata" section. The corresponding sections used for large
pieces of data are ".fardata", ".far" and ".const".
-msdata=all
Put all data, not just small objects, into the sections reserved
for small data, and use addressing relative to the "B14" register
to access them.
-msdata=none
Make no use of the sections reserved for small data, and use
absolute addresses to access all data. Put all initialized
global and static data in the ".fardata" section, and all
uninitialized data in the ".far" section. Put all constant data
into the ".const" section.
CRIS Options
These options are defined specifically for the CRIS ports.
-march=architecture-type
-mcpu=architecture-type
Generate code for the specified architecture. The choices for
architecture-type are v3, v8 and v10 for respectively ETRAX 4,
ETRAX 100, and ETRAX 100 LX. Default is v0 except for cris-axis-
linux-gnu, where the default is v10.
-mtune=architecture-type
Tune to architecture-type everything applicable about the
generated code, except for the ABI and the set of available
instructions. The choices for architecture-type are the same as
for -march=architecture-type.
-mmax-stack-frame=n
Warn when the stack frame of a function exceeds n bytes.
-metrax4
-metrax100
The options -metrax4 and -metrax100 are synonyms for -march=v3
and -march=v8 respectively.
-mmul-bug-workaround
-mno-mul-bug-workaround
Work around a bug in the "muls" and "mulu" instructions for CPU
models where it applies. This option is active by default.
-mpdebug
Enable CRIS-specific verbose debug-related information in the
assembly code. This option also has the effect of turning off
the #NO_APP formatted-code indicator to the assembler at the
beginning of the assembly file.
-mcc-init
Do not use condition-code results from previous instruction;
always emit compare and test instructions before use of condition
codes.
-mno-side-effects
Do not emit instructions with side effects in addressing modes
other than post-increment.
-mstack-align
-mno-stack-align
-mdata-align
-mno-data-align
-mconst-align
-mno-const-align
These options (no- options) arrange (eliminate arrangements) for
the stack frame, individual data and constants to be aligned for
the maximum single data access size for the chosen CPU model.
The default is to arrange for 32-bit alignment. ABI details such
as structure layout are not affected by these options.
-m32-bit
-m16-bit
-m8-bit
Similar to the stack- data- and const-align options above, these
options arrange for stack frame, writable data and constants to
all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit
alignment.
-mno-prologue-epilogue
-mprologue-epilogue
With -mno-prologue-epilogue, the normal function prologue and
epilogue which set up the stack frame are omitted and no return
instructions or return sequences are generated in the code. Use
this option only together with visual inspection of the compiled
code: no warnings or errors are generated when call-saved
registers must be saved, or storage for local variables needs to
be allocated.
-mno-gotplt
-mgotplt
With -fpic and -fPIC, don't generate (do generate) instruction
sequences that load addresses for functions from the PLT part of
the GOT rather than (traditional on other architectures) calls to
the PLT. The default is -mgotplt.
-melf
Legacy no-op option only recognized with the cris-axis-elf and
cris-axis-linux-gnu targets.
-mlinux
Legacy no-op option only recognized with the cris-axis-linux-gnu
target.
-sim
This option, recognized for the cris-axis-elf, arranges to link
with input-output functions from a simulator library. Code,
initialized data and zero-initialized data are allocated
consecutively.
-sim2
Like -sim, but pass linker options to locate initialized data at
0x40000000 and zero-initialized data at 0x80000000.
CR16 Options
These options are defined specifically for the CR16 ports.
-mmac
Enable the use of multiply-accumulate instructions. Disabled by
default.
-mcr16cplus
-mcr16c
Generate code for CR16C or CR16C+ architecture. CR16C+
architecture is default.
-msim
Links the library libsim.a which is in compatible with simulator.
Applicable to ELF compiler only.
-mint32
Choose integer type as 32-bit wide.
-mbit-ops
Generates "sbit"/"cbit" instructions for bit manipulations.
-mdata-model=model
Choose a data model. The choices for model are near, far or
medium. medium is default. However, far is not valid with
-mcr16c, as the CR16C architecture does not support the far data
model.
Darwin Options
These options are defined for all architectures running the Darwin
operating system.
FSF GCC on Darwin does not create "fat" object files; it creates an
object file for the single architecture that GCC was built to target.
Apple's GCC on Darwin does create "fat" files if multiple -arch
options are used; it does so by running the compiler or linker
multiple times and joining the results together with lipo.
The subtype of the file created (like ppc7400 or ppc970 or i686) is
determined by the flags that specify the ISA that GCC is targeting,
like -mcpu or -march. The -force_cpusubtype_ALL option can be used
to override this.
The Darwin tools vary in their behavior when presented with an ISA
mismatch. The assembler, as, only permits instructions to be used
that are valid for the subtype of the file it is generating, so you
cannot put 64-bit instructions in a ppc750 object file. The linker
for shared libraries, /usr/bin/libtool, fails and prints an error if
asked to create a shared library with a less restrictive subtype than
its input files (for instance, trying to put a ppc970 object file in
a ppc7400 library). The linker for executables, ld, quietly gives
the executable the most restrictive subtype of any of its input
files.
-Fdir
Add the framework directory dir to the head of the list of
directories to be searched for header files. These directories
are interleaved with those specified by -I options and are
scanned in a left-to-right order.
A framework directory is a directory with frameworks in it. A
framework is a directory with a Headers and/or PrivateHeaders
directory contained directly in it that ends in .framework. The
name of a framework is the name of this directory excluding the
.framework. Headers associated with the framework are found in
one of those two directories, with Headers being searched first.
A subframework is a framework directory that is in a framework's
Frameworks directory. Includes of subframework headers can only
appear in a header of a framework that contains the subframework,
or in a sibling subframework header. Two subframeworks are
siblings if they occur in the same framework. A subframework
should not have the same name as a framework; a warning is issued
if this is violated. Currently a subframework cannot have
subframeworks; in the future, the mechanism may be extended to
support this. The standard frameworks can be found in
/System/Library/Frameworks and /Library/Frameworks. An example
include looks like "#include <Framework/header.h>", where
Framework denotes the name of the framework and header.h is found
in the PrivateHeaders or Headers directory.
-iframeworkdir
Like -F except the directory is a treated as a system directory.
The main difference between this -iframework and -F is that with
-iframework the compiler does not warn about constructs contained
within header files found via dir. This option is valid only for
the C family of languages.
-gused
Emit debugging information for symbols that are used. For stabs
debugging format, this enables -feliminate-unused-debug-symbols.
This is by default ON.
-gfull
Emit debugging information for all symbols and types.
-mmacosx-version-min=version
The earliest version of MacOS X that this executable will run on
is version. Typical values of version include 10.1, 10.2, and
10.3.9.
If the compiler was built to use the system's headers by default,
then the default for this option is the system version on which
the compiler is running, otherwise the default is to make choices
that are compatible with as many systems and code bases as
possible.
-mkernel
Enable kernel development mode. The -mkernel option sets
-static, -fno-common, -fno-use-cxa-atexit, -fno-exceptions,
-fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti
where applicable. This mode also sets -mno-altivec,
-msoft-float, -fno-builtin and -mlong-branch for PowerPC targets.
-mone-byte-bool
Override the defaults for "bool" so that "sizeof(bool)==1". By
default "sizeof(bool)" is 4 when compiling for Darwin/PowerPC and
1 when compiling for Darwin/x86, so this option has no effect on
x86.
Warning: The -mone-byte-bool switch causes GCC to generate code
that is not binary compatible with code generated without that
switch. Using this switch may require recompiling all other
modules in a program, including system libraries. Use this
switch to conform to a non-default data model.
-mfix-and-continue
-ffix-and-continue
-findirect-data
Generate code suitable for fast turnaround development, such as
to allow GDB to dynamically load .o files into already-running
programs. -findirect-data and -ffix-and-continue are provided
for backwards compatibility.
-all_load
Loads all members of static archive libraries. See man ld(1) for
more information.
-arch_errors_fatal
Cause the errors having to do with files that have the wrong
architecture to be fatal.
-bind_at_load
Causes the output file to be marked such that the dynamic linker
will bind all undefined references when the file is loaded or
launched.
-bundle
Produce a Mach-o bundle format file. See man ld(1) for more
information.
-bundle_loader executable
This option specifies the executable that will load the build
output file being linked. See man ld(1) for more information.
-dynamiclib
When passed this option, GCC produces a dynamic library instead
of an executable when linking, using the Darwin libtool command.
-force_cpusubtype_ALL
This causes GCC's output file to have the ALL subtype, instead of
one controlled by the -mcpu or -march option.
-allowable_client client_name
-client_name
-compatibility_version
-current_version
-dead_strip
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-exported_symbols_list
-filelist
-flat_namespace
-force_flat_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-no_dead_strip_inits_and_terms
-nofixprebinding
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-segaddr
-segs_read_only_addr
-segs_read_write_addr
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded
These options are passed to the Darwin linker. The Darwin linker
man page describes them in detail.
DEC Alpha Options
These -m options are defined for the DEC Alpha implementations:
-mno-soft-float
-msoft-float
Use (do not use) the hardware floating-point instructions for
floating-point operations. When -msoft-float is specified,
functions in libgcc.a are used to perform floating-point
operations. Unless they are replaced by routines that emulate
the floating-point operations, or compiled in such a way as to
call such emulations routines, these routines issue floating-
point operations. If you are compiling for an Alpha without
floating-point operations, you must ensure that the library is
built so as not to call them.
Note that Alpha implementations without floating-point operations
are required to have floating-point registers.
-mfp-reg
-mno-fp-regs
Generate code that uses (does not use) the floating-point
register set. -mno-fp-regs implies -msoft-float. If the
floating-point register set is not used, floating-point operands
are passed in integer registers as if they were integers and
floating-point results are passed in $0 instead of $f0. This is
a non-standard calling sequence, so any function with a floating-
point argument or return value called by code compiled with
-mno-fp-regs must also be compiled with that option.
A typical use of this option is building a kernel that does not
use, and hence need not save and restore, any floating-point
registers.
-mieee
The Alpha architecture implements floating-point hardware
optimized for maximum performance. It is mostly compliant with
the IEEE floating-point standard. However, for full compliance,
software assistance is required. This option generates code
fully IEEE-compliant code except that the inexact-flag is not
maintained (see below). If this option is turned on, the
preprocessor macro "_IEEE_FP" is defined during compilation. The
resulting code is less efficient but is able to correctly support
denormalized numbers and exceptional IEEE values such as not-a-
number and plus/minus infinity. Other Alpha compilers call this
option -ieee_with_no_inexact.
-mieee-with-inexact
This is like -mieee except the generated code also maintains the
IEEE inexact-flag. Turning on this option causes the generated
code to implement fully-compliant IEEE math. In addition to
"_IEEE_FP", "_IEEE_FP_EXACT" is defined as a preprocessor macro.
On some Alpha implementations the resulting code may execute
significantly slower than the code generated by default. Since
there is very little code that depends on the inexact-flag, you
should normally not specify this option. Other Alpha compilers
call this option -ieee_with_inexact.
-mfp-trap-mode=trap-mode
This option controls what floating-point related traps are
enabled. Other Alpha compilers call this option -fptm trap-mode.
The trap mode can be set to one of four values:
n This is the default (normal) setting. The only traps that
are enabled are the ones that cannot be disabled in software
(e.g., division by zero trap).
u In addition to the traps enabled by n, underflow traps are
enabled as well.
su Like u, but the instructions are marked to be safe for
software completion (see Alpha architecture manual for
details).
sui Like su, but inexact traps are enabled as well.
-mfp-rounding-mode=rounding-mode
Selects the IEEE rounding mode. Other Alpha compilers call this
option -fprm rounding-mode. The rounding-mode can be one of:
n Normal IEEE rounding mode. Floating-point numbers are
rounded towards the nearest machine number or towards the
even machine number in case of a tie.
m Round towards minus infinity.
c Chopped rounding mode. Floating-point numbers are rounded
towards zero.
d Dynamic rounding mode. A field in the floating-point control
register (fpcr, see Alpha architecture reference manual)
controls the rounding mode in effect. The C library
initializes this register for rounding towards plus infinity.
Thus, unless your program modifies the fpcr, d corresponds to
round towards plus infinity.
-mtrap-precision=trap-precision
In the Alpha architecture, floating-point traps are imprecise.
This means without software assistance it is impossible to
recover from a floating trap and program execution normally needs
to be terminated. GCC can generate code that can assist
operating system trap handlers in determining the exact location
that caused a floating-point trap. Depending on the requirements
of an application, different levels of precisions can be
selected:
p Program precision. This option is the default and means a
trap handler can only identify which program caused a
floating-point exception.
f Function precision. The trap handler can determine the
function that caused a floating-point exception.
i Instruction precision. The trap handler can determine the
exact instruction that caused a floating-point exception.
Other Alpha compilers provide the equivalent options called
-scope_safe and -resumption_safe.
-mieee-conformant
This option marks the generated code as IEEE conformant. You
must not use this option unless you also specify
-mtrap-precision=i and either -mfp-trap-mode=su or
-mfp-trap-mode=sui. Its only effect is to emit the line .eflag
48 in the function prologue of the generated assembly file.
-mbuild-constants
Normally GCC examines a 32- or 64-bit integer constant to see if
it can construct it from smaller constants in two or three
instructions. If it cannot, it outputs the constant as a literal
and generates code to load it from the data segment at run time.
Use this option to require GCC to construct all integer constants
using code, even if it takes more instructions (the maximum is
six).
You typically use this option to build a shared library dynamic
loader. Itself a shared library, it must relocate itself in
memory before it can find the variables and constants in its own
data segment.
-mbwx
-mno-bwx
-mcix
-mno-cix
-mfix
-mno-fix
-mmax
-mno-max
Indicate whether GCC should generate code to use the optional
BWX, CIX, FIX and MAX instruction sets. The default is to use
the instruction sets supported by the CPU type specified via
-mcpu= option or that of the CPU on which GCC was built if none
is specified.
-mfloat-vax
-mfloat-ieee
Generate code that uses (does not use) VAX F and G floating-point
arithmetic instead of IEEE single and double precision.
-mexplicit-relocs
-mno-explicit-relocs
Older Alpha assemblers provided no way to generate symbol
relocations except via assembler macros. Use of these macros
does not allow optimal instruction scheduling. GNU binutils as
of version 2.12 supports a new syntax that allows the compiler to
explicitly mark which relocations should apply to which
instructions. This option is mostly useful for debugging, as GCC
detects the capabilities of the assembler when it is built and
sets the default accordingly.
-msmall-data
-mlarge-data
When -mexplicit-relocs is in effect, static data is accessed via
gp-relative relocations. When -msmall-data is used, objects 8
bytes long or smaller are placed in a small data area (the
".sdata" and ".sbss" sections) and are accessed via 16-bit
relocations off of the $gp register. This limits the size of the
small data area to 64KB, but allows the variables to be directly
accessed via a single instruction.
The default is -mlarge-data. With this option the data area is
limited to just below 2GB. Programs that require more than 2GB
of data must use "malloc" or "mmap" to allocate the data in the
heap instead of in the program's data segment.
When generating code for shared libraries, -fpic implies
-msmall-data and -fPIC implies -mlarge-data.
-msmall-text
-mlarge-text
When -msmall-text is used, the compiler assumes that the code of
the entire program (or shared library) fits in 4MB, and is thus
reachable with a branch instruction. When -msmall-data is used,
the compiler can assume that all local symbols share the same $gp
value, and thus reduce the number of instructions required for a
function call from 4 to 1.
The default is -mlarge-text.
-mcpu=cpu_type
Set the instruction set and instruction scheduling parameters for
machine type cpu_type. You can specify either the EV style name
or the corresponding chip number. GCC supports scheduling
parameters for the EV4, EV5 and EV6 family of processors and
chooses the default values for the instruction set from the
processor you specify. If you do not specify a processor type,
GCC defaults to the processor on which the compiler was built.
Supported values for cpu_type are
ev4
ev45
21064
Schedules as an EV4 and has no instruction set extensions.
ev5
21164
Schedules as an EV5 and has no instruction set extensions.
ev56
21164a
Schedules as an EV5 and supports the BWX extension.
pca56
21164pc
21164PC
Schedules as an EV5 and supports the BWX and MAX extensions.
ev6
21264
Schedules as an EV6 and supports the BWX, FIX, and MAX
extensions.
ev67
21264a
Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX
extensions.
Native toolchains also support the value native, which selects
the best architecture option for the host processor.
-mcpu=native has no effect if GCC does not recognize the
processor.
-mtune=cpu_type
Set only the instruction scheduling parameters for machine type
cpu_type. The instruction set is not changed.
Native toolchains also support the value native, which selects
the best architecture option for the host processor.
-mtune=native has no effect if GCC does not recognize the
processor.
-mmemory-latency=time
Sets the latency the scheduler should assume for typical memory
references as seen by the application. This number is highly
dependent on the memory access patterns used by the application
and the size of the external cache on the machine.
Valid options for time are
number
A decimal number representing clock cycles.
L1
L2
L3
main
The compiler contains estimates of the number of clock cycles
for "typical" EV4 & EV5 hardware for the Level 1, 2 & 3
caches (also called Dcache, Scache, and Bcache), as well as
to main memory. Note that L3 is only valid for EV5.
FR30 Options
These options are defined specifically for the FR30 port.
-msmall-model
Use the small address space model. This can produce smaller
code, but it does assume that all symbolic values and addresses
fit into a 20-bit range.
-mno-lsim
Assume that runtime support has been provided and so there is no
need to include the simulator library (libsim.a) on the linker
command line.
FT32 Options
These options are defined specifically for the FT32 port.
-msim
Specifies that the program will be run on the simulator. This
causes an alternate runtime startup and library to be linked.
You must not use this option when generating programs that will
run on real hardware; you must provide your own runtime library
for whatever I/O functions are needed.
-mlra
Enable Local Register Allocation. This is still experimental for
FT32, so by default the compiler uses standard reload.
-mnodiv
Do not use div and mod instructions.
FRV Options
-mgpr-32
Only use the first 32 general-purpose registers.
-mgpr-64
Use all 64 general-purpose registers.
-mfpr-32
Use only the first 32 floating-point registers.
-mfpr-64
Use all 64 floating-point registers.
-mhard-float
Use hardware instructions for floating-point operations.
-msoft-float
Use library routines for floating-point operations.
-malloc-cc
Dynamically allocate condition code registers.
-mfixed-cc
Do not try to dynamically allocate condition code registers, only
use "icc0" and "fcc0".
-mdword
Change ABI to use double word insns.
-mno-dword
Do not use double word instructions.
-mdouble
Use floating-point double instructions.
-mno-double
Do not use floating-point double instructions.
-mmedia
Use media instructions.
-mno-media
Do not use media instructions.
-mmuladd
Use multiply and add/subtract instructions.
-mno-muladd
Do not use multiply and add/subtract instructions.
-mfdpic
Select the FDPIC ABI, which uses function descriptors to
represent pointers to functions. Without any PIC/PIE-related
options, it implies -fPIE. With -fpic or -fpie, it assumes GOT
entries and small data are within a 12-bit range from the GOT
base address; with -fPIC or -fPIE, GOT offsets are computed with
32 bits. With a bfin-elf target, this option implies -msim.
-minline-plt
Enable inlining of PLT entries in function calls to functions
that are not known to bind locally. It has no effect without
-mfdpic. It's enabled by default if optimizing for speed and
compiling for shared libraries (i.e., -fPIC or -fpic), or when an
optimization option such as -O3 or above is present in the
command line.
-mTLS
Assume a large TLS segment when generating thread-local code.
-mtls
Do not assume a large TLS segment when generating thread-local
code.
-mgprel-ro
Enable the use of "GPREL" relocations in the FDPIC ABI for data
that is known to be in read-only sections. It's enabled by
default, except for -fpic or -fpie: even though it may help make
the global offset table smaller, it trades 1 instruction for 4.
With -fPIC or -fPIE, it trades 3 instructions for 4, one of which
may be shared by multiple symbols, and it avoids the need for a
GOT entry for the referenced symbol, so it's more likely to be a
win. If it is not, -mno-gprel-ro can be used to disable it.
-multilib-library-pic
Link with the (library, not FD) pic libraries. It's implied by
-mlibrary-pic, as well as by -fPIC and -fpic without -mfdpic.
You should never have to use it explicitly.
-mlinked-fp
Follow the EABI requirement of always creating a frame pointer
whenever a stack frame is allocated. This option is enabled by
default and can be disabled with -mno-linked-fp.
-mlong-calls
Use indirect addressing to call functions outside the current
compilation unit. This allows the functions to be placed
anywhere within the 32-bit address space.
-malign-labels
Try to align labels to an 8-byte boundary by inserting NOPs into
the previous packet. This option only has an effect when VLIW
packing is enabled. It doesn't create new packets; it merely
adds NOPs to existing ones.
-mlibrary-pic
Generate position-independent EABI code.
-macc-4
Use only the first four media accumulator registers.
-macc-8
Use all eight media accumulator registers.
-mpack
Pack VLIW instructions.
-mno-pack
Do not pack VLIW instructions.
-mno-eflags
Do not mark ABI switches in e_flags.
-mcond-move
Enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will likely
be removed in a future version.
-mno-cond-move
Disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will likely
be removed in a future version.
-mscc
Enable the use of conditional set instructions (default).
This switch is mainly for debugging the compiler and will likely
be removed in a future version.
-mno-scc
Disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and will likely
be removed in a future version.
-mcond-exec
Enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and will likely
be removed in a future version.
-mno-cond-exec
Disable the use of conditional execution.
This switch is mainly for debugging the compiler and will likely
be removed in a future version.
-mvliw-branch
Run a pass to pack branches into VLIW instructions (default).
This switch is mainly for debugging the compiler and will likely
be removed in a future version.
-mno-vliw-branch
Do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will likely
be removed in a future version.
-mmulti-cond-exec
Enable optimization of "&&" and "||" in conditional execution
(default).
This switch is mainly for debugging the compiler and will likely
be removed in a future version.
-mno-multi-cond-exec
Disable optimization of "&&" and "||" in conditional execution.
This switch is mainly for debugging the compiler and will likely
be removed in a future version.
-mnested-cond-exec
Enable nested conditional execution optimizations (default).
This switch is mainly for debugging the compiler and will likely
be removed in a future version.
-mno-nested-cond-exec
Disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will likely
be removed in a future version.
-moptimize-membar
This switch removes redundant "membar" instructions from the
compiler-generated code. It is enabled by default.
-mno-optimize-membar
This switch disables the automatic removal of redundant "membar"
instructions from the generated code.
-mtomcat-stats
Cause gas to print out tomcat statistics.
-mcpu=cpu
Select the processor type for which to generate code. Possible
values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300
and simple.
GNU/Linux Options
These -m options are defined for GNU/Linux targets:
-mglibc
Use the GNU C library. This is the default except on
*-*-linux-*uclibc*, *-*-linux-*musl* and *-*-linux-*android*
targets.
-muclibc
Use uClibc C library. This is the default on *-*-linux-*uclibc*
targets.
-mmusl
Use the musl C library. This is the default on *-*-linux-*musl*
targets.
-mbionic
Use Bionic C library. This is the default on *-*-linux-*android*
targets.
-mandroid
Compile code compatible with Android platform. This is the
default on *-*-linux-*android* targets.
When compiling, this option enables -mbionic, -fPIC,
-fno-exceptions and -fno-rtti by default. When linking, this
option makes the GCC driver pass Android-specific options to the
linker. Finally, this option causes the preprocessor macro
"__ANDROID__" to be defined.
-tno-android-cc
Disable compilation effects of -mandroid, i.e., do not enable
-mbionic, -fPIC, -fno-exceptions and -fno-rtti by default.
-tno-android-ld
Disable linking effects of -mandroid, i.e., pass standard Linux
linking options to the linker.
H8/300 Options
These -m options are defined for the H8/300 implementations:
-mrelax
Shorten some address references at link time, when possible; uses
the linker option -relax.
-mh Generate code for the H8/300H.
-ms Generate code for the H8S.
-mn Generate code for the H8S and H8/300H in the normal mode. This
switch must be used either with -mh or -ms.
-ms2600
Generate code for the H8S/2600. This switch must be used with
-ms.
-mexr
Extended registers are stored on stack before execution of
function with monitor attribute. Default option is -mexr. This
option is valid only for H8S targets.
-mno-exr
Extended registers are not stored on stack before execution of
function with monitor attribute. Default option is -mno-exr.
This option is valid only for H8S targets.
-mint32
Make "int" data 32 bits by default.
-malign-300
On the H8/300H and H8S, use the same alignment rules as for the
H8/300. The default for the H8/300H and H8S is to align longs
and floats on 4-byte boundaries. -malign-300 causes them to be
aligned on 2-byte boundaries. This option has no effect on the
H8/300.
HPPA Options
These -m options are defined for the HPPA family of computers:
-march=architecture-type
Generate code for the specified architecture. The choices for
architecture-type are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0 for
PA 2.0 processors. Refer to /usr/lib/sched.models on an HP-UX
system to determine the proper architecture option for your
machine. Code compiled for lower numbered architectures runs on
higher numbered architectures, but not the other way around.
-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.
-mcaller-copies
The caller copies function arguments passed by hidden reference.
This option should be used with care as it is not compatible with
the default 32-bit runtime. However, only aggregates larger than
eight bytes are passed by hidden reference and the option
provides better compatibility with OpenMP.
-mjump-in-delay
This option is ignored and provided for compatibility purposes
only.
-mdisable-fpregs
Prevent floating-point registers from being used in any manner.
This is necessary for compiling kernels that perform lazy context
switching of floating-point registers. If you use this option
and attempt to perform floating-point operations, the compiler
aborts.
-mdisable-indexing
Prevent the compiler from using indexing address modes. This
avoids some rather obscure problems when compiling MIG generated
code under MACH.
-mno-space-regs
Generate code that assumes the target has no space registers.
This allows GCC to generate faster indirect calls and use
unscaled index address modes.
Such code is suitable for level 0 PA systems and kernels.
-mfast-indirect-calls
Generate code that assumes calls never cross space boundaries.
This allows GCC to emit code that performs faster indirect calls.
This option does not work in the presence of shared libraries or
nested functions.
-mfixed-range=register-range
Generate code treating the given register range as fixed
registers. A fixed register is one that the register allocator
cannot use. This is useful when compiling kernel code. A
register range is specified as two registers separated by a dash.
Multiple register ranges can be specified separated by a comma.
-mlong-load-store
Generate 3-instruction load and store sequences as sometimes
required by the HP-UX 10 linker. This is equivalent to the +k
option to the HP compilers.
-mportable-runtime
Use the portable calling conventions proposed by HP for ELF
systems.
-mgas
Enable the use of assembler directives only GAS understands.
-mschedule=cpu-type
Schedule code according to the constraints for the machine type
cpu-type. The choices for cpu-type are 700 7100, 7100LC, 7200,
7300 and 8000. Refer to /usr/lib/sched.models on an HP-UX system
to determine the proper scheduling option for your machine. The
default scheduling is 8000.
-mlinker-opt
Enable the optimization pass in the HP-UX linker. Note this
makes symbolic debugging impossible. It also triggers a bug in
the HP-UX 8 and HP-UX 9 linkers in which they give bogus error
messages when linking some programs.
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all HPPA
targets. Normally the facilities of the machine's usual C
compiler are used, but this cannot be done directly in cross-
compilation. You must make your own arrangements to provide
suitable library functions for cross-compilation.
-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for this
to work.
-msio
Generate the predefine, "_SIO", for server IO. The default is
-mwsio. This generates the predefines, "__hp9000s700",
"__hp9000s700__" and "_WSIO", for workstation IO. These options
are available under HP-UX and HI-UX.
-mgnu-ld
Use options specific to GNU ld. This passes -shared to ld when
building a shared library. It is the default when GCC is
configured, explicitly or implicitly, with the GNU linker. This
option does not affect which ld is called; it only changes what
parameters are passed to that ld. The ld that is called is
determined by the --with-ld configure option, GCC's program
search path, and finally by the user's PATH. The linker used by
GCC can be printed using which `gcc -print-prog-name=ld`. This
option is only available on the 64-bit HP-UX GCC, i.e. configured
with hppa*64*-*-hpux*.
-mhp-ld
Use options specific to HP ld. This passes -b to ld when
building a shared library and passes +Accept TypeMismatch to ld
on all links. It is the default when GCC is configured,
explicitly or implicitly, with the HP linker. This option does
not affect which ld is called; it only changes what parameters
are passed to that ld. The ld that is called is determined by
the --with-ld configure option, GCC's program search path, and
finally by the user's PATH. The linker used by GCC can be
printed using which `gcc -print-prog-name=ld`. This option is
only available on the 64-bit HP-UX GCC, i.e. configured with
hppa*64*-*-hpux*.
-mlong-calls
Generate code that uses long call sequences. This ensures that a
call is always able to reach linker generated stubs. The default
is to generate long calls only when the distance from the call
site to the beginning of the function or translation unit, as the
case may be, exceeds a predefined limit set by the branch type
being used. The limits for normal calls are 7,600,000 and
240,000 bytes, respectively for the PA 2.0 and PA 1.X
architectures. Sibcalls are always limited at 240,000 bytes.
Distances are measured from the beginning of functions when using
the -ffunction-sections option, or when using the -mgas and
-mno-portable-runtime options together under HP-UX with the SOM
linker.
It is normally not desirable to use this option as it degrades
performance. However, it may be useful in large applications,
particularly when partial linking is used to build the
application.
The types of long calls used depends on the capabilities of the
assembler and linker, and the type of code being generated. The
impact on systems that support long absolute calls, and long pic
symbol-difference or pc-relative calls should be relatively
small. However, an indirect call is used on 32-bit ELF systems
in pic code and it is quite long.
-munix=unix-std
Generate compiler predefines and select a startfile for the
specified UNIX standard. The choices for unix-std are 93, 95 and
98. 93 is supported on all HP-UX versions. 95 is available on
HP-UX 10.10 and later. 98 is available on HP-UX 11.11 and later.
The default values are 93 for HP-UX 10.00, 95 for HP-UX 10.10
though to 11.00, and 98 for HP-UX 11.11 and later.
-munix=93 provides the same predefines as GCC 3.3 and 3.4.
-munix=95 provides additional predefines for "XOPEN_UNIX" and
"_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o. -munix=98
provides additional predefines for "_XOPEN_UNIX",
"_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and
"_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.
It is important to note that this option changes the interfaces
for various library routines. It also affects the operational
behavior of the C library. Thus, extreme care is needed in using
this option.
Library code that is intended to operate with more than one UNIX
standard must test, set and restore the variable
"__xpg4_extended_mask" as appropriate. Most GNU software doesn't
provide this capability.
-nolibdld
Suppress the generation of link options to search libdld.sl when
the -static option is specified on HP-UX 10 and later.
-static
The HP-UX implementation of setlocale in libc has a dependency on
libdld.sl. There isn't an archive version of libdld.sl. Thus,
when the -static option is specified, special link options are
needed to resolve this dependency.
On HP-UX 10 and later, the GCC driver adds the necessary options
to link with libdld.sl when the -static option is specified.
This causes the resulting binary to be dynamic. On the 64-bit
port, the linkers generate dynamic binaries by default in any
case. The -nolibdld option can be used to prevent the GCC driver
from adding these link options.
-threads
Add support for multithreading with the dce thread library under
HP-UX. This option sets flags for both the preprocessor and
linker.
IA-64 Options
These are the -m options defined for the Intel IA-64 architecture.
-mbig-endian
Generate code for a big-endian target. This is the default for
HP-UX.
-mlittle-endian
Generate code for a little-endian target. This is the default
for AIX5 and GNU/Linux.
-mgnu-as
-mno-gnu-as
Generate (or don't) code for the GNU assembler. This is the
default.
-mgnu-ld
-mno-gnu-ld
Generate (or don't) code for the GNU linker. This is the
default.
-mno-pic
Generate code that does not use a global pointer register. The
result is not position independent code, and violates the IA-64
ABI.
-mvolatile-asm-stop
-mno-volatile-asm-stop
Generate (or don't) a stop bit immediately before and after
volatile asm statements.
-mregister-names
-mno-register-names
Generate (or don't) in, loc, and out register names for the
stacked registers. This may make assembler output more readable.
-mno-sdata
-msdata
Disable (or enable) optimizations that use the small data
section. This may be useful for working around optimizer bugs.
-mconstant-gp
Generate code that uses a single constant global pointer value.
This is useful when compiling kernel code.
-mauto-pic
Generate code that is self-relocatable. This implies
-mconstant-gp. This is useful when compiling firmware code.
-minline-float-divide-min-latency
Generate code for inline divides of floating-point values using
the minimum latency algorithm.
-minline-float-divide-max-throughput
Generate code for inline divides of floating-point values using
the maximum throughput algorithm.
-mno-inline-float-divide
Do not generate inline code for divides of floating-point values.
-minline-int-divide-min-latency
Generate code for inline divides of integer values using the
minimum latency algorithm.
-minline-int-divide-max-throughput
Generate code for inline divides of integer values using the
maximum throughput algorithm.
-mno-inline-int-divide
Do not generate inline code for divides of integer values.
-minline-sqrt-min-latency
Generate code for inline square roots using the minimum latency
algorithm.
-minline-sqrt-max-throughput
Generate code for inline square roots using the maximum
throughput algorithm.
-mno-inline-sqrt
Do not generate inline code for "sqrt".
-mfused-madd
-mno-fused-madd
Do (don't) generate code that uses the fused multiply/add or
multiply/subtract instructions. The default is to use these
instructions.
-mno-dwarf2-asm
-mdwarf2-asm
Don't (or do) generate assembler code for the DWARF line number
debugging info. This may be useful when not using the GNU
assembler.
-mearly-stop-bits
-mno-early-stop-bits
Allow stop bits to be placed earlier than immediately preceding
the instruction that triggered the stop bit. This can improve
instruction scheduling, but does not always do so.
-mfixed-range=register-range
Generate code treating the given register range as fixed
registers. A fixed register is one that the register allocator
cannot use. This is useful when compiling kernel code. A
register range is specified as two registers separated by a dash.
Multiple register ranges can be specified separated by a comma.
-mtls-size=tls-size
Specify bit size of immediate TLS offsets. Valid values are 14,
22, and 64.
-mtune=cpu-type
Tune the instruction scheduling for a particular CPU, Valid
values are itanium, itanium1, merced, itanium2, and mckinley.
-milp32
-mlp64
Generate code for a 32-bit or 64-bit environment. The 32-bit
environment sets int, long and pointer to 32 bits. The 64-bit
environment sets int to 32 bits and long and pointer to 64 bits.
These are HP-UX specific flags.
-mno-sched-br-data-spec
-msched-br-data-spec
(Dis/En)able data speculative scheduling before reload. This
results in generation of "ld.a" instructions and the
corresponding check instructions ("ld.c" / "chk.a"). The default
setting is disabled.
-msched-ar-data-spec
-mno-sched-ar-data-spec
(En/Dis)able data speculative scheduling after reload. This
results in generation of "ld.a" instructions and the
corresponding check instructions ("ld.c" / "chk.a"). The default
setting is enabled.
-mno-sched-control-spec
-msched-control-spec
(Dis/En)able control speculative scheduling. This feature is
available only during region scheduling (i.e. before reload).
This results in generation of the "ld.s" instructions and the
corresponding check instructions "chk.s". The default setting is
disabled.
-msched-br-in-data-spec
-mno-sched-br-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are
dependent on the data speculative loads before reload. This is
effective only with -msched-br-data-spec enabled. The default
setting is enabled.
-msched-ar-in-data-spec
-mno-sched-ar-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are
dependent on the data speculative loads after reload. This is
effective only with -msched-ar-data-spec enabled. The default
setting is enabled.
-msched-in-control-spec
-mno-sched-in-control-spec
(En/Dis)able speculative scheduling of the instructions that are
dependent on the control speculative loads. This is effective
only with -msched-control-spec enabled. The default setting is
enabled.
-mno-sched-prefer-non-data-spec-insns
-msched-prefer-non-data-spec-insns
If enabled, data-speculative instructions are chosen for schedule
only if there are no other choices at the moment. This makes the
use of the data speculation much more conservative. The default
setting is disabled.
-mno-sched-prefer-non-control-spec-insns
-msched-prefer-non-control-spec-insns
If enabled, control-speculative instructions are chosen for
schedule only if there are no other choices at the moment. This
makes the use of the control speculation much more conservative.
The default setting is disabled.
-mno-sched-count-spec-in-critical-path
-msched-count-spec-in-critical-path
If enabled, speculative dependencies are considered during
computation of the instructions priorities. This makes the use
of the speculation a bit more conservative. The default setting
is disabled.
-msched-spec-ldc
Use a simple data speculation check. This option is on by
default.
-msched-control-spec-ldc
Use a simple check for control speculation. This option is on by
default.
-msched-stop-bits-after-every-cycle
Place a stop bit after every cycle when scheduling. This option
is on by default.
-msched-fp-mem-deps-zero-cost
Assume that floating-point stores and loads are not likely to
cause a conflict when placed into the same instruction group.
This option is disabled by default.
-msel-sched-dont-check-control-spec
Generate checks for control speculation in selective scheduling.
This flag is disabled by default.
-msched-max-memory-insns=max-insns
Limit on the number of memory insns per instruction group, giving
lower priority to subsequent memory insns attempting to schedule
in the same instruction group. Frequently useful to prevent cache
bank conflicts. The default value is 1.
-msched-max-memory-insns-hard-limit
Makes the limit specified by msched-max-memory-insns a hard
limit, disallowing more than that number in an instruction group.
Otherwise, the limit is "soft", meaning that non-memory
operations are preferred when the limit is reached, but memory
operations may still be scheduled.
LM32 Options
These -m options are defined for the LatticeMico32 architecture:
-mbarrel-shift-enabled
Enable barrel-shift instructions.
-mdivide-enabled
Enable divide and modulus instructions.
-mmultiply-enabled
Enable multiply instructions.
-msign-extend-enabled
Enable sign extend instructions.
-muser-enabled
Enable user-defined instructions.
M32C Options
-mcpu=name
Select the CPU for which code is generated. name may be one of
r8c for the R8C/Tiny series, m16c for the M16C (up to /60)
series, m32cm for the M16C/80 series, or m32c for the M32C/80
series.
-msim
Specifies that the program will be run on the simulator. This
causes an alternate runtime library to be linked in which
supports, for example, file I/O. You must not use this option
when generating programs that will run on real hardware; you must
provide your own runtime library for whatever I/O functions are
needed.
-memregs=number
Specifies the number of memory-based pseudo-registers GCC uses
during code generation. These pseudo-registers are used like
real registers, so there is a tradeoff between GCC's ability to
fit the code into available registers, and the performance
penalty of using memory instead of registers. Note that all
modules in a program must be compiled with the same value for
this option. Because of that, you must not use this option with
GCC's default runtime libraries.
M32R/D Options
These -m options are defined for Renesas M32R/D architectures:
-m32r2
Generate code for the M32R/2.
-m32rx
Generate code for the M32R/X.
-m32r
Generate code for the M32R. This is the default.
-mmodel=small
Assume all objects live in the lower 16MB of memory (so that
their addresses can be loaded with the "ld24" instruction), and
assume all subroutines are reachable with the "bl" instruction.
This is the default.
The addressability of a particular object can be set with the
"model" attribute.
-mmodel=medium
Assume objects may be anywhere in the 32-bit address space (the
compiler generates "seth/add3" instructions to load their
addresses), and assume all subroutines are reachable with the
"bl" instruction.
-mmodel=large
Assume objects may be anywhere in the 32-bit address space (the
compiler generates "seth/add3" instructions to load their
addresses), and assume subroutines may not be reachable with the
"bl" instruction (the compiler generates the much slower
"seth/add3/jl" instruction sequence).
-msdata=none
Disable use of the small data area. Variables are put into one
of ".data", ".bss", or ".rodata" (unless the "section" attribute
has been specified). This is the default.
The small data area consists of sections ".sdata" and ".sbss".
Objects may be explicitly put in the small data area with the
"section" attribute using one of these sections.
-msdata=sdata
Put small global and static data in the small data area, but do
not generate special code to reference them.
-msdata=use
Put small global and static data in the small data area, and
generate special instructions to reference them.
-G num
Put global and static objects less than or equal to num bytes
into the small data or BSS sections instead of the normal data or
BSS sections. The default value of num is 8. The -msdata option
must be set to one of sdata or use for this option to have any
effect.
All modules should be compiled with the same -G num value.
Compiling with different values of num may or may not work; if it
doesn't the linker gives an error message---incorrect code is not
generated.
-mdebug
Makes the M32R-specific code in the compiler display some
statistics that might help in debugging programs.
-malign-loops
Align all loops to a 32-byte boundary.
-mno-align-loops
Do not enforce a 32-byte alignment for loops. This is the
default.
-missue-rate=number
Issue number instructions per cycle. number can only be 1 or 2.
-mbranch-cost=number
number can only be 1 or 2. If it is 1 then branches are
preferred over conditional code, if it is 2, then the opposite
applies.
-mflush-trap=number
Specifies the trap number to use to flush the cache. The default
is 12. Valid numbers are between 0 and 15 inclusive.
-mno-flush-trap
Specifies that the cache cannot be flushed by using a trap.
-mflush-func=name
Specifies the name of the operating system function to call to
flush the cache. The default is _flush_cache, but a function
call is only used if a trap is not available.
-mno-flush-func
Indicates that there is no OS function for flushing the cache.
M680x0 Options
These are the -m options defined for M680x0 and ColdFire processors.
The default settings depend on which architecture was selected when
the compiler was configured; the defaults for the most common choices
are given below.
-march=arch
Generate code for a specific M680x0 or ColdFire instruction set
architecture. Permissible values of arch for M680x0
architectures are: 68000, 68010, 68020, 68030, 68040, 68060 and
cpu32. ColdFire architectures are selected according to
Freescale's ISA classification and the permissible values are:
isaa, isaaplus, isab and isac.
GCC defines a macro "__mcfarch__" whenever it is generating code
for a ColdFire target. The arch in this macro is one of the
-march arguments given above.
When used together, -march and -mtune select code that runs on a
family of similar processors but that is optimized for a
particular microarchitecture.
-mcpu=cpu
Generate code for a specific M680x0 or ColdFire processor. The
M680x0 cpus are: 68000, 68010, 68020, 68030, 68040, 68060, 68302,
68332 and cpu32. The ColdFire cpus are given by the table below,
which also classifies the CPUs into families:
Family : -mcpu arguments
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483
5484 5485
-mcpu=cpu overrides -march=arch if arch is compatible with cpu.
Other combinations of -mcpu and -march are rejected.
GCC defines the macro "__mcf_cpu_cpu" when ColdFire target cpu is
selected. It also defines "__mcf_family_family", where the value
of family is given by the table above.
-mtune=tune
Tune the code for a particular microarchitecture within the
constraints set by -march and -mcpu. The M680x0
microarchitectures are: 68000, 68010, 68020, 68030, 68040, 68060
and cpu32. The ColdFire microarchitectures are: cfv1, cfv2,
cfv3, cfv4 and cfv4e.
You can also use -mtune=68020-40 for code that needs to run
relatively well on 68020, 68030 and 68040 targets.
-mtune=68020-60 is similar but includes 68060 targets as well.
These two options select the same tuning decisions as -m68020-40
and -m68020-60 respectively.
GCC defines the macros "__mcarch" and "__mcarch__" when tuning
for 680x0 architecture arch. It also defines "mcarch" unless
either -ansi or a non-GNU -std option is used. If GCC is tuning
for a range of architectures, as selected by -mtune=68020-40 or
-mtune=68020-60, it defines the macros for every architecture in
the range.
GCC also defines the macro "__muarch__" when tuning for ColdFire
microarchitecture uarch, where uarch is one of the arguments
given above.
-m68000
-mc68000
Generate output for a 68000. This is the default when the
compiler is configured for 68000-based systems. It is equivalent
to -march=68000.
Use this option for microcontrollers with a 68000 or EC000 core,
including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
-m68010
Generate output for a 68010. This is the default when the
compiler is configured for 68010-based systems. It is equivalent
to -march=68010.
-m68020
-mc68020
Generate output for a 68020. This is the default when the
compiler is configured for 68020-based systems. It is equivalent
to -march=68020.
-m68030
Generate output for a 68030. This is the default when the
compiler is configured for 68030-based systems. It is equivalent
to -march=68030.
-m68040
Generate output for a 68040. This is the default when the
compiler is configured for 68040-based systems. It is equivalent
to -march=68040.
This option inhibits the use of 68881/68882 instructions that
have to be emulated by software on the 68040. Use this option if
your 68040 does not have code to emulate those instructions.
-m68060
Generate output for a 68060. This is the default when the
compiler is configured for 68060-based systems. It is equivalent
to -march=68060.
This option inhibits the use of 68020 and 68881/68882
instructions that have to be emulated by software on the 68060.
Use this option if your 68060 does not have code to emulate those
instructions.
-mcpu32
Generate output for a CPU32. This is the default when the
compiler is configured for CPU32-based systems. It is equivalent
to -march=cpu32.
Use this option for microcontrollers with a CPU32 or CPU32+ core,
including the 68330, 68331, 68332, 68333, 68334, 68336, 68340,
68341, 68349 and 68360.
-m5200
Generate output for a 520X ColdFire CPU. This is the default
when the compiler is configured for 520X-based systems. It is
equivalent to -mcpu=5206, and is now deprecated in favor of that
option.
Use this option for microcontroller with a 5200 core, including
the MCF5202, MCF5203, MCF5204 and MCF5206.
-m5206e
Generate output for a 5206e ColdFire CPU. The option is now
deprecated in favor of the equivalent -mcpu=5206e.
-m528x
Generate output for a member of the ColdFire 528X family. The
option is now deprecated in favor of the equivalent -mcpu=528x.
-m5307
Generate output for a ColdFire 5307 CPU. The option is now
deprecated in favor of the equivalent -mcpu=5307.
-m5407
Generate output for a ColdFire 5407 CPU. The option is now
deprecated in favor of the equivalent -mcpu=5407.
-mcfv4e
Generate output for a ColdFire V4e family CPU (e.g. 547x/548x).
This includes use of hardware floating-point instructions. The
option is equivalent to -mcpu=547x, and is now deprecated in
favor of that option.
-m68020-40
Generate output for a 68040, without using any of the new
instructions. This results in code that can run relatively
efficiently on either a 68020/68881 or a 68030 or a 68040. The
generated code does use the 68881 instructions that are emulated
on the 68040.
The option is equivalent to -march=68020 -mtune=68020-40.
-m68020-60
Generate output for a 68060, without using any of the new
instructions. This results in code that can run relatively
efficiently on either a 68020/68881 or a 68030 or a 68040. The
generated code does use the 68881 instructions that are emulated
on the 68060.
The option is equivalent to -march=68020 -mtune=68020-60.
-mhard-float
-m68881
Generate floating-point instructions. This is the default for
68020 and above, and for ColdFire devices that have an FPU. It
defines the macro "__HAVE_68881__" on M680x0 targets and
"__mcffpu__" on ColdFire targets.
-msoft-float
Do not generate floating-point instructions; use library calls
instead. This is the default for 68000, 68010, and 68832
targets. It is also the default for ColdFire devices that have
no FPU.
-mdiv
-mno-div
Generate (do not generate) ColdFire hardware divide and remainder
instructions. If -march is used without -mcpu, the default is
"on" for ColdFire architectures and "off" for M680x0
architectures. Otherwise, the default is taken from the target
CPU (either the default CPU, or the one specified by -mcpu). For
example, the default is "off" for -mcpu=5206 and "on" for
-mcpu=5206e.
GCC defines the macro "__mcfhwdiv__" when this option is enabled.
-mshort
Consider type "int" to be 16 bits wide, like "short int".
Additionally, parameters passed on the stack are also aligned to
a 16-bit boundary even on targets whose API mandates promotion to
32-bit.
-mno-short
Do not consider type "int" to be 16 bits wide. This is the
default.
-mnobitfield
-mno-bitfield
Do not use the bit-field instructions. The -m68000, -mcpu32 and
-m5200 options imply -mnobitfield.
-mbitfield
Do use the bit-field instructions. The -m68020 option implies
-mbitfield. This is the default if you use a configuration
designed for a 68020.
-mrtd
Use a different function-calling convention, in which functions
that take a fixed number of arguments return with the "rtd"
instruction, which pops their arguments while returning. This
saves one instruction in the caller since there is no need to pop
the arguments there.
This calling convention is incompatible with the one normally
used on Unix, so you cannot use it if you need to call libraries
compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including "printf");
otherwise incorrect code is generated for calls to those
functions.
In addition, seriously incorrect code results if you call a
function with too many arguments. (Normally, extra arguments are
harmlessly ignored.)
The "rtd" instruction is supported by the 68010, 68020, 68030,
68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
-mno-rtd
Do not use the calling conventions selected by -mrtd. This is
the default.
-malign-int
-mno-align-int
Control whether GCC aligns "int", "long", "long long", "float",
"double", and "long double" variables on a 32-bit boundary
(-malign-int) or a 16-bit boundary (-mno-align-int). Aligning
variables on 32-bit boundaries produces code that runs somewhat
faster on processors with 32-bit busses at the expense of more
memory.
Warning: if you use the -malign-int switch, GCC aligns structures
containing the above types differently than most published
application binary interface specifications for the m68k.
-mpcrel
Use the pc-relative addressing mode of the 68000 directly,
instead of using a global offset table. At present, this option
implies -fpic, allowing at most a 16-bit offset for pc-relative
addressing. -fPIC is not presently supported with -mpcrel,
though this could be supported for 68020 and higher processors.
-mno-strict-align
-mstrict-align
Do not (do) assume that unaligned memory references are handled
by the system.
-msep-data
Generate code that allows the data segment to be located in a
different area of memory from the text segment. This allows for
execute-in-place in an environment without virtual memory
management. This option implies -fPIC.
-mno-sep-data
Generate code that assumes that the data segment follows the text
segment. This is the default.
-mid-shared-library
Generate code that supports shared libraries via the library ID
method. This allows for execute-in-place and shared libraries in
an environment without virtual memory management. This option
implies -fPIC.
-mno-id-shared-library
Generate code that doesn't assume ID-based shared libraries are
being used. This is the default.
-mshared-library-id=n
Specifies the identification number of the ID-based shared
library being compiled. Specifying a value of 0 generates more
compact code; specifying other values forces the allocation of
that number to the current library, but is no more space- or
time-efficient than omitting this option.
-mxgot
-mno-xgot
When generating position-independent code for ColdFire, generate
code that works if the GOT has more than 8192 entries. This code
is larger and slower than code generated without this option. On
M680x0 processors, this option is not needed; -fPIC suffices.
GCC normally uses a single instruction to load values from the
GOT. While this is relatively efficient, it only works if the
GOT is smaller than about 64k. Anything larger causes the linker
to report an error such as:
relocation truncated to fit: R_68K_GOT16O foobar
If this happens, you should recompile your code with -mxgot. It
should then work with very large GOTs. However, code generated
with -mxgot is less efficient, since it takes 4 instructions to
fetch the value of a global symbol.
Note that some linkers, including newer versions of the GNU
linker, can create multiple GOTs and sort GOT entries. If you
have such a linker, you should only need to use -mxgot when
compiling a single object file that accesses more than 8192 GOT
entries. Very few do.
These options have no effect unless GCC is generating position-
independent code.
-mlong-jump-table-offsets
Use 32-bit offsets in "switch" tables. The default is to use
16-bit offsets.
MCore Options
These are the -m options defined for the Motorola M*Core processors.
-mhardlit
-mno-hardlit
Inline constants into the code stream if it can be done in two
instructions or less.
-mdiv
-mno-div
Use the divide instruction. (Enabled by default).
-mrelax-immediate
-mno-relax-immediate
Allow arbitrary-sized immediates in bit operations.
-mwide-bitfields
-mno-wide-bitfields
Always treat bit-fields as "int"-sized.
-m4byte-functions
-mno-4byte-functions
Force all functions to be aligned to a 4-byte boundary.
-mcallgraph-data
-mno-callgraph-data
Emit callgraph information.
-mslow-bytes
-mno-slow-bytes
Prefer word access when reading byte quantities.
-mlittle-endian
-mbig-endian
Generate code for a little-endian target.
-m210
-m340
Generate code for the 210 processor.
-mno-lsim
Assume that runtime support has been provided and so omit the
simulator library (libsim.a) from the linker command line.
-mstack-increment=size
Set the maximum amount for a single stack increment operation.
Large values can increase the speed of programs that contain
functions that need a large amount of stack space, but they can
also trigger a segmentation fault if the stack is extended too
much. The default value is 0x1000.
MeP Options
-mabsdiff
Enables the "abs" instruction, which is the absolute difference
between two registers.
-mall-opts
Enables all the optional instructions---average, multiply,
divide, bit operations, leading zero, absolute difference,
min/max, clip, and saturation.
-maverage
Enables the "ave" instruction, which computes the average of two
registers.
-mbased=n
Variables of size n bytes or smaller are placed in the ".based"
section by default. Based variables use the $tp register as a
base register, and there is a 128-byte limit to the ".based"
section.
-mbitops
Enables the bit operation instructions---bit test ("btstm"), set
("bsetm"), clear ("bclrm"), invert ("bnotm"), and test-and-set
("tas").
-mc=name
Selects which section constant data is placed in. name may be
tiny, near, or far.
-mclip
Enables the "clip" instruction. Note that -mclip is not useful
unless you also provide -mminmax.
-mconfig=name
Selects one of the built-in core configurations. Each MeP chip
has one or more modules in it; each module has a core CPU and a
variety of coprocessors, optional instructions, and peripherals.
The "MeP-Integrator" tool, not part of GCC, provides these
configurations through this option; using this option is the same
as using all the corresponding command-line options. The default
configuration is default.
-mcop
Enables the coprocessor instructions. By default, this is a
32-bit coprocessor. Note that the coprocessor is normally
enabled via the -mconfig= option.
-mcop32
Enables the 32-bit coprocessor's instructions.
-mcop64
Enables the 64-bit coprocessor's instructions.
-mivc2
Enables IVC2 scheduling. IVC2 is a 64-bit VLIW coprocessor.
-mdc
Causes constant variables to be placed in the ".near" section.
-mdiv
Enables the "div" and "divu" instructions.
-meb
Generate big-endian code.
-mel
Generate little-endian code.
-mio-volatile
Tells the compiler that any variable marked with the "io"
attribute is to be considered volatile.
-ml Causes variables to be assigned to the ".far" section by default.
-mleadz
Enables the "leadz" (leading zero) instruction.
-mm Causes variables to be assigned to the ".near" section by
default.
-mminmax
Enables the "min" and "max" instructions.
-mmult
Enables the multiplication and multiply-accumulate instructions.
-mno-opts
Disables all the optional instructions enabled by -mall-opts.
-mrepeat
Enables the "repeat" and "erepeat" instructions, used for low-
overhead looping.
-ms Causes all variables to default to the ".tiny" section. Note
that there is a 65536-byte limit to this section. Accesses to
these variables use the %gp base register.
-msatur
Enables the saturation instructions. Note that the compiler does
not currently generate these itself, but this option is included
for compatibility with other tools, like "as".
-msdram
Link the SDRAM-based runtime instead of the default ROM-based
runtime.
-msim
Link the simulator run-time libraries.
-msimnovec
Link the simulator runtime libraries, excluding built-in support
for reset and exception vectors and tables.
-mtf
Causes all functions to default to the ".far" section. Without
this option, functions default to the ".near" section.
-mtiny=n
Variables that are n bytes or smaller are allocated to the
".tiny" section. These variables use the $gp base register. The
default for this option is 4, but note that there's a 65536-byte
limit to the ".tiny" section.
MicroBlaze Options
-msoft-float
Use software emulation for floating point (default).
-mhard-float
Use hardware floating-point instructions.
-mmemcpy
Do not optimize block moves, use "memcpy".
-mno-clearbss
This option is deprecated. Use -fno-zero-initialized-in-bss
instead.
-mcpu=cpu-type
Use features of, and schedule code for, the given CPU. Supported
values are in the format vX.YY.Z, where X is a major version, YY
is the minor version, and Z is compatibility code. Example
values are v3.00.a, v4.00.b, v5.00.a, v5.00.b, v5.00.b, v6.00.a.
-mxl-soft-mul
Use software multiply emulation (default).
-mxl-soft-div
Use software emulation for divides (default).
-mxl-barrel-shift
Use the hardware barrel shifter.
-mxl-pattern-compare
Use pattern compare instructions.
-msmall-divides
Use table lookup optimization for small signed integer divisions.
-mxl-stack-check
This option is deprecated. Use -fstack-check instead.
-mxl-gp-opt
Use GP-relative ".sdata"/".sbss" sections.
-mxl-multiply-high
Use multiply high instructions for high part of 32x32 multiply.
-mxl-float-convert
Use hardware floating-point conversion instructions.
-mxl-float-sqrt
Use hardware floating-point square root instruction.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target.
-mxl-reorder
Use reorder instructions (swap and byte reversed load/store).
-mxl-mode-app-model
Select application model app-model. Valid models are
executable
normal executable (default), uses startup code crt0.o.
xmdstub
for use with Xilinx Microprocessor Debugger (XMD) based
software intrusive debug agent called xmdstub. This uses
startup file crt1.o and sets the start address of the program
to 0x800.
bootstrap
for applications that are loaded using a bootloader. This
model uses startup file crt2.o which does not contain a
processor reset vector handler. This is suitable for
transferring control on a processor reset to the bootloader
rather than the application.
novectors
for applications that do not require any of the MicroBlaze
vectors. This option may be useful for applications running
within a monitoring application. This model uses crt3.o as a
startup file.
Option -xl-mode-app-model is a deprecated alias for
-mxl-mode-app-model.
MIPS Options
-EB Generate big-endian code.
-EL Generate little-endian code. This is the default for mips*el-*-*
configurations.
-march=arch
Generate code that runs on arch, which can be the name of a
generic MIPS ISA, or the name of a particular processor. The ISA
names are: mips1, mips2, mips3, mips4, mips32, mips32r2,
mips32r3, mips32r5, mips32r6, mips64, mips64r2, mips64r3,
mips64r5 and mips64r6. The processor names are: 4kc, 4km, 4kp,
4ksc, 4kec, 4kem, 4kep, 4ksd, 5kc, 5kf, 20kc, 24kc, 24kf2_1,
24kf1_1, 24kec, 24kef2_1, 24kef1_1, 34kc, 34kf2_1, 34kf1_1, 34kn,
74kc, 74kf2_1, 74kf1_1, 74kf3_2, 1004kc, 1004kf2_1, 1004kf1_1,
i6400, interaptiv, loongson2e, loongson2f, loongson3a, m4k, m14k,
m14kc, m14ke, m14kec, m5100, m5101, octeon, octeon+, octeon2,
octeon3, orion, p5600, r2000, r3000, r3900, r4000, r4400, r4600,
r4650, r4700, r6000, r8000, rm7000, rm9000, r10000, r12000,
r14000, r16000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130,
vr4300, vr5000, vr5400, vr5500, xlr and xlp. The special value
from-abi selects the most compatible architecture for the
selected ABI (that is, mips1 for 32-bit ABIs and mips3 for 64-bit
ABIs).
The native Linux/GNU toolchain also supports the value native,
which selects the best architecture option for the host
processor. -march=native has no effect if GCC does not recognize
the processor.
In processor names, a final 000 can be abbreviated as k (for
example, -march=r2k). Prefixes are optional, and vr may be
written r.
Names of the form nf2_1 refer to processors with FPUs clocked at
half the rate of the core, names of the form nf1_1 refer to
processors with FPUs clocked at the same rate as the core, and
names of the form nf3_2 refer to processors with FPUs clocked a
ratio of 3:2 with respect to the core. For compatibility
reasons, nf is accepted as a synonym for nf2_1 while nx and bfx
are accepted as synonyms for nf1_1.
GCC defines two macros based on the value of this option. The
first is "_MIPS_ARCH", which gives the name of target
architecture, as a string. The second has the form
"_MIPS_ARCH_foo", where foo is the capitalized value of
"_MIPS_ARCH". For example, -march=r2000 sets "_MIPS_ARCH" to
"r2000" and defines the macro "_MIPS_ARCH_R2000".
Note that the "_MIPS_ARCH" macro uses the processor names given
above. In other words, it has the full prefix and does not
abbreviate 000 as k. In the case of from-abi, the macro names
the resolved architecture (either "mips1" or "mips3"). It names
the default architecture when no -march option is given.
-mtune=arch
Optimize for arch. Among other things, this option controls the
way instructions are scheduled, and the perceived cost of
arithmetic operations. The list of arch values is the same as
for -march.
When this option is not used, GCC optimizes for the processor
specified by -march. By using -march and -mtune together, it is
possible to generate code that runs on a family of processors,
but optimize the code for one particular member of that family.
-mtune defines the macros "_MIPS_TUNE" and "_MIPS_TUNE_foo",
which work in the same way as the -march ones described above.
-mips1
Equivalent to -march=mips1.
-mips2
Equivalent to -march=mips2.
-mips3
Equivalent to -march=mips3.
-mips4
Equivalent to -march=mips4.
-mips32
Equivalent to -march=mips32.
-mips32r3
Equivalent to -march=mips32r3.
-mips32r5
Equivalent to -march=mips32r5.
-mips32r6
Equivalent to -march=mips32r6.
-mips64
Equivalent to -march=mips64.
-mips64r2
Equivalent to -march=mips64r2.
-mips64r3
Equivalent to -march=mips64r3.
-mips64r5
Equivalent to -march=mips64r5.
-mips64r6
Equivalent to -march=mips64r6.
-mips16
-mno-mips16
Generate (do not generate) MIPS16 code. If GCC is targeting a
MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE.
MIPS16 code generation can also be controlled on a per-function
basis by means of "mips16" and "nomips16" attributes.
-mflip-mips16
Generate MIPS16 code on alternating functions. This option is
provided for regression testing of mixed MIPS16/non-MIPS16 code
generation, and is not intended for ordinary use in compiling
user code.
-minterlink-compressed
-mno-interlink-compressed
Require (do not require) that code using the standard
(uncompressed) MIPS ISA be link-compatible with MIPS16 and
microMIPS code, and vice versa.
For example, code using the standard ISA encoding cannot jump
directly to MIPS16 or microMIPS code; it must either use a call
or an indirect jump. -minterlink-compressed therefore disables
direct jumps unless GCC knows that the target of the jump is not
compressed.
-minterlink-mips16
-mno-interlink-mips16
Aliases of -minterlink-compressed and -mno-interlink-compressed.
These options predate the microMIPS ASE and are retained for
backwards compatibility.
-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi
Generate code for the given ABI.
Note that the EABI has a 32-bit and a 64-bit variant. GCC
normally generates 64-bit code when you select a 64-bit
architecture, but you can use -mgp32 to get 32-bit code instead.
For information about the O64 ABI, see
<http://gcc.gnu.org/projects/mipso64-abi.html >.
GCC supports a variant of the o32 ABI in which floating-point
registers are 64 rather than 32 bits wide. You can select this
combination with -mabi=32 -mfp64. This ABI relies on the "mthc1"
and "mfhc1" instructions and is therefore only supported for
MIPS32R2, MIPS32R3 and MIPS32R5 processors.
The register assignments for arguments and return values remain
the same, but each scalar value is passed in a single 64-bit
register rather than a pair of 32-bit registers. For example,
scalar floating-point values are returned in $f0 only, not a
$f0/$f1 pair. The set of call-saved registers also remains the
same in that the even-numbered double-precision registers are
saved.
Two additional variants of the o32 ABI are supported to enable a
transition from 32-bit to 64-bit registers. These are FPXX
(-mfpxx) and FP64A (-mfp64 -mno-odd-spreg). The FPXX extension
mandates that all code must execute correctly when run using
32-bit or 64-bit registers. The code can be interlinked with
either FP32 or FP64, but not both. The FP64A extension is
similar to the FP64 extension but forbids the use of odd-numbered
single-precision registers. This can be used in conjunction with
the "FRE" mode of FPUs in MIPS32R5 processors and allows both
FP32 and FP64A code to interlink and run in the same process
without changing FPU modes.
-mabicalls
-mno-abicalls
Generate (do not generate) code that is suitable for SVR4-style
dynamic objects. -mabicalls is the default for SVR4-based
systems.
-mshared
-mno-shared
Generate (do not generate) code that is fully position-
independent, and that can therefore be linked into shared
libraries. This option only affects -mabicalls.
All -mabicalls code has traditionally been position-independent,
regardless of options like -fPIC and -fpic. However, as an
extension, the GNU toolchain allows executables to use absolute
accesses for locally-binding symbols. It can also use shorter GP
initialization sequences and generate direct calls to locally-
defined functions. This mode is selected by -mno-shared.
-mno-shared depends on binutils 2.16 or higher and generates
objects that can only be linked by the GNU linker. However, the
option does not affect the ABI of the final executable; it only
affects the ABI of relocatable objects. Using -mno-shared
generally makes executables both smaller and quicker.
-mshared is the default.
-mplt
-mno-plt
Assume (do not assume) that the static and dynamic linkers
support PLTs and copy relocations. This option only affects
-mno-shared -mabicalls. For the n64 ABI, this option has no
effect without -msym32.
You can make -mplt the default by configuring GCC with
--with-mips-plt. The default is -mno-plt otherwise.
-mxgot
-mno-xgot
Lift (do not lift) the usual restrictions on the size of the
global offset table.
GCC normally uses a single instruction to load values from the
GOT. While this is relatively efficient, it only works if the
GOT is smaller than about 64k. Anything larger causes the linker
to report an error such as:
relocation truncated to fit: R_MIPS_GOT16 foobar
If this happens, you should recompile your code with -mxgot.
This works with very large GOTs, although the code is also less
efficient, since it takes three instructions to fetch the value
of a global symbol.
Note that some linkers can create multiple GOTs. If you have
such a linker, you should only need to use -mxgot when a single
object file accesses more than 64k's worth of GOT entries. Very
few do.
These options have no effect unless GCC is generating position
independent code.
-mgp32
Assume that general-purpose registers are 32 bits wide.
-mgp64
Assume that general-purpose registers are 64 bits wide.
-mfp32
Assume that floating-point registers are 32 bits wide.
-mfp64
Assume that floating-point registers are 64 bits wide.
-mfpxx
Do not assume the width of floating-point registers.
-mhard-float
Use floating-point coprocessor instructions.
-msoft-float
Do not use floating-point coprocessor instructions. Implement
floating-point calculations using library calls instead.
-mno-float
Equivalent to -msoft-float, but additionally asserts that the
program being compiled does not perform any floating-point
operations. This option is presently supported only by some
bare-metal MIPS configurations, where it may select a special set
of libraries that lack all floating-point support (including, for
example, the floating-point "printf" formats). If code compiled
with -mno-float accidentally contains floating-point operations,
it is likely to suffer a link-time or run-time failure.
-msingle-float
Assume that the floating-point coprocessor only supports single-
precision operations.
-mdouble-float
Assume that the floating-point coprocessor supports double-
precision operations. This is the default.
-modd-spreg
-mno-odd-spreg
Enable the use of odd-numbered single-precision floating-point
registers for the o32 ABI. This is the default for processors
that are known to support these registers. When using the o32
FPXX ABI, -mno-odd-spreg is set by default.
-mabs=2008
-mabs=legacy
These options control the treatment of the special not-a-number
(NaN) IEEE 754 floating-point data with the "abs.fmt" and
"neg.fmt" machine instructions.
By default or when -mabs=legacy is used the legacy treatment is
selected. In this case these instructions are considered
arithmetic and avoided where correct operation is required and
the input operand might be a NaN. A longer sequence of
instructions that manipulate the sign bit of floating-point datum
manually is used instead unless the -ffinite-math-only option has
also been specified.
The -mabs=2008 option selects the IEEE 754-2008 treatment. In
this case these instructions are considered non-arithmetic and
therefore operating correctly in all cases, including in
particular where the input operand is a NaN. These instructions
are therefore always used for the respective operations.
-mnan=2008
-mnan=legacy
These options control the encoding of the special not-a-number
(NaN) IEEE 754 floating-point data.
The -mnan=legacy option selects the legacy encoding. In this
case quiet NaNs (qNaNs) are denoted by the first bit of their
trailing significand field being 0, whereas signaling NaNs
(sNaNs) are denoted by the first bit of their trailing
significand field being 1.
The -mnan=2008 option selects the IEEE 754-2008 encoding. In
this case qNaNs are denoted by the first bit of their trailing
significand field being 1, whereas sNaNs are denoted by the first
bit of their trailing significand field being 0.
The default is -mnan=legacy unless GCC has been configured with
--with-nan=2008.
-mllsc
-mno-llsc
Use (do not use) ll, sc, and sync instructions to implement
atomic memory built-in functions. When neither option is
specified, GCC uses the instructions if the target architecture
supports them.
-mllsc is useful if the runtime environment can emulate the
instructions and -mno-llsc can be useful when compiling for
nonstandard ISAs. You can make either option the default by
configuring GCC with --with-llsc and --without-llsc respectively.
--with-llsc is the default for some configurations; see the
installation documentation for details.
-mdsp
-mno-dsp
Use (do not use) revision 1 of the MIPS DSP ASE.
This option defines the preprocessor macro "__mips_dsp". It
also defines "__mips_dsp_rev" to 1.
-mdspr2
-mno-dspr2
Use (do not use) revision 2 of the MIPS DSP ASE.
This option defines the preprocessor macros "__mips_dsp" and
"__mips_dspr2". It also defines "__mips_dsp_rev" to 2.
-msmartmips
-mno-smartmips
Use (do not use) the MIPS SmartMIPS ASE.
-mpaired-single
-mno-paired-single
Use (do not use) paired-single floating-point instructions.
This option requires hardware floating-point support to be
enabled.
-mdmx
-mno-mdmx
Use (do not use) MIPS Digital Media Extension instructions. This
option can only be used when generating 64-bit code and requires
hardware floating-point support to be enabled.
-mips3d
-mno-mips3d
Use (do not use) the MIPS-3D ASE. The option -mips3d implies
-mpaired-single.
-mmicromips
-mno-micromips
Generate (do not generate) microMIPS code.
MicroMIPS code generation can also be controlled on a per-
function basis by means of "micromips" and "nomicromips"
attributes.
-mmt
-mno-mt
Use (do not use) MT Multithreading instructions.
-mmcu
-mno-mcu
Use (do not use) the MIPS MCU ASE instructions.
-meva
-mno-eva
Use (do not use) the MIPS Enhanced Virtual Addressing
instructions.
-mvirt
-mno-virt
Use (do not use) the MIPS Virtualization (VZ) instructions.
-mxpa
-mno-xpa
Use (do not use) the MIPS eXtended Physical Address (XPA)
instructions.
-mlong64
Force "long" types to be 64 bits wide. See -mlong32 for an
explanation of the default and the way that the pointer size is
determined.
-mlong32
Force "long", "int", and pointer types to be 32 bits wide.
The default size of "int"s, "long"s and pointers depends on the
ABI. All the supported ABIs use 32-bit "int"s. The n64 ABI uses
64-bit "long"s, as does the 64-bit EABI; the others use 32-bit
"long"s. Pointers are the same size as "long"s, or the same size
as integer registers, whichever is smaller.
-msym32
-mno-sym32
Assume (do not assume) that all symbols have 32-bit values,
regardless of the selected ABI. This option is useful in
combination with -mabi=64 and -mno-abicalls because it allows GCC
to generate shorter and faster references to symbolic addresses.
-G num
Put definitions of externally-visible data in a small data
section if that data is no bigger than num bytes. GCC can then
generate more efficient accesses to the data; see -mgpopt for
details.
The default -G option depends on the configuration.
-mlocal-sdata
-mno-local-sdata
Extend (do not extend) the -G behavior to local data too, such as
to static variables in C. -mlocal-sdata is the default for all
configurations.
If the linker complains that an application is using too much
small data, you might want to try rebuilding the less
performance-critical parts with -mno-local-sdata. You might also
want to build large libraries with -mno-local-sdata, so that the
libraries leave more room for the main program.
-mextern-sdata
-mno-extern-sdata
Assume (do not assume) that externally-defined data is in a small
data section if the size of that data is within the -G limit.
-mextern-sdata is the default for all configurations.
If you compile a module Mod with -mextern-sdata -G num -mgpopt,
and Mod references a variable Var that is no bigger than num
bytes, you must make sure that Var is placed in a small data
section. If Var is defined by another module, you must either
compile that module with a high-enough -G setting or attach a
"section" attribute to Var's definition. If Var is common, you
must link the application with a high-enough -G setting.
The easiest way of satisfying these restrictions is to compile
and link every module with the same -G option. However, you may
wish to build a library that supports several different small
data limits. You can do this by compiling the library with the
highest supported -G setting and additionally using
-mno-extern-sdata to stop the library from making assumptions
about externally-defined data.
-mgpopt
-mno-gpopt
Use (do not use) GP-relative accesses for symbols that are known
to be in a small data section; see -G, -mlocal-sdata and
-mextern-sdata. -mgpopt is the default for all configurations.
-mno-gpopt is useful for cases where the $gp register might not
hold the value of "_gp". For example, if the code is part of a
library that might be used in a boot monitor, programs that call
boot monitor routines pass an unknown value in $gp. (In such
situations, the boot monitor itself is usually compiled with
-G0.)
-mno-gpopt implies -mno-local-sdata and -mno-extern-sdata.
-membedded-data
-mno-embedded-data
Allocate variables to the read-only data section first if
possible, then next in the small data section if possible,
otherwise in data. This gives slightly slower code than the
default, but reduces the amount of RAM required when executing,
and thus may be preferred for some embedded systems.
-muninit-const-in-rodata
-mno-uninit-const-in-rodata
Put uninitialized "const" variables in the read-only data
section. This option is only meaningful in conjunction with
-membedded-data.
-mcode-readable=setting
Specify whether GCC may generate code that reads from executable
sections. There are three possible settings:
-mcode-readable=yes
Instructions may freely access executable sections. This is
the default setting.
-mcode-readable=pcrel
MIPS16 PC-relative load instructions can access executable
sections, but other instructions must not do so. This option
is useful on 4KSc and 4KSd processors when the code TLBs have
the Read Inhibit bit set. It is also useful on processors
that can be configured to have a dual instruction/data SRAM
interface and that, like the M4K, automatically redirect PC-
relative loads to the instruction RAM.
-mcode-readable=no
Instructions must not access executable sections. This
option can be useful on targets that are configured to have a
dual instruction/data SRAM interface but that (unlike the
M4K) do not automatically redirect PC-relative loads to the
instruction RAM.
-msplit-addresses
-mno-split-addresses
Enable (disable) use of the "%hi()" and "%lo()" assembler
relocation operators. This option has been superseded by
-mexplicit-relocs but is retained for backwards compatibility.
-mexplicit-relocs
-mno-explicit-relocs
Use (do not use) assembler relocation operators when dealing with
symbolic addresses. The alternative, selected by
-mno-explicit-relocs, is to use assembler macros instead.
-mexplicit-relocs is the default if GCC was configured to use an
assembler that supports relocation operators.
-mcheck-zero-division
-mno-check-zero-division
Trap (do not trap) on integer division by zero.
The default is -mcheck-zero-division.
-mdivide-traps
-mdivide-breaks
MIPS systems check for division by zero by generating either a
conditional trap or a break instruction. Using traps results in
smaller code, but is only supported on MIPS II and later. Also,
some versions of the Linux kernel have a bug that prevents trap
from generating the proper signal ("SIGFPE"). Use -mdivide-traps
to allow conditional traps on architectures that support them and
-mdivide-breaks to force the use of breaks.
The default is usually -mdivide-traps, but this can be overridden
at configure time using --with-divide=breaks. Divide-by-zero
checks can be completely disabled using -mno-check-zero-division.
-mload-store-pairs
-mno-load-store-pairs
Enable (disable) an optimization that pairs consecutive load or
store instructions to enable load/store bonding. This option is
enabled by default but only takes effect when the selected
architecture is known to support bonding.
-mmemcpy
-mno-memcpy
Force (do not force) the use of "memcpy" for non-trivial block
moves. The default is -mno-memcpy, which allows GCC to inline
most constant-sized copies.
-mlong-calls
-mno-long-calls
Disable (do not disable) use of the "jal" instruction. Calling
functions using "jal" is more efficient but requires the caller
and callee to be in the same 256 megabyte segment.
This option has no effect on abicalls code. The default is
-mno-long-calls.
-mmad
-mno-mad
Enable (disable) use of the "mad", "madu" and "mul" instructions,
as provided by the R4650 ISA.
-mimadd
-mno-imadd
Enable (disable) use of the "madd" and "msub" integer
instructions. The default is -mimadd on architectures that
support "madd" and "msub" except for the 74k architecture where
it was found to generate slower code.
-mfused-madd
-mno-fused-madd
Enable (disable) use of the floating-point multiply-accumulate
instructions, when they are available. The default is
-mfused-madd.
On the R8000 CPU when multiply-accumulate instructions are used,
the intermediate product is calculated to infinite precision and
is not subject to the FCSR Flush to Zero bit. This may be
undesirable in some circumstances. On other processors the
result is numerically identical to the equivalent computation
using separate multiply, add, subtract and negate instructions.
-nocpp
Tell the MIPS assembler to not run its preprocessor over user
assembler files (with a .s suffix) when assembling them.
-mfix-24k
-mno-fix-24k
Work around the 24K E48 (lost data on stores during refill)
errata. The workarounds are implemented by the assembler rather
than by GCC.
-mfix-r4000
-mno-fix-r4000
Work around certain R4000 CPU errata:
- A double-word or a variable shift may give an incorrect
result if executed immediately after starting an integer
division.
- A double-word or a variable shift may give an incorrect
result if executed while an integer multiplication is in
progress.
- An integer division may give an incorrect result if started
in a delay slot of a taken branch or a jump.
-mfix-r4400
-mno-fix-r4400
Work around certain R4400 CPU errata:
- A double-word or a variable shift may give an incorrect
result if executed immediately after starting an integer
division.
-mfix-r10000
-mno-fix-r10000
Work around certain R10000 errata:
- "ll"/"sc" sequences may not behave atomically on revisions
prior to 3.0. They may deadlock on revisions 2.6 and
earlier.
This option can only be used if the target architecture supports
branch-likely instructions. -mfix-r10000 is the default when
-march=r10000 is used; -mno-fix-r10000 is the default otherwise.
-mfix-rm7000
-mno-fix-rm7000
Work around the RM7000 "dmult"/"dmultu" errata. The workarounds
are implemented by the assembler rather than by GCC.
-mfix-vr4120
-mno-fix-vr4120
Work around certain VR4120 errata:
- "dmultu" does not always produce the correct result.
- "div" and "ddiv" do not always produce the correct result if
one of the operands is negative.
The workarounds for the division errata rely on special functions
in libgcc.a. At present, these functions are only provided by
the "mips64vr*-elf" configurations.
Other VR4120 errata require a NOP to be inserted between certain
pairs of instructions. These errata are handled by the
assembler, not by GCC itself.
-mfix-vr4130
Work around the VR4130 "mflo"/"mfhi" errata. The workarounds are
implemented by the assembler rather than by GCC, although GCC
avoids using "mflo" and "mfhi" if the VR4130 "macc", "macchi",
"dmacc" and "dmacchi" instructions are available instead.
-mfix-sb1
-mno-fix-sb1
Work around certain SB-1 CPU core errata. (This flag currently
works around the SB-1 revision 2 "F1" and "F2" floating-point
errata.)
-mr10k-cache-barrier=setting
Specify whether GCC should insert cache barriers to avoid the
side-effects of speculation on R10K processors.
In common with many processors, the R10K tries to predict the
outcome of a conditional branch and speculatively executes
instructions from the "taken" branch. It later aborts these
instructions if the predicted outcome is wrong. However, on the
R10K, even aborted instructions can have side effects.
This problem only affects kernel stores and, depending on the
system, kernel loads. As an example, a speculatively-executed
store may load the target memory into cache and mark the cache
line as dirty, even if the store itself is later aborted. If a
DMA operation writes to the same area of memory before the
"dirty" line is flushed, the cached data overwrites the DMA-ed
data. See the R10K processor manual for a full description,
including other potential problems.
One workaround is to insert cache barrier instructions before
every memory access that might be speculatively executed and that
might have side effects even if aborted.
-mr10k-cache-barrier=setting controls GCC's implementation of
this workaround. It assumes that aborted accesses to any byte in
the following regions does not have side effects:
1. the memory occupied by the current function's stack frame;
2. the memory occupied by an incoming stack argument;
3. the memory occupied by an object with a link-time-constant
address.
It is the kernel's responsibility to ensure that speculative
accesses to these regions are indeed safe.
If the input program contains a function declaration such as:
void foo (void);
then the implementation of "foo" must allow "j foo" and "jal foo"
to be executed speculatively. GCC honors this restriction for
functions it compiles itself. It expects non-GCC functions (such
as hand-written assembly code) to do the same.
The option has three forms:
-mr10k-cache-barrier=load-store
Insert a cache barrier before a load or store that might be
speculatively executed and that might have side effects even
if aborted.
-mr10k-cache-barrier=store
Insert a cache barrier before a store that might be
speculatively executed and that might have side effects even
if aborted.
-mr10k-cache-barrier=none
Disable the insertion of cache barriers. This is the default
setting.
-mflush-func=func
-mno-flush-func
Specifies the function to call to flush the I and D caches, or to
not call any such function. If called, the function must take
the same arguments as the common "_flush_func", that is, the
address of the memory range for which the cache is being flushed,
the size of the memory range, and the number 3 (to flush both
caches). The default depends on the target GCC was configured
for, but commonly is either "_flush_func" or "__cpu_flush".
mbranch-cost=num
Set the cost of branches to roughly num "simple" instructions.
This cost is only a heuristic and is not guaranteed to produce
consistent results across releases. A zero cost redundantly
selects the default, which is based on the -mtune setting.
-mbranch-likely
-mno-branch-likely
Enable or disable use of Branch Likely instructions, regardless
of the default for the selected architecture. By default, Branch
Likely instructions may be generated if they are supported by the
selected architecture. An exception is for the MIPS32 and MIPS64
architectures and processors that implement those architectures;
for those, Branch Likely instructions are not be generated by
default because the MIPS32 and MIPS64 architectures specifically
deprecate their use.
-mcompact-branches=never
-mcompact-branches=optimal
-mcompact-branches=always
These options control which form of branches will be generated.
The default is -mcompact-branches=optimal.
The -mcompact-branches=never option ensures that compact branch
instructions will never be generated.
The -mcompact-branches=always option ensures that a compact
branch instruction will be generated if available. If a compact
branch instruction is not available, a delay slot form of the
branch will be used instead.
This option is supported from MIPS Release 6 onwards.
The -mcompact-branches=optimal option will cause a delay slot
branch to be used if one is available in the current ISA and the
delay slot is successfully filled. If the delay slot is not
filled, a compact branch will be chosen if one is available.
-mfp-exceptions
-mno-fp-exceptions
Specifies whether FP exceptions are enabled. This affects how FP
instructions are scheduled for some processors. The default is
that FP exceptions are enabled.
For instance, on the SB-1, if FP exceptions are disabled, and we
are emitting 64-bit code, then we can use both FP pipes.
Otherwise, we can only use one FP pipe.
-mvr4130-align
-mno-vr4130-align
The VR4130 pipeline is two-way superscalar, but can only issue
two instructions together if the first one is 8-byte aligned.
When this option is enabled, GCC aligns pairs of instructions
that it thinks should execute in parallel.
This option only has an effect when optimizing for the VR4130.
It normally makes code faster, but at the expense of making it
bigger. It is enabled by default at optimization level -O3.
-msynci
-mno-synci
Enable (disable) generation of "synci" instructions on
architectures that support it. The "synci" instructions (if
enabled) are generated when "__builtin___clear_cache" is
compiled.
This option defaults to -mno-synci, but the default can be
overridden by configuring GCC with --with-synci.
When compiling code for single processor systems, it is generally
safe to use "synci". However, on many multi-core (SMP) systems,
it does not invalidate the instruction caches on all cores and
may lead to undefined behavior.
-mrelax-pic-calls
-mno-relax-pic-calls
Try to turn PIC calls that are normally dispatched via register
$25 into direct calls. This is only possible if the linker can
resolve the destination at link time and if the destination is
within range for a direct call.
-mrelax-pic-calls is the default if GCC was configured to use an
assembler and a linker that support the ".reloc" assembly
directive and -mexplicit-relocs is in effect. With
-mno-explicit-relocs, this optimization can be performed by the
assembler and the linker alone without help from the compiler.
-mmcount-ra-address
-mno-mcount-ra-address
Emit (do not emit) code that allows "_mcount" to modify the
calling function's return address. When enabled, this option
extends the usual "_mcount" interface with a new ra-address
parameter, which has type "intptr_t *" and is passed in register
$12. "_mcount" can then modify the return address by doing both
of the following:
* Returning the new address in register $31.
* Storing the new address in "*ra-address", if ra-address is
nonnull.
The default is -mno-mcount-ra-address.
-mframe-header-opt
-mno-frame-header-opt
Enable (disable) frame header optimization in the o32 ABI. When
using the o32 ABI, calling functions will allocate 16 bytes on
the stack for the called function to write out register
arguments. When enabled, this optimization will suppress the
allocation of the frame header if it can be determined that it is
unused.
This optimization is off by default at all optimization levels.
-mlxc1-sxc1
-mno-lxc1-sxc1
When applicable, enable (disable) the generation of "lwxc1",
"swxc1", "ldxc1", "sdxc1" instructions. Enabled by default.
-mmadd4
-mno-madd4
When applicable, enable (disable) the generation of 4-operand
"madd.s", "madd.d" and related instructions. Enabled by default.
MMIX Options
These options are defined for the MMIX:
-mlibfuncs
-mno-libfuncs
Specify that intrinsic library functions are being compiled,
passing all values in registers, no matter the size.
-mepsilon
-mno-epsilon
Generate floating-point comparison instructions that compare with
respect to the "rE" epsilon register.
-mabi=mmixware
-mabi=gnu
Generate code that passes function parameters and return values
that (in the called function) are seen as registers $0 and up, as
opposed to the GNU ABI which uses global registers $231 and up.
-mzero-extend
-mno-zero-extend
When reading data from memory in sizes shorter than 64 bits, use
(do not use) zero-extending load instructions by default, rather
than sign-extending ones.
-mknuthdiv
-mno-knuthdiv
Make the result of a division yielding a remainder have the same
sign as the divisor. With the default, -mno-knuthdiv, the sign
of the remainder follows the sign of the dividend. Both methods
are arithmetically valid, the latter being almost exclusively
used.
-mtoplevel-symbols
-mno-toplevel-symbols
Prepend (do not prepend) a : to all global symbols, so the
assembly code can be used with the "PREFIX" assembly directive.
-melf
Generate an executable in the ELF format, rather than the default
mmo format used by the mmix simulator.
-mbranch-predict
-mno-branch-predict
Use (do not use) the probable-branch instructions, when static
branch prediction indicates a probable branch.
-mbase-addresses
-mno-base-addresses
Generate (do not generate) code that uses base addresses. Using
a base address automatically generates a request (handled by the
assembler and the linker) for a constant to be set up in a global
register. The register is used for one or more base address
requests within the range 0 to 255 from the value held in the
register. The generally leads to short and fast code, but the
number of different data items that can be addressed is limited.
This means that a program that uses lots of static data may
require -mno-base-addresses.
-msingle-exit
-mno-single-exit
Force (do not force) generated code to have a single exit point
in each function.
MN10300 Options
These -m options are defined for Matsushita MN10300 architectures:
-mmult-bug
Generate code to avoid bugs in the multiply instructions for the
MN10300 processors. This is the default.
-mno-mult-bug
Do not generate code to avoid bugs in the multiply instructions
for the MN10300 processors.
-mam33
Generate code using features specific to the AM33 processor.
-mno-am33
Do not generate code using features specific to the AM33
processor. This is the default.
-mam33-2
Generate code using features specific to the AM33/2.0 processor.
-mam34
Generate code using features specific to the AM34 processor.
-mtune=cpu-type
Use the timing characteristics of the indicated CPU type when
scheduling instructions. This does not change the targeted
processor type. The CPU type must be one of mn10300, am33,
am33-2 or am34.
-mreturn-pointer-on-d0
When generating a function that returns a pointer, return the
pointer in both "a0" and "d0". Otherwise, the pointer is
returned only in "a0", and attempts to call such functions
without a prototype result in errors. Note that this option is
on by default; use -mno-return-pointer-on-d0 to disable it.
-mno-crt0
Do not link in the C run-time initialization object file.
-mrelax
Indicate to the linker that it should perform a relaxation
optimization pass to shorten branches, calls and absolute memory
addresses. This option only has an effect when used on the
command line for the final link step.
This option makes symbolic debugging impossible.
-mliw
Allow the compiler to generate Long Instruction Word instructions
if the target is the AM33 or later. This is the default. This
option defines the preprocessor macro "__LIW__".
-mnoliw
Do not allow the compiler to generate Long Instruction Word
instructions. This option defines the preprocessor macro
"__NO_LIW__".
-msetlb
Allow the compiler to generate the SETLB and Lcc instructions if
the target is the AM33 or later. This is the default. This
option defines the preprocessor macro "__SETLB__".
-mnosetlb
Do not allow the compiler to generate SETLB or Lcc instructions.
This option defines the preprocessor macro "__NO_SETLB__".
Moxie Options
-meb
Generate big-endian code. This is the default for moxie-*-*
configurations.
-mel
Generate little-endian code.
-mmul.x
Generate mul.x and umul.x instructions. This is the default for
moxiebox-*-* configurations.
-mno-crt0
Do not link in the C run-time initialization object file.
MSP430 Options
These options are defined for the MSP430:
-masm-hex
Force assembly output to always use hex constants. Normally such
constants are signed decimals, but this option is available for
testsuite and/or aesthetic purposes.
-mmcu=
Select the MCU to target. This is used to create a C
preprocessor symbol based upon the MCU name, converted to upper
case and pre- and post-fixed with __. This in turn is used by
the msp430.h header file to select an MCU-specific supplementary
header file.
The option also sets the ISA to use. If the MCU name is one that
is known to only support the 430 ISA then that is selected,
otherwise the 430X ISA is selected. A generic MCU name of msp430
can also be used to select the 430 ISA. Similarly the generic
msp430x MCU name selects the 430X ISA.
In addition an MCU-specific linker script is added to the linker
command line. The script's name is the name of the MCU with .ld
appended. Thus specifying -mmcu=xxx on the gcc command line
defines the C preprocessor symbol "__XXX__" and cause the linker
to search for a script called xxx.ld.
This option is also passed on to the assembler.
-mwarn-mcu
-mno-warn-mcu
This option enables or disables warnings about conflicts between
the MCU name specified by the -mmcu option and the ISA set by the
-mcpu option and/or the hardware multiply support set by the
-mhwmult option. It also toggles warnings about unrecognized MCU
names. This option is on by default.
-mcpu=
Specifies the ISA to use. Accepted values are msp430, msp430x
and msp430xv2. This option is deprecated. The -mmcu= option
should be used to select the ISA.
-msim
Link to the simulator runtime libraries and linker script.
Overrides any scripts that would be selected by the -mmcu=
option.
-mlarge
Use large-model addressing (20-bit pointers, 32-bit "size_t").
-msmall
Use small-model addressing (16-bit pointers, 16-bit "size_t").
-mrelax
This option is passed to the assembler and linker, and allows the
linker to perform certain optimizations that cannot be done until
the final link.
mhwmult=
Describes the type of hardware multiply supported by the target.
Accepted values are none for no hardware multiply, 16bit for the
original 16-bit-only multiply supported by early MCUs. 32bit for
the 16/32-bit multiply supported by later MCUs and f5series for
the 16/32-bit multiply supported by F5-series MCUs. A value of
auto can also be given. This tells GCC to deduce the hardware
multiply support based upon the MCU name provided by the -mmcu
option. If no -mmcu option is specified or if the MCU name is
not recognized then no hardware multiply support is assumed.
"auto" is the default setting.
Hardware multiplies are normally performed by calling a library
routine. This saves space in the generated code. When compiling
at -O3 or higher however the hardware multiplier is invoked
inline. This makes for bigger, but faster code.
The hardware multiply routines disable interrupts whilst running
and restore the previous interrupt state when they finish. This
makes them safe to use inside interrupt handlers as well as in
normal code.
-minrt
Enable the use of a minimum runtime environment - no static
initializers or constructors. This is intended for memory-
constrained devices. The compiler includes special symbols in
some objects that tell the linker and runtime which code
fragments are required.
-mcode-region=
-mdata-region=
These options tell the compiler where to place functions and data
that do not have one of the "lower", "upper", "either" or
"section" attributes. Possible values are "lower", "upper",
"either" or "any". The first three behave like the corresponding
attribute. The fourth possible value - "any" - is the default.
It leaves placement entirely up to the linker script and how it
assigns the standard sections (".text", ".data", etc) to the
memory regions.
-msilicon-errata=
This option passes on a request to assembler to enable the fixes
for the named silicon errata.
-msilicon-errata-warn=
This option passes on a request to the assembler to enable
warning messages when a silicon errata might need to be applied.
NDS32 Options
These options are defined for NDS32 implementations:
-mbig-endian
Generate code in big-endian mode.
-mlittle-endian
Generate code in little-endian mode.
-mreduced-regs
Use reduced-set registers for register allocation.
-mfull-regs
Use full-set registers for register allocation.
-mcmov
Generate conditional move instructions.
-mno-cmov
Do not generate conditional move instructions.
-mperf-ext
Generate performance extension instructions.
-mno-perf-ext
Do not generate performance extension instructions.
-mv3push
Generate v3 push25/pop25 instructions.
-mno-v3push
Do not generate v3 push25/pop25 instructions.
-m16-bit
Generate 16-bit instructions.
-mno-16-bit
Do not generate 16-bit instructions.
-misr-vector-size=num
Specify the size of each interrupt vector, which must be 4 or 16.
-mcache-block-size=num
Specify the size of each cache block, which must be a power of 2
between 4 and 512.
-march=arch
Specify the name of the target architecture.
-mcmodel=code-model
Set the code model to one of
small
All the data and read-only data segments must be within 512KB
addressing space. The text segment must be within 16MB
addressing space.
medium
The data segment must be within 512KB while the read-only
data segment can be within 4GB addressing space. The text
segment should be still within 16MB addressing space.
large
All the text and data segments can be within 4GB addressing
space.
-mctor-dtor
Enable constructor/destructor feature.
-mrelax
Guide linker to relax instructions.
Nios II Options
These are the options defined for the Altera Nios II processor.
-G num
Put global and static objects less than or equal to num bytes
into the small data or BSS sections instead of the normal data or
BSS sections. The default value of num is 8.
-mgpopt=option
-mgpopt
-mno-gpopt
Generate (do not generate) GP-relative accesses. The following
option names are recognized:
none
Do not generate GP-relative accesses.
local
Generate GP-relative accesses for small data objects that are
not external, weak, or uninitialized common symbols. Also
use GP-relative addressing for objects that have been
explicitly placed in a small data section via a "section"
attribute.
global
As for local, but also generate GP-relative accesses for
small data objects that are external, weak, or common. If
you use this option, you must ensure that all parts of your
program (including libraries) are compiled with the same -G
setting.
data
Generate GP-relative accesses for all data objects in the
program. If you use this option, the entire data and BSS
segments of your program must fit in 64K of memory and you
must use an appropriate linker script to allocate them within
the addressable range of the global pointer.
all Generate GP-relative addresses for function pointers as well
as data pointers. If you use this option, the entire text,
data, and BSS segments of your program must fit in 64K of
memory and you must use an appropriate linker script to
allocate them within the addressable range of the global
pointer.
-mgpopt is equivalent to -mgpopt=local, and -mno-gpopt is
equivalent to -mgpopt=none.
The default is -mgpopt except when -fpic or -fPIC is specified to
generate position-independent code. Note that the Nios II ABI
does not permit GP-relative accesses from shared libraries.
You may need to specify -mno-gpopt explicitly when building
programs that include large amounts of small data, including
large GOT data sections. In this case, the 16-bit offset for GP-
relative addressing may not be large enough to allow access to
the entire small data section.
-mel
-meb
Generate little-endian (default) or big-endian (experimental)
code, respectively.
-march=arch
This specifies the name of the target Nios II architecture. GCC
uses this name to determine what kind of instructions it can emit
when generating assembly code. Permissible names are: r1, r2.
The preprocessor macro "__nios2_arch__" is available to programs,
with value 1 or 2, indicating the targeted ISA level.
-mbypass-cache
-mno-bypass-cache
Force all load and store instructions to always bypass cache by
using I/O variants of the instructions. The default is not to
bypass the cache.
-mno-cache-volatile
-mcache-volatile
Volatile memory access bypass the cache using the I/O variants of
the load and store instructions. The default is not to bypass the
cache.
-mno-fast-sw-div
-mfast-sw-div
Do not use table-based fast divide for small numbers. The default
is to use the fast divide at -O3 and above.
-mno-hw-mul
-mhw-mul
-mno-hw-mulx
-mhw-mulx
-mno-hw-div
-mhw-div
Enable or disable emitting "mul", "mulx" and "div" family of
instructions by the compiler. The default is to emit "mul" and
not emit "div" and "mulx".
-mbmx
-mno-bmx
-mcdx
-mno-cdx
Enable or disable generation of Nios II R2 BMX (bit manipulation)
and CDX (code density) instructions. Enabling these instructions
also requires -march=r2. Since these instructions are optional
extensions to the R2 architecture, the default is not to emit
them.
-mcustom-insn=N
-mno-custom-insn
Each -mcustom-insn=N option enables use of a custom instruction
with encoding N when generating code that uses insn. For
example, -mcustom-fadds=253 generates custom instruction 253 for
single-precision floating-point add operations instead of the
default behavior of using a library call.
The following values of insn are supported. Except as otherwise
noted, floating-point operations are expected to be implemented
with normal IEEE 754 semantics and correspond directly to the C
operators or the equivalent GCC built-in functions.
Single-precision floating point:
fadds, fsubs, fdivs, fmuls
Binary arithmetic operations.
fnegs
Unary negation.
fabss
Unary absolute value.
fcmpeqs, fcmpges, fcmpgts, fcmples, fcmplts, fcmpnes
Comparison operations.
fmins, fmaxs
Floating-point minimum and maximum. These instructions are
only generated if -ffinite-math-only is specified.
fsqrts
Unary square root operation.
fcoss, fsins, ftans, fatans, fexps, flogs
Floating-point trigonometric and exponential functions.
These instructions are only generated if
-funsafe-math-optimizations is also specified.
Double-precision floating point:
faddd, fsubd, fdivd, fmuld
Binary arithmetic operations.
fnegd
Unary negation.
fabsd
Unary absolute value.
fcmpeqd, fcmpged, fcmpgtd, fcmpled, fcmpltd, fcmpned
Comparison operations.
fmind, fmaxd
Double-precision minimum and maximum. These instructions are
only generated if -ffinite-math-only is specified.
fsqrtd
Unary square root operation.
fcosd, fsind, ftand, fatand, fexpd, flogd
Double-precision trigonometric and exponential functions.
These instructions are only generated if
-funsafe-math-optimizations is also specified.
Conversions:
fextsd
Conversion from single precision to double precision.
ftruncds
Conversion from double precision to single precision.
fixsi, fixsu, fixdi, fixdu
Conversion from floating point to signed or unsigned integer
types, with truncation towards zero.
round
Conversion from single-precision floating point to signed
integer, rounding to the nearest integer and ties away from
zero. This corresponds to the "__builtin_lroundf" function
when -fno-math-errno is used.
floatis, floatus, floatid, floatud
Conversion from signed or unsigned integer types to floating-
point types.
In addition, all of the following transfer instructions for
internal registers X and Y must be provided to use any of the
double-precision floating-point instructions. Custom
instructions taking two double-precision source operands expect
the first operand in the 64-bit register X. The other operand
(or only operand of a unary operation) is given to the custom
arithmetic instruction with the least significant half in source
register src1 and the most significant half in src2. A custom
instruction that returns a double-precision result returns the
most significant 32 bits in the destination register and the
other half in 32-bit register Y. GCC automatically generates the
necessary code sequences to write register X and/or read register
Y when double-precision floating-point instructions are used.
fwrx
Write src1 into the least significant half of X and src2 into
the most significant half of X.
fwry
Write src1 into Y.
frdxhi, frdxlo
Read the most or least (respectively) significant half of X
and store it in dest.
frdy
Read the value of Y and store it into dest.
Note that you can gain more local control over generation of Nios
II custom instructions by using the "target("custom-insn=N")" and
"target("no-custom-insn")" function attributes or pragmas.
-mcustom-fpu-cfg=name
This option enables a predefined, named set of custom instruction
encodings (see -mcustom-insn above). Currently, the following
sets are defined:
-mcustom-fpu-cfg=60-1 is equivalent to: -mcustom-fmuls=252
-mcustom-fadds=253 -mcustom-fsubs=254 -fsingle-precision-constant
-mcustom-fpu-cfg=60-2 is equivalent to: -mcustom-fmuls=252
-mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255
-fsingle-precision-constant
-mcustom-fpu-cfg=72-3 is equivalent to: -mcustom-floatus=243
-mcustom-fixsi=244 -mcustom-floatis=245 -mcustom-fcmpgts=246
-mcustom-fcmples=249 -mcustom-fcmpeqs=250 -mcustom-fcmpnes=251
-mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254
-mcustom-fdivs=255 -fsingle-precision-constant
Custom instruction assignments given by individual -mcustom-insn=
options override those given by -mcustom-fpu-cfg=, regardless of
the order of the options on the command line.
Note that you can gain more local control over selection of a FPU
configuration by using the "target("custom-fpu-cfg=name")"
function attribute or pragma.
These additional -m options are available for the Altera Nios II ELF
(bare-metal) target:
-mhal
Link with HAL BSP. This suppresses linking with the GCC-provided
C runtime startup and termination code, and is typically used in
conjunction with -msys-crt0= to specify the location of the
alternate startup code provided by the HAL BSP.
-msmallc
Link with a limited version of the C library, -lsmallc, rather
than Newlib.
-msys-crt0=startfile
startfile is the file name of the startfile (crt0) to use when
linking. This option is only useful in conjunction with -mhal.
-msys-lib=systemlib
systemlib is the library name of the library that provides low-
level system calls required by the C library, e.g. "read" and
"write". This option is typically used to link with a library
provided by a HAL BSP.
Nvidia PTX Options
These options are defined for Nvidia PTX:
-m32
-m64
Generate code for 32-bit or 64-bit ABI.
-mmainkernel
Link in code for a __main kernel. This is for stand-alone
instead of offloading execution.
-moptimize
Apply partitioned execution optimizations. This is the default
when any level of optimization is selected.
-msoft-stack
Generate code that does not use ".local" memory directly for
stack storage. Instead, a per-warp stack pointer is maintained
explicitly. This enables variable-length stack allocation (with
variable-length arrays or "alloca"), and when global memory is
used for underlying storage, makes it possible to access
automatic variables from other threads, or with atomic
instructions. This code generation variant is used for OpenMP
offloading, but the option is exposed on its own for the purpose
of testing the compiler; to generate code suitable for linking
into programs using OpenMP offloading, use option -mgomp.
-muniform-simt
Switch to code generation variant that allows to execute all
threads in each warp, while maintaining memory state and side
effects as if only one thread in each warp was active outside of
OpenMP SIMD regions. All atomic operations and calls to runtime
(malloc, free, vprintf) are conditionally executed (iff current
lane index equals the master lane index), and the register being
assigned is copied via a shuffle instruction from the master
lane. Outside of SIMD regions lane 0 is the master; inside, each
thread sees itself as the master. Shared memory array "int
__nvptx_uni[]" stores all-zeros or all-ones bitmasks for each
warp, indicating current mode (0 outside of SIMD regions). Each
thread can bitwise-and the bitmask at position "tid.y" with
current lane index to compute the master lane index.
-mgomp
Generate code for use in OpenMP offloading: enables -msoft-stack
and -muniform-simt options, and selects corresponding multilib
variant.
PDP-11 Options
These options are defined for the PDP-11:
-mfpu
Use hardware FPP floating point. This is the default. (FIS
floating point on the PDP-11/40 is not supported.)
-msoft-float
Do not use hardware floating point.
-mac0
Return floating-point results in ac0 (fr0 in Unix assembler
syntax).
-mno-ac0
Return floating-point results in memory. This is the default.
-m40
Generate code for a PDP-11/40.
-m45
Generate code for a PDP-11/45. This is the default.
-m10
Generate code for a PDP-11/10.
-mbcopy-builtin
Use inline "movmemhi" patterns for copying memory. This is the
default.
-mbcopy
Do not use inline "movmemhi" patterns for copying memory.
-mint16
-mno-int32
Use 16-bit "int". This is the default.
-mint32
-mno-int16
Use 32-bit "int".
-mfloat64
-mno-float32
Use 64-bit "float". This is the default.
-mfloat32
-mno-float64
Use 32-bit "float".
-mabshi
Use "abshi2" pattern. This is the default.
-mno-abshi
Do not use "abshi2" pattern.
-mbranch-expensive
Pretend that branches are expensive. This is for experimenting
with code generation only.
-mbranch-cheap
Do not pretend that branches are expensive. This is the default.
-munix-asm
Use Unix assembler syntax. This is the default when configured
for pdp11-*-bsd.
-mdec-asm
Use DEC assembler syntax. This is the default when configured
for any PDP-11 target other than pdp11-*-bsd.
picoChip Options
These -m options are defined for picoChip implementations:
-mae=ae_type
Set the instruction set, register set, and instruction scheduling
parameters for array element type ae_type. Supported values for
ae_type are ANY, MUL, and MAC.
-mae=ANY selects a completely generic AE type. Code generated
with this option runs on any of the other AE types. The code is
not as efficient as it would be if compiled for a specific AE
type, and some types of operation (e.g., multiplication) do not
work properly on all types of AE.
-mae=MUL selects a MUL AE type. This is the most useful AE type
for compiled code, and is the default.
-mae=MAC selects a DSP-style MAC AE. Code compiled with this
option may suffer from poor performance of byte (char)
manipulation, since the DSP AE does not provide hardware support
for byte load/stores.
-msymbol-as-address
Enable the compiler to directly use a symbol name as an address
in a load/store instruction, without first loading it into a
register. Typically, the use of this option generates larger
programs, which run faster than when the option isn't used.
However, the results vary from program to program, so it is left
as a user option, rather than being permanently enabled.
-mno-inefficient-warnings
Disables warnings about the generation of inefficient code.
These warnings can be generated, for example, when compiling code
that performs byte-level memory operations on the MAC AE type.
The MAC AE has no hardware support for byte-level memory
operations, so all byte load/stores must be synthesized from word
load/store operations. This is inefficient and a warning is
generated to indicate that you should rewrite the code to avoid
byte operations, or to target an AE type that has the necessary
hardware support. This option disables these warnings.
PowerPC Options
These are listed under
RISC-V Options
These command-line options are defined for RISC-V targets:
-mbranch-cost=n
Set the cost of branches to roughly n instructions.
-mmemcpy
-mno-memcpy
Don't optimize block moves.
-mplt
-mno-plt
When generating PIC code, allow the use of PLTs. Ignored for non-
PIC.
-mabi=ABI-string
Specify integer and floating-point calling convention. This
defaults to the natural calling convention: e.g. LP64 for RV64I,
ILP32 for RV32I, LP64D for RV64G.
-mfdiv
-mno-fdiv
Use hardware floating-point divide and square root instructions.
This requires the F or D extensions for floating-point registers.
-mdiv
-mno-div
Use hardware instructions for integer division. This requires
the M extension.
-march=ISA-string
Generate code for given RISC-V ISA (e.g. rv64im). ISA strings
must be lower-case. Examples include rv64i, rv32g, and rv32imaf.
-mtune=processor-string
Optimize the output for the given processor, specified by
microarchitecture name.
-msmall-data-limit=n
Put global and static data smaller than n bytes into a special
section (on some targets).
-msave-restore
-mno-save-restore
Use smaller but slower prologue and epilogue code.
-mcmodel=code-model
Specify the code model.
RL78 Options
-msim
Links in additional target libraries to support operation within
a simulator.
-mmul=none
-mmul=g10
-mmul=g13
-mmul=g14
-mmul=rl78
Specifies the type of hardware multiplication and division
support to be used. The simplest is "none", which uses software
for both multiplication and division. This is the default. The
"g13" value is for the hardware multiply/divide peripheral found
on the RL78/G13 (S2 core) targets. The "g14" value selects the
use of the multiplication and division instructions supported by
the RL78/G14 (S3 core) parts. The value "rl78" is an alias for
"g14" and the value "mg10" is an alias for "none".
In addition a C preprocessor macro is defined, based upon the
setting of this option. Possible values are:
"__RL78_MUL_NONE__", "__RL78_MUL_G13__" or "__RL78_MUL_G14__".
-mcpu=g10
-mcpu=g13
-mcpu=g14
-mcpu=rl78
Specifies the RL78 core to target. The default is the G14 core,
also known as an S3 core or just RL78. The G13 or S2 core does
not have multiply or divide instructions, instead it uses a
hardware peripheral for these operations. The G10 or S1 core
does not have register banks, so it uses a different calling
convention.
If this option is set it also selects the type of hardware
multiply support to use, unless this is overridden by an explicit
-mmul=none option on the command line. Thus specifying -mcpu=g13
enables the use of the G13 hardware multiply peripheral and
specifying -mcpu=g10 disables the use of hardware multiplications
altogether.
Note, although the RL78/G14 core is the default target,
specifying -mcpu=g14 or -mcpu=rl78 on the command line does
change the behavior of the toolchain since it also enables G14
hardware multiply support. If these options are not specified on
the command line then software multiplication routines will be
used even though the code targets the RL78 core. This is for
backwards compatibility with older toolchains which did not have
hardware multiply and divide support.
In addition a C preprocessor macro is defined, based upon the
setting of this option. Possible values are: "__RL78_G10__",
"__RL78_G13__" or "__RL78_G14__".
-mg10
-mg13
-mg14
-mrl78
These are aliases for the corresponding -mcpu= option. They are
provided for backwards compatibility.
-mallregs
Allow the compiler to use all of the available registers. By
default registers "r24..r31" are reserved for use in interrupt
handlers. With this option enabled these registers can be used
in ordinary functions as well.
-m64bit-doubles
-m32bit-doubles
Make the "double" data type be 64 bits (-m64bit-doubles) or 32
bits (-m32bit-doubles) in size. The default is -m32bit-doubles.
-msave-mduc-in-interrupts
-mno-save-mduc-in-interrupts
Specifies that interrupt handler functions should preserve the
MDUC registers. This is only necessary if normal code might use
the MDUC registers, for example because it performs
multiplication and division operations. The default is to ignore
the MDUC registers as this makes the interrupt handlers faster.
The target option -mg13 needs to be passed for this to work as
this feature is only available on the G13 target (S2 core). The
MDUC registers will only be saved if the interrupt handler
performs a multiplication or division operation or it calls
another function.
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
-mmfcrf
-mno-mfcrf
-mpopcntb
-mno-popcntb
-mpopcntd
-mno-popcntd
-mfprnd
-mno-fprnd
-mcmpb
-mno-cmpb
-mmfpgpr
-mno-mfpgpr
-mhard-dfp
-mno-hard-dfp
You use these options to specify which instructions are available
on the processor you are using. The default value of these
options is determined when configuring GCC. Specifying the
-mcpu=cpu_type overrides the specification of these options. We
recommend you use the -mcpu=cpu_type option rather than the
options listed above.
Specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC
architecture instructions in the General Purpose group, including
floating-point square root. Specifying -mpowerpc-gfxopt allows
GCC to use the optional PowerPC architecture instructions in the
Graphics group, including floating-point select.
The -mmfcrf option allows GCC to generate the move from condition
register field instruction implemented on the POWER4 processor
and other processors that support the PowerPC V2.01 architecture.
The -mpopcntb option allows GCC to generate the popcount and
double-precision FP reciprocal estimate instruction implemented
on the POWER5 processor and other processors that support the
PowerPC V2.02 architecture. The -mpopcntd option allows GCC to
generate the popcount instruction implemented on the POWER7
processor and other processors that support the PowerPC V2.06
architecture. The -mfprnd option allows GCC to generate the FP
round to integer instructions implemented on the POWER5+
processor and other processors that support the PowerPC V2.03
architecture. The -mcmpb option allows GCC to generate the
compare bytes instruction implemented on the POWER6 processor and
other processors that support the PowerPC V2.05 architecture.
The -mmfpgpr option allows GCC to generate the FP move to/from
general-purpose register instructions implemented on the POWER6X
processor and other processors that support the extended PowerPC
V2.05 architecture. The -mhard-dfp option allows GCC to generate
the decimal floating-point instructions implemented on some POWER
processors.
The -mpowerpc64 option allows GCC to generate the additional
64-bit instructions that are found in the full PowerPC64
architecture and to treat GPRs as 64-bit, doubleword quantities.
GCC defaults to -mno-powerpc64.
-mcpu=cpu_type
Set architecture type, register usage, and instruction scheduling
parameters for machine type cpu_type. Supported values for
cpu_type are 401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476,
476fp, 505, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400,
7450, 750, 801, 821, 823, 860, 970, 8540, a2, e300c2, e300c3,
e500mc, e500mc64, e5500, e6500, ec603e, G3, G4, G5, titan,
power3, power4, power5, power5+, power6, power6x, power7, power8,
power9, powerpc, powerpc64, powerpc64le, and rs64.
-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify
pure 32-bit PowerPC (either endian), 64-bit big endian PowerPC
and 64-bit little endian PowerPC architecture machine types, with
an appropriate, generic processor model assumed for scheduling
purposes.
The other options specify a specific processor. Code generated
under those options runs best on that processor, and may not run
at all on others.
The -mcpu options automatically enable or disable the following
options:
-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple -mpopcntb
-mpopcntd -mpowerpc64 -mpowerpc-gpopt -mpowerpc-gfxopt
-msingle-float -mdouble-float -msimple-fpu -mstring -mmulhw
-mdlmzb -mmfpgpr -mvsx -mcrypto -mdirect-move -mhtm
-mpower8-fusion -mpower8-vector -mquad-memory
-mquad-memory-atomic -mfloat128 -mfloat128-hardware
The particular options set for any particular CPU varies between
compiler versions, depending on what setting seems to produce
optimal code for that CPU; it doesn't necessarily reflect the
actual hardware's capabilities. If you wish to set an individual
option to a particular value, you may specify it after the -mcpu
option, like -mcpu=970 -mno-altivec.
On AIX, the -maltivec and -mpowerpc64 options are not enabled or
disabled by the -mcpu option at present because AIX does not have
full support for these options. You may still enable or disable
them individually if you're sure it'll work in your environment.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type
cpu_type, but do not set the architecture type or register usage,
as -mcpu=cpu_type does. The same values for cpu_type are used
for -mtune as for -mcpu. If both are specified, the code
generated uses the architecture and registers set by -mcpu, but
the scheduling parameters set by -mtune.
-mcmodel=small
Generate PowerPC64 code for the small model: The TOC is limited
to 64k.
-mcmodel=medium
Generate PowerPC64 code for the medium model: The TOC and other
static data may be up to a total of 4G in size. This is the
default for 64-bit Linux.
-mcmodel=large
Generate PowerPC64 code for the large model: The TOC may be up to
4G in size. Other data and code is only limited by the 64-bit
address space.
-maltivec
-mno-altivec
Generate code that uses (does not use) AltiVec instructions, and
also enable the use of built-in functions that allow more direct
access to the AltiVec instruction set. You may also need to set
-mabi=altivec to adjust the current ABI with AltiVec ABI
enhancements.
When -maltivec is used, rather than -maltivec=le or -maltivec=be,
the element order for AltiVec intrinsics such as "vec_splat",
"vec_extract", and "vec_insert" match array element order
corresponding to the endianness of the target. That is, element
zero identifies the leftmost element in a vector register when
targeting a big-endian platform, and identifies the rightmost
element in a vector register when targeting a little-endian
platform.
-maltivec=be
Generate AltiVec instructions using big-endian element order,
regardless of whether the target is big- or little-endian. This
is the default when targeting a big-endian platform.
The element order is used to interpret element numbers in AltiVec
intrinsics such as "vec_splat", "vec_extract", and "vec_insert".
By default, these match array element order corresponding to the
endianness for the target.
-maltivec=le
Generate AltiVec instructions using little-endian element order,
regardless of whether the target is big- or little-endian. This
is the default when targeting a little-endian platform. This
option is currently ignored when targeting a big-endian platform.
The element order is used to interpret element numbers in AltiVec
intrinsics such as "vec_splat", "vec_extract", and "vec_insert".
By default, these match array element order corresponding to the
endianness for the target.
-mvrsave
-mno-vrsave
Generate VRSAVE instructions when generating AltiVec code.
-mgen-cell-microcode
Generate Cell microcode instructions.
-mwarn-cell-microcode
Warn when a Cell microcode instruction is emitted. An example of
a Cell microcode instruction is a variable shift.
-msecure-plt
Generate code that allows ld and ld.so to build executables and
shared libraries with non-executable ".plt" and ".got" sections.
This is a PowerPC 32-bit SYSV ABI option.
-mbss-plt
Generate code that uses a BSS ".plt" section that ld.so fills in,
and requires ".plt" and ".got" sections that are both writable
and executable. This is a PowerPC 32-bit SYSV ABI option.
-misel
-mno-isel
This switch enables or disables the generation of ISEL
instructions.
-misel=yes/no
This switch has been deprecated. Use -misel and -mno-isel
instead.
-mlra
Enable Local Register Allocation. By default the port uses LRA.
(i.e. -mno-lra).
-mspe
-mno-spe
This switch enables or disables the generation of SPE simd
instructions.
-mpaired
-mno-paired
This switch enables or disables the generation of PAIRED simd
instructions.
-mspe=yes/no
This option has been deprecated. Use -mspe and -mno-spe instead.
-mvsx
-mno-vsx
Generate code that uses (does not use) vector/scalar (VSX)
instructions, and also enable the use of built-in functions that
allow more direct access to the VSX instruction set.
-mcrypto
-mno-crypto
Enable the use (disable) of the built-in functions that allow
direct access to the cryptographic instructions that were added
in version 2.07 of the PowerPC ISA.
-mdirect-move
-mno-direct-move
Generate code that uses (does not use) the instructions to move
data between the general purpose registers and the vector/scalar
(VSX) registers that were added in version 2.07 of the PowerPC
ISA.
-mhtm
-mno-htm
Enable (disable) the use of the built-in functions that allow
direct access to the Hardware Transactional Memory (HTM)
instructions that were added in version 2.07 of the PowerPC ISA.
-mpower8-fusion
-mno-power8-fusion
Generate code that keeps (does not keeps) some integer operations
adjacent so that the instructions can be fused together on power8
and later processors.
-mpower8-vector
-mno-power8-vector
Generate code that uses (does not use) the vector and scalar
instructions that were added in version 2.07 of the PowerPC ISA.
Also enable the use of built-in functions that allow more direct
access to the vector instructions.
-mquad-memory
-mno-quad-memory
Generate code that uses (does not use) the non-atomic quad word
memory instructions. The -mquad-memory option requires use of
64-bit mode.
-mquad-memory-atomic
-mno-quad-memory-atomic
Generate code that uses (does not use) the atomic quad word
memory instructions. The -mquad-memory-atomic option requires
use of 64-bit mode.
-mupper-regs-di
-mno-upper-regs-di
Generate code that uses (does not use) the scalar instructions
that target all 64 registers in the vector/scalar floating point
register set that were added in version 2.06 of the PowerPC ISA
when processing integers. -mupper-regs-di is turned on by
default if you use any of the -mcpu=power7, -mcpu=power8,
-mcpu=power9, or -mvsx options.
-mupper-regs-df
-mno-upper-regs-df
Generate code that uses (does not use) the scalar double
precision instructions that target all 64 registers in the
vector/scalar floating point register set that were added in
version 2.06 of the PowerPC ISA. -mupper-regs-df is turned on by
default if you use any of the -mcpu=power7, -mcpu=power8,
-mcpu=power9, or -mvsx options.
-mupper-regs-sf
-mno-upper-regs-sf
Generate code that uses (does not use) the scalar single
precision instructions that target all 64 registers in the
vector/scalar floating point register set that were added in
version 2.07 of the PowerPC ISA. -mupper-regs-sf is turned on by
default if you use either of the -mcpu=power8, -mpower8-vector,
or -mcpu=power9 options.
-mupper-regs
-mno-upper-regs
Generate code that uses (does not use) the scalar instructions
that target all 64 registers in the vector/scalar floating point
register set, depending on the model of the machine.
If the -mno-upper-regs option is used, it turns off both
-mupper-regs-sf and -mupper-regs-df options.
-mfloat128
-mno-float128
Enable/disable the __float128 keyword for IEEE 128-bit floating
point and use either software emulation for IEEE 128-bit floating
point or hardware instructions.
The VSX instruction set (-mvsx, -mcpu=power7, or -mcpu=power8)
must be enabled to use the -mfloat128 option. The -mfloat128
option only works on PowerPC 64-bit Linux systems.
If you use the ISA 3.0 instruction set (-mcpu=power9), the
-mfloat128 option will also enable the generation of ISA 3.0 IEEE
128-bit floating point instructions. Otherwise, IEEE 128-bit
floating point will be done with software emulation.
-mfloat128-hardware
-mno-float128-hardware
Enable/disable using ISA 3.0 hardware instructions to support the
__float128 data type.
If you use -mfloat128-hardware, it will enable the option
-mfloat128 as well.
If you select ISA 3.0 instructions with -mcpu=power9, but do not
use either -mfloat128 or -mfloat128-hardware, the IEEE 128-bit
floating point support will not be enabled.
-mfloat-gprs=yes/single/double/no
-mfloat-gprs
This switch enables or disables the generation of floating-point
operations on the general-purpose registers for architectures
that support it.
The argument yes or single enables the use of single-precision
floating-point operations.
The argument double enables the use of single and double-
precision floating-point operations.
The argument no disables floating-point operations on the
general-purpose registers.
This option is currently only available on the MPC854x.
-m32
-m64
Generate code for 32-bit or 64-bit environments of Darwin and
SVR4 targets (including GNU/Linux). The 32-bit environment sets
int, long and pointer to 32 bits and generates code that runs on
any PowerPC variant. The 64-bit environment sets int to 32 bits
and long and pointer to 64 bits, and generates code for
PowerPC64, as for -mpowerpc64.
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is
created for every executable file. The -mfull-toc option is
selected by default. In that case, GCC allocates at least one
TOC entry for each unique non-automatic variable reference in
your program. GCC also places floating-point constants in the
TOC. However, only 16,384 entries are available in the TOC.
If you receive a linker error message that saying you have
overflowed the available TOC space, you can reduce the amount of
TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc
options. -mno-fp-in-toc prevents GCC from putting floating-point
constants in the TOC and -mno-sum-in-toc forces GCC to generate
code to calculate the sum of an address and a constant at run
time instead of putting that sum into the TOC. You may specify
one or both of these options. Each causes GCC to produce very
slightly slower and larger code at the expense of conserving TOC
space.
If you still run out of space in the TOC even when you specify
both of these options, specify -mminimal-toc instead. This
option causes GCC to make only one TOC entry for every file.
When you specify this option, GCC produces code that is slower
and larger but which uses extremely little TOC space. You may
wish to use this option only on files that contain less
frequently-executed code.
-maix64
-maix32
Enable 64-bit AIX ABI and calling convention: 64-bit pointers,
64-bit "long" type, and the infrastructure needed to support
them. Specifying -maix64 implies -mpowerpc64, while -maix32
disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults
to -maix32.
-mxl-compat
-mno-xl-compat
Produce code that conforms more closely to IBM XL compiler
semantics when using AIX-compatible ABI. Pass floating-point
arguments to prototyped functions beyond the register save area
(RSA) on the stack in addition to argument FPRs. Do not assume
that most significant double in 128-bit long double value is
properly rounded when comparing values and converting to double.
Use XL symbol names for long double support routines.
The AIX calling convention was extended but not initially
documented to handle an obscure K&R C case of calling a function
that takes the address of its arguments with fewer arguments than
declared. IBM XL compilers access floating-point arguments that
do not fit in the RSA from the stack when a subroutine is
compiled without optimization. Because always storing floating-
point arguments on the stack is inefficient and rarely needed,
this option is not enabled by default and only is necessary when
calling subroutines compiled by IBM XL compilers without
optimization.
-mpe
Support IBM RS/6000 SP Parallel Environment (PE). Link an
application written to use message passing with special startup
code to enable the application to run. The system must have PE
installed in the standard location (/usr/lpp/ppe.poe/), or the
specs file must be overridden with the -specs= option to specify
the appropriate directory location. The Parallel Environment
does not support threads, so the -mpe option and the -pthread
option are incompatible.
-malign-natural
-malign-power
On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option
-malign-natural overrides the ABI-defined alignment of larger
types, such as floating-point doubles, on their natural size-
based boundary. The option -malign-power instructs GCC to follow
the ABI-specified alignment rules. GCC defaults to the standard
alignment defined in the ABI.
On 64-bit Darwin, natural alignment is the default, and
-malign-power is not supported.
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point
register set. Software floating-point emulation is provided if
you use the -msoft-float option, and pass the option to GCC when
linking.
-msingle-float
-mdouble-float
Generate code for single- or double-precision floating-point
operations. -mdouble-float implies -msingle-float.
-msimple-fpu
Do not generate "sqrt" and "div" instructions for hardware
floating-point unit.
-mfpu=name
Specify type of floating-point unit. Valid values for name are
sp_lite (equivalent to -msingle-float -msimple-fpu), dp_lite
(equivalent to -mdouble-float -msimple-fpu), sp_full (equivalent
to -msingle-float), and dp_full (equivalent to -mdouble-float).
-mxilinx-fpu
Perform optimizations for the floating-point unit on Xilinx PPC
405/440.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word
instructions and the store multiple word instructions. These
instructions are generated by default on POWER systems, and not
generated on PowerPC systems. Do not use -mmultiple on little-
endian PowerPC systems, since those instructions do not work when
the processor is in little-endian mode. The exceptions are
PPC740 and PPC750 which permit these instructions in little-
endian mode.
-mstring
-mno-string
Generate code that uses (does not use) the load string
instructions and the store string word instructions to save
multiple registers and do small block moves. These instructions
are generated by default on POWER systems, and not generated on
PowerPC systems. Do not use -mstring on little-endian PowerPC
systems, since those instructions do not work when the processor
is in little-endian mode. The exceptions are PPC740 and PPC750
which permit these instructions in little-endian mode.
-mupdate
-mno-update
Generate code that uses (does not use) the load or store
instructions that update the base register to the address of the
calculated memory location. These instructions are generated by
default. If you use -mno-update, there is a small window between
the time that the stack pointer is updated and the address of the
previous frame is stored, which means code that walks the stack
frame across interrupts or signals may get corrupted data.
-mavoid-indexed-addresses
-mno-avoid-indexed-addresses
Generate code that tries to avoid (not avoid) the use of indexed
load or store instructions. These instructions can incur a
performance penalty on Power6 processors in certain situations,
such as when stepping through large arrays that cross a 16M
boundary. This option is enabled by default when targeting
Power6 and disabled otherwise.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point
multiply and accumulate instructions. These instructions are
generated by default if hardware floating point is used. The
machine-dependent -mfused-madd option is now mapped to the
machine-independent -ffp-contract=fast option, and
-mno-fused-madd is mapped to -ffp-contract=off.
-mmulhw
-mno-mulhw
Generate code that uses (does not use) the half-word multiply and
multiply-accumulate instructions on the IBM 405, 440, 464 and 476
processors. These instructions are generated by default when
targeting those processors.
-mdlmzb
-mno-dlmzb
Generate code that uses (does not use) the string-search dlmzb
instruction on the IBM 405, 440, 464 and 476 processors. This
instruction is generated by default when targeting those
processors.
-mno-bit-align
-mbit-align
On System V.4 and embedded PowerPC systems do not (do) force
structures and unions that contain bit-fields to be aligned to
the base type of the bit-field.
For example, by default a structure containing nothing but 8
"unsigned" bit-fields of length 1 is aligned to a 4-byte boundary
and has a size of 4 bytes. By using -mno-bit-align, the
structure is aligned to a 1-byte boundary and is 1 byte in size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume
that unaligned memory references are handled by the system.
-mrelocatable
-mno-relocatable
Generate code that allows (does not allow) a static executable to
be relocated to a different address at run time. A simple
embedded PowerPC system loader should relocate the entire
contents of ".got2" and 4-byte locations listed in the ".fixup"
section, a table of 32-bit addresses generated by this option.
For this to work, all objects linked together must be compiled
with -mrelocatable or -mrelocatable-lib. -mrelocatable code
aligns the stack to an 8-byte boundary.
-mrelocatable-lib
-mno-relocatable-lib
Like -mrelocatable, -mrelocatable-lib generates a ".fixup"
section to allow static executables to be relocated at run time,
but -mrelocatable-lib does not use the smaller stack alignment of
-mrelocatable. Objects compiled with -mrelocatable-lib may be
linked with objects compiled with any combination of the
-mrelocatable options.
-mno-toc
-mtoc
On System V.4 and embedded PowerPC systems do not (do) assume
that register 2 contains a pointer to a global area pointing to
the addresses used in the program.
-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for the
processor in little-endian mode. The -mlittle-endian option is
the same as -mlittle.
-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code for the
processor in big-endian mode. The -mbig-endian option is the
same as -mbig.
-mdynamic-no-pic
On Darwin and Mac OS X systems, compile code so that it is not
relocatable, but that its external references are relocatable.
The resulting code is suitable for applications, but not shared
libraries.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather
than loading it in the prologue for each function. The runtime
system is responsible for initializing this register with an
appropriate value before execution begins.
-mprioritize-restricted-insns=priority
This option controls the priority that is assigned to dispatch-
slot restricted instructions during the second scheduling pass.
The argument priority takes the value 0, 1, or 2 to assign no,
highest, or second-highest (respectively) priority to dispatch-
slot restricted instructions.
-msched-costly-dep=dependence_type
This option controls which dependences are considered costly by
the target during instruction scheduling. The argument
dependence_type takes one of the following values:
no No dependence is costly.
all All dependences are costly.
true_store_to_load
A true dependence from store to load is costly.
store_to_load
Any dependence from store to load is costly.
number
Any dependence for which the latency is greater than or equal
to number is costly.
-minsert-sched-nops=scheme
This option controls which NOP insertion scheme is used during
the second scheduling pass. The argument scheme takes one of the
following values:
no Don't insert NOPs.
pad Pad with NOPs any dispatch group that has vacant issue slots,
according to the scheduler's grouping.
regroup_exact
Insert NOPs to force costly dependent insns into separate
groups. Insert exactly as many NOPs as needed to force an
insn to a new group, according to the estimated processor
grouping.
number
Insert NOPs to force costly dependent insns into separate
groups. Insert number NOPs to force an insn to a new group.
-mcall-sysv
On System V.4 and embedded PowerPC systems compile code using
calling conventions that adhere to the March 1995 draft of the
System V Application Binary Interface, PowerPC processor
supplement. This is the default unless you configured GCC using
powerpc-*-eabiaix.
-mcall-sysv-eabi
-mcall-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-aixdesc
On System V.4 and embedded PowerPC systems compile code for the
AIX operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the
Linux-based GNU system.
-mcall-freebsd
On System V.4 and embedded PowerPC systems compile code for the
FreeBSD operating system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the
NetBSD operating system.
-mcall-openbsd
On System V.4 and embedded PowerPC systems compile code for the
OpenBSD operating system.
-maix-struct-return
Return all structures in memory (as specified by the AIX ABI).
-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as specified
by the SVR4 ABI).
-mabi=abi-type
Extend the current ABI with a particular extension, or remove
such extension. Valid values are altivec, no-altivec, spe, no-
spe, ibmlongdouble, ieeelongdouble, elfv1, elfv2.
-mabi=spe
Extend the current ABI with SPE ABI extensions. This does not
change the default ABI, instead it adds the SPE ABI extensions to
the current ABI.
-mabi=no-spe
Disable Book-E SPE ABI extensions for the current ABI.
-mabi=ibmlongdouble
Change the current ABI to use IBM extended-precision long double.
This is a PowerPC 32-bit SYSV ABI option.
-mabi=ieeelongdouble
Change the current ABI to use IEEE extended-precision long
double. This is a PowerPC 32-bit Linux ABI option.
-mabi=elfv1
Change the current ABI to use the ELFv1 ABI. This is the default
ABI for big-endian PowerPC 64-bit Linux. Overriding the default
ABI requires special system support and is likely to fail in
spectacular ways.
-mabi=elfv2
Change the current ABI to use the ELFv2 ABI. This is the default
ABI for little-endian PowerPC 64-bit Linux. Overriding the
default ABI requires special system support and is likely to fail
in spectacular ways.
-mgnu-attribute
-mno-gnu-attribute
Emit .gnu_attribute assembly directives to set tag/value pairs in
a .gnu.attributes section that specify ABI variations in function
parameters or return values.
-mprototype
-mno-prototype
On System V.4 and embedded PowerPC systems assume that all calls
to variable argument functions are properly prototyped.
Otherwise, the compiler must insert an instruction before every
non-prototyped call to set or clear bit 6 of the condition code
register ("CR") to indicate whether floating-point values are
passed in the floating-point registers in case the function takes
variable arguments. With -mprototype, only calls to prototyped
variable argument functions set or clear the bit.
-msim
On embedded PowerPC systems, assume that the startup module is
called sim-crt0.o and that the standard C libraries are libsim.a
and libc.a. This is the default for powerpc-*-eabisim
configurations.
-mmvme
On embedded PowerPC systems, assume that the startup module is
called crt0.o and the standard C libraries are libmvme.a and
libc.a.
-mads
On embedded PowerPC systems, assume that the startup module is
called crt0.o and the standard C libraries are libads.a and
libc.a.
-myellowknife
On embedded PowerPC systems, assume that the startup module is
called crt0.o and the standard C libraries are libyk.a and
libc.a.
-mvxworks
On System V.4 and embedded PowerPC systems, specify that you are
compiling for a VxWorks system.
-memb
On embedded PowerPC systems, set the "PPC_EMB" bit in the ELF
flags header to indicate that eabi extended relocations are used.
-meabi
-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere to
the Embedded Applications Binary Interface (EABI), which is a set
of modifications to the System V.4 specifications. Selecting
-meabi means that the stack is aligned to an 8-byte boundary, a
function "__eabi" is called from "main" to set up the EABI
environment, and the -msdata option can use both "r2" and "r13"
to point to two separate small data areas. Selecting -mno-eabi
means that the stack is aligned to a 16-byte boundary, no EABI
initialization function is called from "main", and the -msdata
option only uses "r13" to point to a single small data area. The
-meabi option is on by default if you configured GCC using one of
the powerpc*-*-eabi* options.
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small initialized
"const" global and static data in the ".sdata2" section, which is
pointed to by register "r2". Put small initialized non-"const"
global and static data in the ".sdata" section, which is pointed
to by register "r13". Put small uninitialized global and static
data in the ".sbss" section, which is adjacent to the ".sdata"
section. The -msdata=eabi option is incompatible with the
-mrelocatable option. The -msdata=eabi option also sets the
-memb option.
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global and
static data in the ".sdata" section, which is pointed to by
register "r13". Put small uninitialized global and static data
in the ".sbss" section, which is adjacent to the ".sdata"
section. The -msdata=sysv option is incompatible with the
-mrelocatable option.
-msdata=default
-msdata
On System V.4 and embedded PowerPC systems, if -meabi is used,
compile code the same as -msdata=eabi, otherwise compile code the
same as -msdata=sysv.
-msdata=data
On System V.4 and embedded PowerPC systems, put small global data
in the ".sdata" section. Put small uninitialized global data in
the ".sbss" section. Do not use register "r13" to address small
data however. This is the default behavior unless other -msdata
options are used.
-msdata=none
-mno-sdata
On embedded PowerPC systems, put all initialized global and
static data in the ".data" section, and all uninitialized data in
the ".bss" section.
-mblock-move-inline-limit=num
Inline all block moves (such as calls to "memcpy" or structure
copies) less than or equal to num bytes. The minimum value for
num is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets.
The default value is target-specific.
-G num
On embedded PowerPC systems, put global and static items less
than or equal to num bytes into the small data or BSS sections
instead of the normal data or BSS section. By default, num is 8.
The -G num switch is also passed to the linker. All modules
should be compiled with the same -G num value.
-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit
register names in the assembly language output using symbolic
forms.
-mlongcall
-mno-longcall
By default assume that all calls are far away so that a longer
and more expensive calling sequence is required. This is
required for calls farther than 32 megabytes (33,554,432 bytes)
from the current location. A short call is generated if the
compiler knows the call cannot be that far away. This setting
can be overridden by the "shortcall" function attribute, or by
"#pragma longcall(0)".
Some linkers are capable of detecting out-of-range calls and
generating glue code on the fly. On these systems, long calls
are unnecessary and generate slower code. As of this writing,
the AIX linker can do this, as can the GNU linker for PowerPC/64.
It is planned to add this feature to the GNU linker for 32-bit
PowerPC systems as well.
On Darwin/PPC systems, "#pragma longcall" generates "jbsr callee,
L42", plus a branch island (glue code). The two target addresses
represent the callee and the branch island. The Darwin/PPC
linker prefers the first address and generates a "bl callee" if
the PPC "bl" instruction reaches the callee directly; otherwise,
the linker generates "bl L42" to call the branch island. The
branch island is appended to the body of the calling function; it
computes the full 32-bit address of the callee and jumps to it.
On Mach-O (Darwin) systems, this option directs the compiler emit
to the glue for every direct call, and the Darwin linker decides
whether to use or discard it.
In the future, GCC may ignore all longcall specifications when
the linker is known to generate glue.
-mtls-markers
-mno-tls-markers
Mark (do not mark) calls to "__tls_get_addr" with a relocation
specifying the function argument. The relocation allows the
linker to reliably associate function call with argument setup
instructions for TLS optimization, which in turn allows GCC to
better schedule the sequence.
-mrecip
-mno-recip
This option enables use of the reciprocal estimate and reciprocal
square root estimate instructions with additional Newton-Raphson
steps to increase precision instead of doing a divide or square
root and divide for floating-point arguments. You should use the
-ffast-math option when using -mrecip (or at least
-funsafe-math-optimizations, -ffinite-math-only,
-freciprocal-math and -fno-trapping-math). Note that while the
throughput of the sequence is generally higher than the
throughput of the non-reciprocal instruction, the precision of
the sequence can be decreased by up to 2 ulp (i.e. the inverse of
1.0 equals 0.99999994) for reciprocal square roots.
-mrecip=opt
This option controls which reciprocal estimate instructions may
be used. opt is a comma-separated list of options, which may be
preceded by a "!" to invert the option:
all Enable all estimate instructions.
default
Enable the default instructions, equivalent to -mrecip.
none
Disable all estimate instructions, equivalent to -mno-recip.
div Enable the reciprocal approximation instructions for both
single and double precision.
divf
Enable the single-precision reciprocal approximation
instructions.
divd
Enable the double-precision reciprocal approximation
instructions.
rsqrt
Enable the reciprocal square root approximation instructions
for both single and double precision.
rsqrtf
Enable the single-precision reciprocal square root
approximation instructions.
rsqrtd
Enable the double-precision reciprocal square root
approximation instructions.
So, for example, -mrecip=all,!rsqrtd enables all of the
reciprocal estimate instructions, except for the "FRSQRTE",
"XSRSQRTEDP", and "XVRSQRTEDP" instructions which handle the
double-precision reciprocal square root calculations.
-mrecip-precision
-mno-recip-precision
Assume (do not assume) that the reciprocal estimate instructions
provide higher-precision estimates than is mandated by the
PowerPC ABI. Selecting -mcpu=power6, -mcpu=power7 or
-mcpu=power8 automatically selects -mrecip-precision. The
double-precision square root estimate instructions are not
generated by default on low-precision machines, since they do not
provide an estimate that converges after three steps.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an
external library. The only type supported at present is mass,
which specifies to use IBM's Mathematical Acceleration Subsystem
(MASS) libraries for vectorizing intrinsics using external
libraries. GCC currently emits calls to "acosd2", "acosf4",
"acoshd2", "acoshf4", "asind2", "asinf4", "asinhd2", "asinhf4",
"atan2d2", "atan2f4", "atand2", "atanf4", "atanhd2", "atanhf4",
"cbrtd2", "cbrtf4", "cosd2", "cosf4", "coshd2", "coshf4",
"erfcd2", "erfcf4", "erfd2", "erff4", "exp2d2", "exp2f4",
"expd2", "expf4", "expm1d2", "expm1f4", "hypotd2", "hypotf4",
"lgammad2", "lgammaf4", "log10d2", "log10f4", "log1pd2",
"log1pf4", "log2d2", "log2f4", "logd2", "logf4", "powd2",
"powf4", "sind2", "sinf4", "sinhd2", "sinhf4", "sqrtd2",
"sqrtf4", "tand2", "tanf4", "tanhd2", and "tanhf4" when
generating code for power7. Both -ftree-vectorize and
-funsafe-math-optimizations must also be enabled. The MASS
libraries must be specified at link time.
-mfriz
-mno-friz
Generate (do not generate) the "friz" instruction when the
-funsafe-math-optimizations option is used to optimize rounding
of floating-point values to 64-bit integer and back to floating
point. The "friz" instruction does not return the same value if
the floating-point number is too large to fit in an integer.
-mpointers-to-nested-functions
-mno-pointers-to-nested-functions
Generate (do not generate) code to load up the static chain
register ("r11") when calling through a pointer on AIX and 64-bit
Linux systems where a function pointer points to a 3-word
descriptor giving the function address, TOC value to be loaded in
register "r2", and static chain value to be loaded in register
"r11". The -mpointers-to-nested-functions is on by default. You
cannot call through pointers to nested functions or pointers to
functions compiled in other languages that use the static chain
if you use -mno-pointers-to-nested-functions.
-msave-toc-indirect
-mno-save-toc-indirect
Generate (do not generate) code to save the TOC value in the
reserved stack location in the function prologue if the function
calls through a pointer on AIX and 64-bit Linux systems. If the
TOC value is not saved in the prologue, it is saved just before
the call through the pointer. The -mno-save-toc-indirect option
is the default.
-mcompat-align-parm
-mno-compat-align-parm
Generate (do not generate) code to pass structure parameters with
a maximum alignment of 64 bits, for compatibility with older
versions of GCC.
Older versions of GCC (prior to 4.9.0) incorrectly did not align
a structure parameter on a 128-bit boundary when that structure
contained a member requiring 128-bit alignment. This is
corrected in more recent versions of GCC. This option may be
used to generate code that is compatible with functions compiled
with older versions of GCC.
The -mno-compat-align-parm option is the default.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard. Supported
locations are global for global canary or tls for per-thread
canary in the TLS block (the default with GNU libc version 2.4 or
later).
With the latter choice the options
-mstack-protector-guard-reg=reg and
-mstack-protector-guard-offset=offset furthermore specify which
register to use as base register for reading the canary, and from
what offset from that base register. The default for those is as
specified in the relevant ABI.
RX Options
These command-line options are defined for RX targets:
-m64bit-doubles
-m32bit-doubles
Make the "double" data type be 64 bits (-m64bit-doubles) or 32
bits (-m32bit-doubles) in size. The default is -m32bit-doubles.
Note RX floating-point hardware only works on 32-bit values,
which is why the default is -m32bit-doubles.
-fpu
-nofpu
Enables (-fpu) or disables (-nofpu) the use of RX floating-point
hardware. The default is enabled for the RX600 series and
disabled for the RX200 series.
Floating-point instructions are only generated for 32-bit
floating-point values, however, so the FPU hardware is not used
for doubles if the -m64bit-doubles option is used.
Note If the -fpu option is enabled then
-funsafe-math-optimizations is also enabled automatically. This
is because the RX FPU instructions are themselves unsafe.
-mcpu=name
Selects the type of RX CPU to be targeted. Currently three types
are supported, the generic RX600 and RX200 series hardware and
the specific RX610 CPU. The default is RX600.
The only difference between RX600 and RX610 is that the RX610
does not support the "MVTIPL" instruction.
The RX200 series does not have a hardware floating-point unit and
so -nofpu is enabled by default when this type is selected.
-mbig-endian-data
-mlittle-endian-data
Store data (but not code) in the big-endian format. The default
is -mlittle-endian-data, i.e. to store data in the little-endian
format.
-msmall-data-limit=N
Specifies the maximum size in bytes of global and static
variables which can be placed into the small data area. Using
the small data area can lead to smaller and faster code, but the
size of area is limited and it is up to the programmer to ensure
that the area does not overflow. Also when the small data area
is used one of the RX's registers (usually "r13") is reserved for
use pointing to this area, so it is no longer available for use
by the compiler. This could result in slower and/or larger code
if variables are pushed onto the stack instead of being held in
this register.
Note, common variables (variables that have not been initialized)
and constants are not placed into the small data area as they are
assigned to other sections in the output executable.
The default value is zero, which disables this feature. Note,
this feature is not enabled by default with higher optimization
levels (-O2 etc) because of the potentially detrimental effects
of reserving a register. It is up to the programmer to
experiment and discover whether this feature is of benefit to
their program. See the description of the -mpid option for a
description of how the actual register to hold the small data
area pointer is chosen.
-msim
-mno-sim
Use the simulator runtime. The default is to use the libgloss
board-specific runtime.
-mas100-syntax
-mno-as100-syntax
When generating assembler output use a syntax that is compatible
with Renesas's AS100 assembler. This syntax can also be handled
by the GAS assembler, but it has some restrictions so it is not
generated by default.
-mmax-constant-size=N
Specifies the maximum size, in bytes, of a constant that can be
used as an operand in a RX instruction. Although the RX
instruction set does allow constants of up to 4 bytes in length
to be used in instructions, a longer value equates to a longer
instruction. Thus in some circumstances it can be beneficial to
restrict the size of constants that are used in instructions.
Constants that are too big are instead placed into a constant
pool and referenced via register indirection.
The value N can be between 0 and 4. A value of 0 (the default)
or 4 means that constants of any size are allowed.
-mrelax
Enable linker relaxation. Linker relaxation is a process whereby
the linker attempts to reduce the size of a program by finding
shorter versions of various instructions. Disabled by default.
-mint-register=N
Specify the number of registers to reserve for fast interrupt
handler functions. The value N can be between 0 and 4. A value
of 1 means that register "r13" is reserved for the exclusive use
of fast interrupt handlers. A value of 2 reserves "r13" and
"r12". A value of 3 reserves "r13", "r12" and "r11", and a value
of 4 reserves "r13" through "r10". A value of 0, the default,
does not reserve any registers.
-msave-acc-in-interrupts
Specifies that interrupt handler functions should preserve the
accumulator register. This is only necessary if normal code
might use the accumulator register, for example because it
performs 64-bit multiplications. The default is to ignore the
accumulator as this makes the interrupt handlers faster.
-mpid
-mno-pid
Enables the generation of position independent data. When
enabled any access to constant data is done via an offset from a
base address held in a register. This allows the location of
constant data to be determined at run time without requiring the
executable to be relocated, which is a benefit to embedded
applications with tight memory constraints. Data that can be
modified is not affected by this option.
Note, using this feature reserves a register, usually "r13", for
the constant data base address. This can result in slower and/or
larger code, especially in complicated functions.
The actual register chosen to hold the constant data base address
depends upon whether the -msmall-data-limit and/or the
-mint-register command-line options are enabled. Starting with
register "r13" and proceeding downwards, registers are allocated
first to satisfy the requirements of -mint-register, then -mpid
and finally -msmall-data-limit. Thus it is possible for the
small data area register to be "r8" if both -mint-register=4 and
-mpid are specified on the command line.
By default this feature is not enabled. The default can be
restored via the -mno-pid command-line option.
-mno-warn-multiple-fast-interrupts
-mwarn-multiple-fast-interrupts
Prevents GCC from issuing a warning message if it finds more than
one fast interrupt handler when it is compiling a file. The
default is to issue a warning for each extra fast interrupt
handler found, as the RX only supports one such interrupt.
-mallow-string-insns
-mno-allow-string-insns
Enables or disables the use of the string manipulation
instructions "SMOVF", "SCMPU", "SMOVB", "SMOVU", "SUNTIL"
"SWHILE" and also the "RMPA" instruction. These instructions may
prefetch data, which is not safe to do if accessing an I/O
register. (See section 12.2.7 of the RX62N Group User's Manual
for more information).
The default is to allow these instructions, but it is not
possible for GCC to reliably detect all circumstances where a
string instruction might be used to access an I/O register, so
their use cannot be disabled automatically. Instead it is
reliant upon the programmer to use the -mno-allow-string-insns
option if their program accesses I/O space.
When the instructions are enabled GCC defines the C preprocessor
symbol "__RX_ALLOW_STRING_INSNS__", otherwise it defines the
symbol "__RX_DISALLOW_STRING_INSNS__".
-mjsr
-mno-jsr
Use only (or not only) "JSR" instructions to access functions.
This option can be used when code size exceeds the range of "BSR"
instructions. Note that -mno-jsr does not mean to not use "JSR"
but instead means that any type of branch may be used.
Note: The generic GCC command-line option -ffixed-reg has special
significance to the RX port when used with the "interrupt" function
attribute. This attribute indicates a function intended to process
fast interrupts. GCC ensures that it only uses the registers "r10",
"r11", "r12" and/or "r13" and only provided that the normal use of
the corresponding registers have been restricted via the -ffixed-reg
or -mint-register command-line options.
S/390 and zSeries Options
These are the -m options defined for the S/390 and zSeries
architecture.
-mhard-float
-msoft-float
Use (do not use) the hardware floating-point instructions and
registers for floating-point operations. When -msoft-float is
specified, functions in libgcc.a are used to perform floating-
point operations. When -mhard-float is specified, the compiler
generates IEEE floating-point instructions. This is the default.
-mhard-dfp
-mno-hard-dfp
Use (do not use) the hardware decimal-floating-point instructions
for decimal-floating-point operations. When -mno-hard-dfp is
specified, functions in libgcc.a are used to perform decimal-
floating-point operations. When -mhard-dfp is specified, the
compiler generates decimal-floating-point hardware instructions.
This is the default for -march=z9-ec or higher.
-mlong-double-64
-mlong-double-128
These switches control the size of "long double" type. A size of
64 bits makes the "long double" type equivalent to the "double"
type. This is the default.
-mbackchain
-mno-backchain
Store (do not store) the address of the caller's frame as
backchain pointer into the callee's stack frame. A backchain may
be needed to allow debugging using tools that do not understand
DWARF call frame information. When -mno-packed-stack is in
effect, the backchain pointer is stored at the bottom of the
stack frame; when -mpacked-stack is in effect, the backchain is
placed into the topmost word of the 96/160 byte register save
area.
In general, code compiled with -mbackchain is call-compatible
with code compiled with -mmo-backchain; however, use of the
backchain for debugging purposes usually requires that the whole
binary is built with -mbackchain. Note that the combination of
-mbackchain, -mpacked-stack and -mhard-float is not supported.
In order to build a linux kernel use -msoft-float.
The default is to not maintain the backchain.
-mpacked-stack
-mno-packed-stack
Use (do not use) the packed stack layout. When -mno-packed-stack
is specified, the compiler uses the all fields of the 96/160 byte
register save area only for their default purpose; unused fields
still take up stack space. When -mpacked-stack is specified,
register save slots are densely packed at the top of the register
save area; unused space is reused for other purposes, allowing
for more efficient use of the available stack space. However,
when -mbackchain is also in effect, the topmost word of the save
area is always used to store the backchain, and the return
address register is always saved two words below the backchain.
As long as the stack frame backchain is not used, code generated
with -mpacked-stack is call-compatible with code generated with
-mno-packed-stack. Note that some non-FSF releases of GCC 2.95
for S/390 or zSeries generated code that uses the stack frame
backchain at run time, not just for debugging purposes. Such
code is not call-compatible with code compiled with
-mpacked-stack. Also, note that the combination of -mbackchain,
-mpacked-stack and -mhard-float is not supported. In order to
build a linux kernel use -msoft-float.
The default is to not use the packed stack layout.
-msmall-exec
-mno-small-exec
Generate (or do not generate) code using the "bras" instruction
to do subroutine calls. This only works reliably if the total
executable size does not exceed 64k. The default is to use the
"basr" instruction instead, which does not have this limitation.
-m64
-m31
When -m31 is specified, generate code compliant to the GNU/Linux
for S/390 ABI. When -m64 is specified, generate code compliant
to the GNU/Linux for zSeries ABI. This allows GCC in particular
to generate 64-bit instructions. For the s390 targets, the
default is -m31, while the s390x targets default to -m64.
-mzarch
-mesa
When -mzarch is specified, generate code using the instructions
available on z/Architecture. When -mesa is specified, generate
code using the instructions available on ESA/390. Note that
-mesa is not possible with -m64. When generating code compliant
to the GNU/Linux for S/390 ABI, the default is -mesa. When
generating code compliant to the GNU/Linux for zSeries ABI, the
default is -mzarch.
-mhtm
-mno-htm
The -mhtm option enables a set of builtins making use of
instructions available with the transactional execution facility
introduced with the IBM zEnterprise EC12 machine generation S/390
System z Built-in Functions. -mhtm is enabled by default when
using -march=zEC12.
-mvx
-mno-vx
When -mvx is specified, generate code using the instructions
available with the vector extension facility introduced with the
IBM z13 machine generation. This option changes the ABI for some
vector type values with regard to alignment and calling
conventions. In case vector type values are being used in an
ABI-relevant context a GAS .gnu_attribute command will be added
to mark the resulting binary with the ABI used. -mvx is enabled
by default when using -march=z13.
-mzvector
-mno-zvector
The -mzvector option enables vector language extensions and
builtins using instructions available with the vector extension
facility introduced with the IBM z13 machine generation. This
option adds support for vector to be used as a keyword to define
vector type variables and arguments. vector is only available
when GNU extensions are enabled. It will not be expanded when
requesting strict standard compliance e.g. with -std=c99. In
addition to the GCC low-level builtins -mzvector enables a set of
builtins added for compatibility with AltiVec-style
implementations like Power and Cell. In order to make use of
these builtins the header file vecintrin.h needs to be included.
-mzvector is disabled by default.
-mmvcle
-mno-mvcle
Generate (or do not generate) code using the "mvcle" instruction
to perform block moves. When -mno-mvcle is specified, use a
"mvc" loop instead. This is the default unless optimizing for
size.
-mdebug
-mno-debug
Print (or do not print) additional debug information when
compiling. The default is to not print debug information.
-march=cpu-type
Generate code that runs on cpu-type, which is the name of a
system representing a certain processor type. Possible values
for cpu-type are z900/arch5, z990/arch6, z9-109, z9-ec/arch7,
z10/arch8, z196/arch9, zEC12, z13/arch11, and native.
The default is -march=z900. g5/arch3 and g6 are deprecated and
will be removed with future releases.
Specifying native as cpu type can be used to select the best
architecture option for the host processor. -march=native has no
effect if GCC does not recognize the processor.
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code,
except for the ABI and the set of available instructions. The
list of cpu-type values is the same as for -march. The default
is the value used for -march.
-mtpf-trace
-mno-tpf-trace
Generate code that adds (does not add) in TPF OS specific
branches to trace routines in the operating system. This option
is off by default, even when compiling for the TPF OS.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point
multiply and accumulate instructions. These instructions are
generated by default if hardware floating point is used.
-mwarn-framesize=framesize
Emit a warning if the current function exceeds the given frame
size. Because this is a compile-time check it doesn't need to be
a real problem when the program runs. It is intended to identify
functions that most probably cause a stack overflow. It is
useful to be used in an environment with limited stack size e.g.
the linux kernel.
-mwarn-dynamicstack
Emit a warning if the function calls "alloca" or uses
dynamically-sized arrays. This is generally a bad idea with a
limited stack size.
-mstack-guard=stack-guard
-mstack-size=stack-size
If these options are provided the S/390 back end emits additional
instructions in the function prologue that trigger a trap if the
stack size is stack-guard bytes above the stack-size (remember
that the stack on S/390 grows downward). If the stack-guard
option is omitted the smallest power of 2 larger than the frame
size of the compiled function is chosen. These options are
intended to be used to help debugging stack overflow problems.
The additionally emitted code causes only little overhead and
hence can also be used in production-like systems without greater
performance degradation. The given values have to be exact
powers of 2 and stack-size has to be greater than stack-guard
without exceeding 64k. In order to be efficient the extra code
makes the assumption that the stack starts at an address aligned
to the value given by stack-size. The stack-guard option can
only be used in conjunction with stack-size.
-mhotpatch=pre-halfwords,post-halfwords
If the hotpatch option is enabled, a "hot-patching" function
prologue is generated for all functions in the compilation unit.
The funtion label is prepended with the given number of two-byte
NOP instructions (pre-halfwords, maximum 1000000). After the
label, 2 * post-halfwords bytes are appended, using the largest
NOP like instructions the architecture allows (maximum 1000000).
If both arguments are zero, hotpatching is disabled.
This option can be overridden for individual functions with the
"hotpatch" attribute.
Score Options
These options are defined for Score implementations:
-meb
Compile code for big-endian mode. This is the default.
-mel
Compile code for little-endian mode.
-mnhwloop
Disable generation of "bcnz" instructions.
-muls
Enable generation of unaligned load and store instructions.
-mmac
Enable the use of multiply-accumulate instructions. Disabled by
default.
-mscore5
Specify the SCORE5 as the target architecture.
-mscore5u
Specify the SCORE5U of the target architecture.
-mscore7
Specify the SCORE7 as the target architecture. This is the
default.
-mscore7d
Specify the SCORE7D as the target architecture.
SH Options
These -m options are defined for the SH implementations:
-m1 Generate code for the SH1.
-m2 Generate code for the SH2.
-m2e
Generate code for the SH2e.
-m2a-nofpu
Generate code for the SH2a without FPU, or for a SH2a-FPU in such
a way that the floating-point unit is not used.
-m2a-single-only
Generate code for the SH2a-FPU, in such a way that no double-
precision floating-point operations are used.
-m2a-single
Generate code for the SH2a-FPU assuming the floating-point unit
is in single-precision mode by default.
-m2a
Generate code for the SH2a-FPU assuming the floating-point unit
is in double-precision mode by default.
-m3 Generate code for the SH3.
-m3e
Generate code for the SH3e.
-m4-nofpu
Generate code for the SH4 without a floating-point unit.
-m4-single-only
Generate code for the SH4 with a floating-point unit that only
supports single-precision arithmetic.
-m4-single
Generate code for the SH4 assuming the floating-point unit is in
single-precision mode by default.
-m4 Generate code for the SH4.
-m4-100
Generate code for SH4-100.
-m4-100-nofpu
Generate code for SH4-100 in such a way that the floating-point
unit is not used.
-m4-100-single
Generate code for SH4-100 assuming the floating-point unit is in
single-precision mode by default.
-m4-100-single-only
Generate code for SH4-100 in such a way that no double-precision
floating-point operations are used.
-m4-200
Generate code for SH4-200.
-m4-200-nofpu
Generate code for SH4-200 without in such a way that the
floating-point unit is not used.
-m4-200-single
Generate code for SH4-200 assuming the floating-point unit is in
single-precision mode by default.
-m4-200-single-only
Generate code for SH4-200 in such a way that no double-precision
floating-point operations are used.
-m4-300
Generate code for SH4-300.
-m4-300-nofpu
Generate code for SH4-300 without in such a way that the
floating-point unit is not used.
-m4-300-single
Generate code for SH4-300 in such a way that no double-precision
floating-point operations are used.
-m4-300-single-only
Generate code for SH4-300 in such a way that no double-precision
floating-point operations are used.
-m4-340
Generate code for SH4-340 (no MMU, no FPU).
-m4-500
Generate code for SH4-500 (no FPU). Passes -isa=sh4-nofpu to the
assembler.
-m4a-nofpu
Generate code for the SH4al-dsp, or for a SH4a in such a way that
the floating-point unit is not used.
-m4a-single-only
Generate code for the SH4a, in such a way that no double-
precision floating-point operations are used.
-m4a-single
Generate code for the SH4a assuming the floating-point unit is in
single-precision mode by default.
-m4a
Generate code for the SH4a.
-m4al
Same as -m4a-nofpu, except that it implicitly passes -dsp to the
assembler. GCC doesn't generate any DSP instructions at the
moment.
-mb Compile code for the processor in big-endian mode.
-ml Compile code for the processor in little-endian mode.
-mdalign
Align doubles at 64-bit boundaries. Note that this changes the
calling conventions, and thus some functions from the standard C
library do not work unless you recompile it first with -mdalign.
-mrelax
Shorten some address references at link time, when possible; uses
the linker option -relax.
-mbigtable
Use 32-bit offsets in "switch" tables. The default is to use
16-bit offsets.
-mbitops
Enable the use of bit manipulation instructions on SH2A.
-mfmovd
Enable the use of the instruction "fmovd". Check -mdalign for
alignment constraints.
-mrenesas
Comply with the calling conventions defined by Renesas.
-mno-renesas
Comply with the calling conventions defined for GCC before the
Renesas conventions were available. This option is the default
for all targets of the SH toolchain.
-mnomacsave
Mark the "MAC" register as call-clobbered, even if -mrenesas is
given.
-mieee
-mno-ieee
Control the IEEE compliance of floating-point comparisons, which
affects the handling of cases where the result of a comparison is
unordered. By default -mieee is implicitly enabled. If
-ffinite-math-only is enabled -mno-ieee is implicitly set, which
results in faster floating-point greater-equal and less-equal
comparisons. The implicit settings can be overridden by
specifying either -mieee or -mno-ieee.
-minline-ic_invalidate
Inline code to invalidate instruction cache entries after setting
up nested function trampolines. This option has no effect if
-musermode is in effect and the selected code generation option
(e.g. -m4) does not allow the use of the "icbi" instruction. If
the selected code generation option does not allow the use of the
"icbi" instruction, and -musermode is not in effect, the inlined
code manipulates the instruction cache address array directly
with an associative write. This not only requires privileged
mode at run time, but it also fails if the cache line had been
mapped via the TLB and has become unmapped.
-misize
Dump instruction size and location in the assembly code.
-mpadstruct
This option is deprecated. It pads structures to multiple of 4
bytes, which is incompatible with the SH ABI.
-matomic-model=model
Sets the model of atomic operations and additional parameters as
a comma separated list. For details on the atomic built-in
functions see __atomic Builtins. The following models and
parameters are supported:
none
Disable compiler generated atomic sequences and emit library
calls for atomic operations. This is the default if the
target is not "sh*-*-linux*".
soft-gusa
Generate GNU/Linux compatible gUSA software atomic sequences
for the atomic built-in functions. The generated atomic
sequences require additional support from the
interrupt/exception handling code of the system and are only
suitable for SH3* and SH4* single-core systems. This option
is enabled by default when the target is "sh*-*-linux*" and
SH3* or SH4*. When the target is SH4A, this option also
partially utilizes the hardware atomic instructions "movli.l"
and "movco.l" to create more efficient code, unless strict is
specified.
soft-tcb
Generate software atomic sequences that use a variable in the
thread control block. This is a variation of the gUSA
sequences which can also be used on SH1* and SH2* targets.
The generated atomic sequences require additional support
from the interrupt/exception handling code of the system and
are only suitable for single-core systems. When using this
model, the gbr-offset= parameter has to be specified as well.
soft-imask
Generate software atomic sequences that temporarily disable
interrupts by setting "SR.IMASK = 1111". This model works
only when the program runs in privileged mode and is only
suitable for single-core systems. Additional support from
the interrupt/exception handling code of the system is not
required. This model is enabled by default when the target
is "sh*-*-linux*" and SH1* or SH2*.
hard-llcs
Generate hardware atomic sequences using the "movli.l" and
"movco.l" instructions only. This is only available on SH4A
and is suitable for multi-core systems. Since the hardware
instructions support only 32 bit atomic variables access to 8
or 16 bit variables is emulated with 32 bit accesses. Code
compiled with this option is also compatible with other
software atomic model interrupt/exception handling systems if
executed on an SH4A system. Additional support from the
interrupt/exception handling code of the system is not
required for this model.
gbr-offset=
This parameter specifies the offset in bytes of the variable
in the thread control block structure that should be used by
the generated atomic sequences when the soft-tcb model has
been selected. For other models this parameter is ignored.
The specified value must be an integer multiple of four and
in the range 0-1020.
strict
This parameter prevents mixed usage of multiple atomic
models, even if they are compatible, and makes the compiler
generate atomic sequences of the specified model only.
-mtas
Generate the "tas.b" opcode for "__atomic_test_and_set". Notice
that depending on the particular hardware and software
configuration this can degrade overall performance due to the
operand cache line flushes that are implied by the "tas.b"
instruction. On multi-core SH4A processors the "tas.b"
instruction must be used with caution since it can result in data
corruption for certain cache configurations.
-mprefergot
When generating position-independent code, emit function calls
using the Global Offset Table instead of the Procedure Linkage
Table.
-musermode
-mno-usermode
Don't allow (allow) the compiler generating privileged mode code.
Specifying -musermode also implies -mno-inline-ic_invalidate if
the inlined code would not work in user mode. -musermode is the
default when the target is "sh*-*-linux*". If the target is SH1*
or SH2* -musermode has no effect, since there is no user mode.
-multcost=number
Set the cost to assume for a multiply insn.
-mdiv=strategy
Set the division strategy to be used for integer division
operations. strategy can be one of:
call-div1
Calls a library function that uses the single-step division
instruction "div1" to perform the operation. Division by
zero calculates an unspecified result and does not trap.
This is the default except for SH4, SH2A and SHcompact.
call-fp
Calls a library function that performs the operation in
double precision floating point. Division by zero causes a
floating-point exception. This is the default for SHcompact
with FPU. Specifying this for targets that do not have a
double precision FPU defaults to "call-div1".
call-table
Calls a library function that uses a lookup table for small
divisors and the "div1" instruction with case distinction for
larger divisors. Division by zero calculates an unspecified
result and does not trap. This is the default for SH4.
Specifying this for targets that do not have dynamic shift
instructions defaults to "call-div1".
When a division strategy has not been specified the default
strategy is selected based on the current target. For SH2A the
default strategy is to use the "divs" and "divu" instructions
instead of library function calls.
-maccumulate-outgoing-args
Reserve space once for outgoing arguments in the function
prologue rather than around each call. Generally beneficial for
performance and size. Also needed for unwinding to avoid
changing the stack frame around conditional code.
-mdivsi3_libfunc=name
Set the name of the library function used for 32-bit signed
division to name. This only affects the name used in the call
division strategies, and the compiler still expects the same sets
of input/output/clobbered registers as if this option were not
present.
-mfixed-range=register-range
Generate code treating the given register range as fixed
registers. A fixed register is one that the register allocator
can not use. This is useful when compiling kernel code. A
register range is specified as two registers separated by a dash.
Multiple register ranges can be specified separated by a comma.
-mbranch-cost=num
Assume num to be the cost for a branch instruction. Higher
numbers make the compiler try to generate more branch-free code
if possible. If not specified the value is selected depending on
the processor type that is being compiled for.
-mzdcbranch
-mno-zdcbranch
Assume (do not assume) that zero displacement conditional branch
instructions "bt" and "bf" are fast. If -mzdcbranch is
specified, the compiler prefers zero displacement branch code
sequences. This is enabled by default when generating code for
SH4 and SH4A. It can be explicitly disabled by specifying
-mno-zdcbranch.
-mcbranch-force-delay-slot
Force the usage of delay slots for conditional branches, which
stuffs the delay slot with a "nop" if a suitable instruction
cannot be found. By default this option is disabled. It can be
enabled to work around hardware bugs as found in the original
SH7055.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point
multiply and accumulate instructions. These instructions are
generated by default if hardware floating point is used. The
machine-dependent -mfused-madd option is now mapped to the
machine-independent -ffp-contract=fast option, and
-mno-fused-madd is mapped to -ffp-contract=off.
-mfsca
-mno-fsca
Allow or disallow the compiler to emit the "fsca" instruction for
sine and cosine approximations. The option -mfsca must be used
in combination with -funsafe-math-optimizations. It is enabled
by default when generating code for SH4A. Using -mno-fsca
disables sine and cosine approximations even if
-funsafe-math-optimizations is in effect.
-mfsrra
-mno-fsrra
Allow or disallow the compiler to emit the "fsrra" instruction
for reciprocal square root approximations. The option -mfsrra
must be used in combination with -funsafe-math-optimizations and
-ffinite-math-only. It is enabled by default when generating
code for SH4A. Using -mno-fsrra disables reciprocal square root
approximations even if -funsafe-math-optimizations and
-ffinite-math-only are in effect.
-mpretend-cmove
Prefer zero-displacement conditional branches for conditional
move instruction patterns. This can result in faster code on the
SH4 processor.
-mfdpic
Generate code using the FDPIC ABI.
Solaris 2 Options
These -m options are supported on Solaris 2:
-mclear-hwcap
-mclear-hwcap tells the compiler to remove the hardware
capabilities generated by the Solaris assembler. This is only
necessary when object files use ISA extensions not supported by
the current machine, but check at runtime whether or not to use
them.
-mimpure-text
-mimpure-text, used in addition to -shared, tells the compiler to
not pass -z text to the linker when linking a shared object.
Using this option, you can link position-dependent code into a
shared object.
-mimpure-text suppresses the "relocations remain against
allocatable but non-writable sections" linker error message.
However, the necessary relocations trigger copy-on-write, and the
shared object is not actually shared across processes. Instead
of using -mimpure-text, you should compile all source code with
-fpic or -fPIC.
These switches are supported in addition to the above on Solaris 2:
-pthreads
This is a synonym for -pthread.
SPARC Options
These -m options are supported on the SPARC:
-mno-app-regs
-mapp-regs
Specify -mapp-regs to generate output using the global registers
2 through 4, which the SPARC SVR4 ABI reserves for applications.
Like the global register 1, each global register 2 through 4 is
then treated as an allocable register that is clobbered by
function calls. This is the default.
To be fully SVR4 ABI-compliant at the cost of some performance
loss, specify -mno-app-regs. You should compile libraries and
system software with this option.
-mflat
-mno-flat
With -mflat, the compiler does not generate save/restore
instructions and uses a "flat" or single register window model.
This model is compatible with the regular register window model.
The local registers and the input registers (0--5) are still
treated as "call-saved" registers and are saved on the stack as
needed.
With -mno-flat (the default), the compiler generates save/restore
instructions (except for leaf functions). This is the normal
operating mode.
-mfpu
-mhard-float
Generate output containing floating-point instructions. This is
the default.
-mno-fpu
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all SPARC
targets. Normally the facilities of the machine's usual C
compiler are used, but this cannot be done directly in cross-
compilation. You must make your own arrangements to provide
suitable library functions for cross-compilation. The embedded
targets sparc-*-aout and sparclite-*-* do provide software
floating-point support.
-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for this
to work.
-mhard-quad-float
Generate output containing quad-word (long double) floating-point
instructions.
-msoft-quad-float
Generate output containing library calls for quad-word (long
double) floating-point instructions. The functions called are
those specified in the SPARC ABI. This is the default.
As of this writing, there are no SPARC implementations that have
hardware support for the quad-word floating-point instructions.
They all invoke a trap handler for one of these instructions, and
then the trap handler emulates the effect of the instruction.
Because of the trap handler overhead, this is much slower than
calling the ABI library routines. Thus the -msoft-quad-float
option is the default.
-mno-unaligned-doubles
-munaligned-doubles
Assume that doubles have 8-byte alignment. This is the default.
With -munaligned-doubles, GCC assumes that doubles have 8-byte
alignment only if they are contained in another type, or if they
have an absolute address. Otherwise, it assumes they have 4-byte
alignment. Specifying this option avoids some rare compatibility
problems with code generated by other compilers. It is not the
default because it results in a performance loss, especially for
floating-point code.
-muser-mode
-mno-user-mode
Do not generate code that can only run in supervisor mode. This
is relevant only for the "casa" instruction emitted for the LEON3
processor. This is the default.
-mfaster-structs
-mno-faster-structs
With -mfaster-structs, the compiler assumes that structures
should have 8-byte alignment. This enables the use of pairs of
"ldd" and "std" instructions for copies in structure assignment,
in place of twice as many "ld" and "st" pairs. However, the use
of this changed alignment directly violates the SPARC ABI. Thus,
it's intended only for use on targets where the developer
acknowledges that their resulting code is not directly in line
with the rules of the ABI.
-mstd-struct-return
-mno-std-struct-return
With -mstd-struct-return, the compiler generates checking code in
functions returning structures or unions to detect size
mismatches between the two sides of function calls, as per the
32-bit ABI.
The default is -mno-std-struct-return. This option has no effect
in 64-bit mode.
-mlra
-mno-lra
Enable Local Register Allocation. This is the default for SPARC
since GCC 7 so -mno-lra needs to be passed to get old Reload.
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling
parameters for machine type cpu_type. Supported values for
cpu_type are v7, cypress, v8, supersparc, hypersparc, leon,
leon3, leon3v7, sparclite, f930, f934, sparclite86x, sparclet,
tsc701, v9, ultrasparc, ultrasparc3, niagara, niagara2, niagara3,
niagara4 and niagara7.
Native Solaris and GNU/Linux toolchains also support the value
native, which selects the best architecture option for the host
processor. -mcpu=native has no effect if GCC does not recognize
the processor.
Default instruction scheduling parameters are used for values
that select an architecture and not an implementation. These are
v7, v8, sparclite, sparclet, v9.
Here is a list of each supported architecture and their supported
implementations.
v7 cypress, leon3v7
v8 supersparc, hypersparc, leon, leon3
sparclite
f930, f934, sparclite86x
sparclet
tsc701
v9 ultrasparc, ultrasparc3, niagara, niagara2, niagara3,
niagara4, niagara7
By default (unless configured otherwise), GCC generates code for
the V7 variant of the SPARC architecture. With -mcpu=cypress,
the compiler additionally optimizes it for the Cypress CY7C602
chip, as used in the SPARCStation/SPARCServer 3xx series. This
is also appropriate for the older SPARCStation 1, 2, IPX etc.
With -mcpu=v8, GCC generates code for the V8 variant of the SPARC
architecture. The only difference from V7 code is that the
compiler emits the integer multiply and integer divide
instructions which exist in SPARC-V8 but not in SPARC-V7. With
-mcpu=supersparc, the compiler additionally optimizes it for the
SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000
series.
With -mcpu=sparclite, GCC generates code for the SPARClite
variant of the SPARC architecture. This adds the integer
multiply, integer divide step and scan ("ffs") instructions which
exist in SPARClite but not in SPARC-V7. With -mcpu=f930, the
compiler additionally optimizes it for the Fujitsu MB86930 chip,
which is the original SPARClite, with no FPU. With -mcpu=f934,
the compiler additionally optimizes it for the Fujitsu MB86934
chip, which is the more recent SPARClite with FPU.
With -mcpu=sparclet, GCC generates code for the SPARClet variant
of the SPARC architecture. This adds the integer multiply,
multiply/accumulate, integer divide step and scan ("ffs")
instructions which exist in SPARClet but not in SPARC-V7. With
-mcpu=tsc701, the compiler additionally optimizes it for the
TEMIC SPARClet chip.
With -mcpu=v9, GCC generates code for the V9 variant of the SPARC
architecture. This adds 64-bit integer and floating-point move
instructions, 3 additional floating-point condition code
registers and conditional move instructions. With
-mcpu=ultrasparc, the compiler additionally optimizes it for the
Sun UltraSPARC I/II/IIi chips. With -mcpu=ultrasparc3, the
compiler additionally optimizes it for the Sun UltraSPARC
III/III+/IIIi/IIIi+/IV/IV+ chips. With -mcpu=niagara, the
compiler additionally optimizes it for Sun UltraSPARC T1 chips.
With -mcpu=niagara2, the compiler additionally optimizes it for
Sun UltraSPARC T2 chips. With -mcpu=niagara3, the compiler
additionally optimizes it for Sun UltraSPARC T3 chips. With
-mcpu=niagara4, the compiler additionally optimizes it for Sun
UltraSPARC T4 chips. With -mcpu=niagara7, the compiler
additionally optimizes it for Oracle SPARC M7 chips.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type
cpu_type, but do not set the instruction set or register set that
the option -mcpu=cpu_type does.
The same values for -mcpu=cpu_type can be used for
-mtune=cpu_type, but the only useful values are those that select
a particular CPU implementation. Those are cypress, supersparc,
hypersparc, leon, leon3, leon3v7, f930, f934, sparclite86x,
tsc701, ultrasparc, ultrasparc3, niagara, niagara2, niagara3,
niagara4 and niagara7. With native Solaris and GNU/Linux
toolchains, native can also be used.
-mv8plus
-mno-v8plus
With -mv8plus, GCC generates code for the SPARC-V8+ ABI. The
difference from the V8 ABI is that the global and out registers
are considered 64 bits wide. This is enabled by default on
Solaris in 32-bit mode for all SPARC-V9 processors.
-mvis
-mno-vis
With -mvis, GCC generates code that takes advantage of the
UltraSPARC Visual Instruction Set extensions. The default is
-mno-vis.
-mvis2
-mno-vis2
With -mvis2, GCC generates code that takes advantage of version
2.0 of the UltraSPARC Visual Instruction Set extensions. The
default is -mvis2 when targeting a cpu that supports such
instructions, such as UltraSPARC-III and later. Setting -mvis2
also sets -mvis.
-mvis3
-mno-vis3
With -mvis3, GCC generates code that takes advantage of version
3.0 of the UltraSPARC Visual Instruction Set extensions. The
default is -mvis3 when targeting a cpu that supports such
instructions, such as niagara-3 and later. Setting -mvis3 also
sets -mvis2 and -mvis.
-mvis4
-mno-vis4
With -mvis4, GCC generates code that takes advantage of version
4.0 of the UltraSPARC Visual Instruction Set extensions. The
default is -mvis4 when targeting a cpu that supports such
instructions, such as niagara-7 and later. Setting -mvis4 also
sets -mvis3, -mvis2 and -mvis.
-mcbcond
-mno-cbcond
With -mcbcond, GCC generates code that takes advantage of the
UltraSPARC Compare-and-Branch-on-Condition instructions. The
default is -mcbcond when targeting a CPU that supports such
instructions, such as Niagara-4 and later.
-mfmaf
-mno-fmaf
With -mfmaf, GCC generates code that takes advantage of the
UltraSPARC Fused Multiply-Add Floating-point instructions. The
default is -mfmaf when targeting a CPU that supports such
instructions, such as Niagara-3 and later.
-mpopc
-mno-popc
With -mpopc, GCC generates code that takes advantage of the
UltraSPARC Population Count instruction. The default is -mpopc
when targeting a CPU that supports such an instruction, such as
Niagara-2 and later.
-msubxc
-mno-subxc
With -msubxc, GCC generates code that takes advantage of the
UltraSPARC Subtract-Extended-with-Carry instruction. The default
is -msubxc when targeting a CPU that supports such an
instruction, such as Niagara-7 and later.
-mfix-at697f
Enable the documented workaround for the single erratum of the
Atmel AT697F processor (which corresponds to erratum #13 of the
AT697E processor).
-mfix-ut699
Enable the documented workarounds for the floating-point errata
and the data cache nullify errata of the UT699 processor.
These -m options are supported in addition to the above on SPARC-V9
processors in 64-bit environments:
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit
environment sets int, long and pointer to 32 bits. The 64-bit
environment sets int to 32 bits and long and pointer to 64 bits.
-mcmodel=which
Set the code model to one of
medlow
The Medium/Low code model: 64-bit addresses, programs must be
linked in the low 32 bits of memory. Programs can be
statically or dynamically linked.
medmid
The Medium/Middle code model: 64-bit addresses, programs must
be linked in the low 44 bits of memory, the text and data
segments must be less than 2GB in size and the data segment
must be located within 2GB of the text segment.
medany
The Medium/Anywhere code model: 64-bit addresses, programs
may be linked anywhere in memory, the text and data segments
must be less than 2GB in size and the data segment must be
located within 2GB of the text segment.
embmedany
The Medium/Anywhere code model for embedded systems: 64-bit
addresses, the text and data segments must be less than 2GB
in size, both starting anywhere in memory (determined at link
time). The global register %g4 points to the base of the
data segment. Programs are statically linked and PIC is not
supported.
-mmemory-model=mem-model
Set the memory model in force on the processor to one of
default
The default memory model for the processor and operating
system.
rmo Relaxed Memory Order
pso Partial Store Order
tso Total Store Order
sc Sequential Consistency
These memory models are formally defined in Appendix D of the
SPARC-V9 architecture manual, as set in the processor's
"PSTATE.MM" field.
-mstack-bias
-mno-stack-bias
With -mstack-bias, GCC assumes that the stack pointer, and frame
pointer if present, are offset by -2047 which must be added back
when making stack frame references. This is the default in
64-bit mode. Otherwise, assume no such offset is present.
SPU Options
These -m options are supported on the SPU:
-mwarn-reloc
-merror-reloc
The loader for SPU does not handle dynamic relocations. By
default, GCC gives an error when it generates code that requires
a dynamic relocation. -mno-error-reloc disables the error,
-mwarn-reloc generates a warning instead.
-msafe-dma
-munsafe-dma
Instructions that initiate or test completion of DMA must not be
reordered with respect to loads and stores of the memory that is
being accessed. With -munsafe-dma you must use the "volatile"
keyword to protect memory accesses, but that can lead to
inefficient code in places where the memory is known to not
change. Rather than mark the memory as volatile, you can use
-msafe-dma to tell the compiler to treat the DMA instructions as
potentially affecting all memory.
-mbranch-hints
By default, GCC generates a branch hint instruction to avoid
pipeline stalls for always-taken or probably-taken branches. A
hint is not generated closer than 8 instructions away from its
branch. There is little reason to disable them, except for
debugging purposes, or to make an object a little bit smaller.
-msmall-mem
-mlarge-mem
By default, GCC generates code assuming that addresses are never
larger than 18 bits. With -mlarge-mem code is generated that
assumes a full 32-bit address.
-mstdmain
By default, GCC links against startup code that assumes the SPU-
style main function interface (which has an unconventional
parameter list). With -mstdmain, GCC links your program against
startup code that assumes a C99-style interface to "main",
including a local copy of "argv" strings.
-mfixed-range=register-range
Generate code treating the given register range as fixed
registers. A fixed register is one that the register allocator
cannot use. This is useful when compiling kernel code. A
register range is specified as two registers separated by a dash.
Multiple register ranges can be specified separated by a comma.
-mea32
-mea64
Compile code assuming that pointers to the PPU address space
accessed via the "__ea" named address space qualifier are either
32 or 64 bits wide. The default is 32 bits. As this is an ABI-
changing option, all object code in an executable must be
compiled with the same setting.
-maddress-space-conversion
-mno-address-space-conversion
Allow/disallow treating the "__ea" address space as superset of
the generic address space. This enables explicit type casts
between "__ea" and generic pointer as well as implicit
conversions of generic pointers to "__ea" pointers. The default
is to allow address space pointer conversions.
-mcache-size=cache-size
This option controls the version of libgcc that the compiler
links to an executable and selects a software-managed cache for
accessing variables in the "__ea" address space with a particular
cache size. Possible options for cache-size are 8, 16, 32, 64
and 128. The default cache size is 64KB.
-matomic-updates
-mno-atomic-updates
This option controls the version of libgcc that the compiler
links to an executable and selects whether atomic updates to the
software-managed cache of PPU-side variables are used. If you
use atomic updates, changes to a PPU variable from SPU code using
the "__ea" named address space qualifier do not interfere with
changes to other PPU variables residing in the same cache line
from PPU code. If you do not use atomic updates, such
interference may occur; however, writing back cache lines is more
efficient. The default behavior is to use atomic updates.
-mdual-nops
-mdual-nops=n
By default, GCC inserts NOPs to increase dual issue when it
expects it to increase performance. n can be a value from 0 to
10. A smaller n inserts fewer NOPs. 10 is the default, 0 is the
same as -mno-dual-nops. Disabled with -Os.
-mhint-max-nops=n
Maximum number of NOPs to insert for a branch hint. A branch
hint must be at least 8 instructions away from the branch it is
affecting. GCC inserts up to n NOPs to enforce this, otherwise
it does not generate the branch hint.
-mhint-max-distance=n
The encoding of the branch hint instruction limits the hint to be
within 256 instructions of the branch it is affecting. By
default, GCC makes sure it is within 125.
-msafe-hints
Work around a hardware bug that causes the SPU to stall
indefinitely. By default, GCC inserts the "hbrp" instruction to
make sure this stall won't happen.
Options for System V
These additional options are available on System V Release 4 for
compatibility with other compilers on those systems:
-G Create a shared object. It is recommended that -symbolic or
-shared be used instead.
-Qy Identify the versions of each tool used by the compiler, in a
".ident" assembler directive in the output.
-Qn Refrain from adding ".ident" directives to the output file (this
is the default).
-YP,dirs
Search the directories dirs, and no others, for libraries
specified with -l.
-Ym,dir
Look in the directory dir to find the M4 preprocessor. The
assembler uses this option.
TILE-Gx Options
These -m options are supported on the TILE-Gx:
-mcmodel=small
Generate code for the small model. The distance for direct calls
is limited to 500M in either direction. PC-relative addresses
are 32 bits. Absolute addresses support the full address range.
-mcmodel=large
Generate code for the large model. There is no limitation on
call distance, pc-relative addresses, or absolute addresses.
-mcpu=name
Selects the type of CPU to be targeted. Currently the only
supported type is tilegx.
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit
environment sets int, long, and pointer to 32 bits. The 64-bit
environment sets int to 32 bits and long and pointer to 64 bits.
-mbig-endian
-mlittle-endian
Generate code in big/little endian mode, respectively.
TILEPro Options
These -m options are supported on the TILEPro:
-mcpu=name
Selects the type of CPU to be targeted. Currently the only
supported type is tilepro.
-m32
Generate code for a 32-bit environment, which sets int, long, and
pointer to 32 bits. This is the only supported behavior so the
flag is essentially ignored.
V850 Options
These -m options are defined for V850 implementations:
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are assumed
to be far away, the compiler always loads the function's address
into a register, and calls indirect through the pointer.
-mno-ep
-mep
Do not optimize (do optimize) basic blocks that use the same
index pointer 4 or more times to copy pointer into the "ep"
register, and use the shorter "sld" and "sst" instructions. The
-mep option is on by default if you optimize.
-mno-prolog-function
-mprolog-function
Do not use (do use) external functions to save and restore
registers at the prologue and epilogue of a function. The
external functions are slower, but use less code space if more
than one function saves the same number of registers. The
-mprolog-function option is on by default if you optimize.
-mspace
Try to make the code as small as possible. At present, this just
turns on the -mep and -mprolog-function options.
-mtda=n
Put static or global variables whose size is n bytes or less into
the tiny data area that register "ep" points to. The tiny data
area can hold up to 256 bytes in total (128 bytes for byte
references).
-msda=n
Put static or global variables whose size is n bytes or less into
the small data area that register "gp" points to. The small data
area can hold up to 64 kilobytes.
-mzda=n
Put static or global variables whose size is n bytes or less into
the first 32 kilobytes of memory.
-mv850
Specify that the target processor is the V850.
-mv850e3v5
Specify that the target processor is the V850E3V5. The
preprocessor constant "__v850e3v5__" is defined if this option is
used.
-mv850e2v4
Specify that the target processor is the V850E3V5. This is an
alias for the -mv850e3v5 option.
-mv850e2v3
Specify that the target processor is the V850E2V3. The
preprocessor constant "__v850e2v3__" is defined if this option is
used.
-mv850e2
Specify that the target processor is the V850E2. The
preprocessor constant "__v850e2__" is defined if this option is
used.
-mv850e1
Specify that the target processor is the V850E1. The
preprocessor constants "__v850e1__" and "__v850e__" are defined
if this option is used.
-mv850es
Specify that the target processor is the V850ES. This is an
alias for the -mv850e1 option.
-mv850e
Specify that the target processor is the V850E. The preprocessor
constant "__v850e__" is defined if this option is used.
If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor
-mv850e2v3 nor -mv850e3v5 are defined then a default target
processor is chosen and the relevant __v850*__ preprocessor
constant is defined.
The preprocessor constants "__v850" and "__v851__" are always
defined, regardless of which processor variant is the target.
-mdisable-callt
-mno-disable-callt
This option suppresses generation of the "CALLT" instruction for
the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the
v850 architecture.
This option is enabled by default when the RH850 ABI is in use
(see -mrh850-abi), and disabled by default when the GCC ABI is in
use. If "CALLT" instructions are being generated then the C
preprocessor symbol "__V850_CALLT__" is defined.
-mrelax
-mno-relax
Pass on (or do not pass on) the -mrelax command-line option to
the assembler.
-mlong-jumps
-mno-long-jumps
Disable (or re-enable) the generation of PC-relative jump
instructions.
-msoft-float
-mhard-float
Disable (or re-enable) the generation of hardware floating point
instructions. This option is only significant when the target
architecture is V850E2V3 or higher. If hardware floating point
instructions are being generated then the C preprocessor symbol
"__FPU_OK__" is defined, otherwise the symbol "__NO_FPU__" is
defined.
-mloop
Enables the use of the e3v5 LOOP instruction. The use of this
instruction is not enabled by default when the e3v5 architecture
is selected because its use is still experimental.
-mrh850-abi
-mghs
Enables support for the RH850 version of the V850 ABI. This is
the default. With this version of the ABI the following rules
apply:
* Integer sized structures and unions are returned via a memory
pointer rather than a register.
* Large structures and unions (more than 8 bytes in size) are
passed by value.
* Functions are aligned to 16-bit boundaries.
* The -m8byte-align command-line option is supported.
* The -mdisable-callt command-line option is enabled by
default. The -mno-disable-callt command-line option is not
supported.
When this version of the ABI is enabled the C preprocessor symbol
"__V850_RH850_ABI__" is defined.
-mgcc-abi
Enables support for the old GCC version of the V850 ABI. With
this version of the ABI the following rules apply:
* Integer sized structures and unions are returned in register
"r10".
* Large structures and unions (more than 8 bytes in size) are
passed by reference.
* Functions are aligned to 32-bit boundaries, unless optimizing
for size.
* The -m8byte-align command-line option is not supported.
* The -mdisable-callt command-line option is supported but not
enabled by default.
When this version of the ABI is enabled the C preprocessor symbol
"__V850_GCC_ABI__" is defined.
-m8byte-align
-mno-8byte-align
Enables support for "double" and "long long" types to be aligned
on 8-byte boundaries. The default is to restrict the alignment
of all objects to at most 4-bytes. When -m8byte-align is in
effect the C preprocessor symbol "__V850_8BYTE_ALIGN__" is
defined.
-mbig-switch
Generate code suitable for big switch tables. Use this option
only if the assembler/linker complain about out of range branches
within a switch table.
-mapp-regs
This option causes r2 and r5 to be used in the code generated by
the compiler. This setting is the default.
-mno-app-regs
This option causes r2 and r5 to be treated as fixed registers.
VAX Options
These -m options are defined for the VAX:
-munix
Do not output certain jump instructions ("aobleq" and so on) that
the Unix assembler for the VAX cannot handle across long ranges.
-mgnu
Do output those jump instructions, on the assumption that the GNU
assembler is being used.
-mg Output code for G-format floating-point numbers instead of
D-format.
Visium Options
-mdebug
A program which performs file I/O and is destined to run on an
MCM target should be linked with this option. It causes the
libraries libc.a and libdebug.a to be linked. The program should
be run on the target under the control of the GDB remote
debugging stub.
-msim
A program which performs file I/O and is destined to run on the
simulator should be linked with option. This causes libraries
libc.a and libsim.a to be linked.
-mfpu
-mhard-float
Generate code containing floating-point instructions. This is
the default.
-mno-fpu
-msoft-float
Generate code containing library calls for floating-point.
-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for this
to work.
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling
parameters for machine type cpu_type. Supported values for
cpu_type are mcm, gr5 and gr6.
mcm is a synonym of gr5 present for backward compatibility.
By default (unless configured otherwise), GCC generates code for
the GR5 variant of the Visium architecture.
With -mcpu=gr6, GCC generates code for the GR6 variant of the
Visium architecture. The only difference from GR5 code is that
the compiler will generate block move instructions.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type
cpu_type, but do not set the instruction set or register set that
the option -mcpu=cpu_type would.
-msv-mode
Generate code for the supervisor mode, where there are no
restrictions on the access to general registers. This is the
default.
-muser-mode
Generate code for the user mode, where the access to some general
registers is forbidden: on the GR5, registers r24 to r31 cannot
be accessed in this mode; on the GR6, only registers r29 to r31
are affected.
VMS Options
These -m options are defined for the VMS implementations:
-mvms-return-codes
Return VMS condition codes from "main". The default is to return
POSIX-style condition (e.g. error) codes.
-mdebug-main=prefix
Flag the first routine whose name starts with prefix as the main
routine for the debugger.
-mmalloc64
Default to 64-bit memory allocation routines.
-mpointer-size=size
Set the default size of pointers. Possible options for size are
32 or short for 32 bit pointers, 64 or long for 64 bit pointers,
and no for supporting only 32 bit pointers. The later option
disables "pragma pointer_size".
VxWorks Options
The options in this section are defined for all VxWorks targets.
Options specific to the target hardware are listed with the other
options for that target.
-mrtp
GCC can generate code for both VxWorks kernels and real time
processes (RTPs). This option switches from the former to the
latter. It also defines the preprocessor macro "__RTP__".
-non-static
Link an RTP executable against shared libraries rather than
static libraries. The options -static and -shared can also be
used for RTPs; -static is the default.
-Bstatic
-Bdynamic
These options are passed down to the linker. They are defined
for compatibility with Diab.
-Xbind-lazy
Enable lazy binding of function calls. This option is equivalent
to -Wl,-z,now and is defined for compatibility with Diab.
-Xbind-now
Disable lazy binding of function calls. This option is the
default and is defined for compatibility with Diab.
x86 Options
These -m options are defined for the x86 family of computers.
-march=cpu-type
Generate instructions for the machine type cpu-type. In contrast
to -mtune=cpu-type, which merely tunes the generated code for the
specified cpu-type, -march=cpu-type allows GCC to generate code
that may not run at all on processors other than the one
indicated. Specifying -march=cpu-type implies -mtune=cpu-type.
The choices for cpu-type are:
native
This selects the CPU to generate code for at compilation time
by determining the processor type of the compiling machine.
Using -march=native enables all instruction subsets supported
by the local machine (hence the result might not run on
different machines). Using -mtune=native produces code
optimized for the local machine under the constraints of the
selected instruction set.
i386
Original Intel i386 CPU.
i486
Intel i486 CPU. (No scheduling is implemented for this
chip.)
i586
pentium
Intel Pentium CPU with no MMX support.
lakemont
Intel Lakemont MCU, based on Intel Pentium CPU.
pentium-mmx
Intel Pentium MMX CPU, based on Pentium core with MMX
instruction set support.
pentiumpro
Intel Pentium Pro CPU.
i686
When used with -march, the Pentium Pro instruction set is
used, so the code runs on all i686 family chips. When used
with -mtune, it has the same meaning as generic.
pentium2
Intel Pentium II CPU, based on Pentium Pro core with MMX
instruction set support.
pentium3
pentium3m
Intel Pentium III CPU, based on Pentium Pro core with MMX and
SSE instruction set support.
pentium-m
Intel Pentium M; low-power version of Intel Pentium III CPU
with MMX, SSE and SSE2 instruction set support. Used by
Centrino notebooks.
pentium4
pentium4m
Intel Pentium 4 CPU with MMX, SSE and SSE2 instruction set
support.
prescott
Improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2
and SSE3 instruction set support.
nocona
Improved version of Intel Pentium 4 CPU with 64-bit
extensions, MMX, SSE, SSE2 and SSE3 instruction set support.
core2
Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3
and SSSE3 instruction set support.
nehalem
Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2 and POPCNT instruction set
support.
westmere
Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES and PCLMUL
instruction set support.
sandybridge
Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES and
PCLMUL instruction set support.
ivybridge
Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES, PCLMUL,
FSGSBASE, RDRND and F16C instruction set support.
haswell
Intel Haswell CPU with 64-bit extensions, MOVBE, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES,
PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2 and F16C instruction
set support.
broadwell
Intel Broadwell CPU with 64-bit extensions, MOVBE, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES,
PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX
and PREFETCHW instruction set support.
skylake
Intel Skylake CPU with 64-bit extensions, MOVBE, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES,
PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX,
PREFETCHW, CLFLUSHOPT, XSAVEC and XSAVES instruction set
support.
bonnell
Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE,
SSE2, SSE3 and SSSE3 instruction set support.
silvermont
Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PCLMUL and
RDRND instruction set support.
knl Intel Knight's Landing CPU with 64-bit extensions, MOVBE,
MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX,
AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C,
RDSEED, ADCX, PREFETCHW, AVX512F, AVX512PF, AVX512ER and
AVX512CD instruction set support.
skylake-avx512
Intel Skylake Server CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX,
AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C,
RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F,
AVX512VL, AVX512BW, AVX512DQ and AVX512CD instruction set
support.
k6 AMD K6 CPU with MMX instruction set support.
k6-2
k6-3
Improved versions of AMD K6 CPU with MMX and 3DNow!
instruction set support.
athlon
athlon-tbird
AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE
prefetch instructions support.
athlon-4
athlon-xp
athlon-mp
Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and
full SSE instruction set support.
k8
opteron
athlon64
athlon-fx
Processors based on the AMD K8 core with x86-64 instruction
set support, including the AMD Opteron, Athlon 64, and Athlon
64 FX processors. (This supersets MMX, SSE, SSE2, 3DNow!,
enhanced 3DNow! and 64-bit instruction set extensions.)
k8-sse3
opteron-sse3
athlon64-sse3
Improved versions of AMD K8 cores with SSE3 instruction set
support.
amdfam10
barcelona
CPUs based on AMD Family 10h cores with x86-64 instruction
set support. (This supersets MMX, SSE, SSE2, SSE3, SSE4A,
3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set
extensions.)
bdver1
CPUs based on AMD Family 15h cores with x86-64 instruction
set support. (This supersets FMA4, AVX, XOP, LWP, AES,
PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1,
SSE4.2, ABM and 64-bit instruction set extensions.)
bdver2
AMD Family 15h core based CPUs with x86-64 instruction set
support. (This supersets BMI, TBM, F16C, FMA, FMA4, AVX,
XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A,
SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set
extensions.)
bdver3
AMD Family 15h core based CPUs with x86-64 instruction set
support. (This supersets BMI, TBM, F16C, FMA, FMA4,
FSGSBASE, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2,
SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit
instruction set extensions.
bdver4
AMD Family 15h core based CPUs with x86-64 instruction set
support. (This supersets BMI, BMI2, TBM, F16C, FMA, FMA4,
FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCL_MUL, CX16, MOVBE,
MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and
64-bit instruction set extensions.
znver1
AMD Family 17h core based CPUs with x86-64 instruction set
support. (This supersets BMI, BMI2, F16C, FMA, FSGSBASE,
AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCL_MUL,
CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1,
SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit
instruction set extensions.
btver1
CPUs based on AMD Family 14h cores with x86-64 instruction
set support. (This supersets MMX, SSE, SSE2, SSE3, SSSE3,
SSE4A, CX16, ABM and 64-bit instruction set extensions.)
btver2
CPUs based on AMD Family 16h cores with x86-64 instruction
set support. This includes MOVBE, F16C, BMI, AVX, PCL_MUL,
AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2,
SSE, MMX and 64-bit instruction set extensions.
winchip-c6
IDT WinChip C6 CPU, dealt in same way as i486 with additional
MMX instruction set support.
winchip2
IDT WinChip 2 CPU, dealt in same way as i486 with additional
MMX and 3DNow! instruction set support.
c3 VIA C3 CPU with MMX and 3DNow! instruction set support. (No
scheduling is implemented for this chip.)
c3-2
VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set
support. (No scheduling is implemented for this chip.)
c7 VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction
set support. (No scheduling is implemented for this chip.)
samuel-2
VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set
support. (No scheduling is implemented for this chip.)
nehemiah
VIA Eden Nehemiah CPU with MMX and SSE instruction set
support. (No scheduling is implemented for this chip.)
esther
VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction
set support. (No scheduling is implemented for this chip.)
eden-x2
VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3
instruction set support. (No scheduling is implemented for
this chip.)
eden-x4
VIA Eden X4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, AVX and AVX2 instruction set support. (No
scheduling is implemented for this chip.)
nano
Generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and
SSSE3 instruction set support. (No scheduling is implemented
for this chip.)
nano-1000
VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3
instruction set support. (No scheduling is implemented for
this chip.)
nano-2000
VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3
instruction set support. (No scheduling is implemented for
this chip.)
nano-3000
VIA Nano 3xxx CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3
and SSE4.1 instruction set support. (No scheduling is
implemented for this chip.)
nano-x2
VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2, SSE3,
SSSE3 and SSE4.1 instruction set support. (No scheduling is
implemented for this chip.)
nano-x4
VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2, SSE3,
SSSE3 and SSE4.1 instruction set support. (No scheduling is
implemented for this chip.)
geode
AMD Geode embedded processor with MMX and 3DNow! instruction
set support.
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code,
except for the ABI and the set of available instructions. While
picking a specific cpu-type schedules things appropriately for
that particular chip, the compiler does not generate any code
that cannot run on the default machine type unless you use a
-march=cpu-type option. For example, if GCC is configured for
i686-pc-linux-gnu then -mtune=pentium4 generates code that is
tuned for Pentium 4 but still runs on i686 machines.
The choices for cpu-type are the same as for -march. In
addition, -mtune supports 2 extra choices for cpu-type:
generic
Produce code optimized for the most common IA32/AMD64/EM64T
processors. If you know the CPU on which your code will run,
then you should use the corresponding -mtune or -march option
instead of -mtune=generic. But, if you do not know exactly
what CPU users of your application will have, then you should
use this option.
As new processors are deployed in the marketplace, the
behavior of this option will change. Therefore, if you
upgrade to a newer version of GCC, code generation controlled
by this option will change to reflect the processors that are
most common at the time that version of GCC is released.
There is no -march=generic option because -march indicates
the instruction set the compiler can use, and there is no
generic instruction set applicable to all processors. In
contrast, -mtune indicates the processor (or, in this case,
collection of processors) for which the code is optimized.
intel
Produce code optimized for the most current Intel processors,
which are Haswell and Silvermont for this version of GCC. If
you know the CPU on which your code will run, then you should
use the corresponding -mtune or -march option instead of
-mtune=intel. But, if you want your application performs
better on both Haswell and Silvermont, then you should use
this option.
As new Intel processors are deployed in the marketplace, the
behavior of this option will change. Therefore, if you
upgrade to a newer version of GCC, code generation controlled
by this option will change to reflect the most current Intel
processors at the time that version of GCC is released.
There is no -march=intel option because -march indicates the
instruction set the compiler can use, and there is no common
instruction set applicable to all processors. In contrast,
-mtune indicates the processor (or, in this case, collection
of processors) for which the code is optimized.
-mcpu=cpu-type
A deprecated synonym for -mtune.
-mfpmath=unit
Generate floating-point arithmetic for selected unit unit. The
choices for unit are:
387 Use the standard 387 floating-point coprocessor present on
the majority of chips and emulated otherwise. Code compiled
with this option runs almost everywhere. The temporary
results are computed in 80-bit precision instead of the
precision specified by the type, resulting in slightly
different results compared to most of other chips. See
-ffloat-store for more detailed description.
This is the default choice for non-Darwin x86-32 targets.
sse Use scalar floating-point instructions present in the SSE
instruction set. This instruction set is supported by
Pentium III and newer chips, and in the AMD line by Athlon-4,
Athlon XP and Athlon MP chips. The earlier version of the
SSE instruction set supports only single-precision
arithmetic, thus the double and extended-precision arithmetic
are still done using 387. A later version, present only in
Pentium 4 and AMD x86-64 chips, supports double-precision
arithmetic too.
For the x86-32 compiler, you must use -march=cpu-type, -msse
or -msse2 switches to enable SSE extensions and make this
option effective. For the x86-64 compiler, these extensions
are enabled by default.
The resulting code should be considerably faster in the
majority of cases and avoid the numerical instability
problems of 387 code, but may break some existing code that
expects temporaries to be 80 bits.
This is the default choice for the x86-64 compiler, Darwin
x86-32 targets, and the default choice for x86-32 targets
with the SSE2 instruction set when -ffast-math is enabled.
sse,387
sse+387
both
Attempt to utilize both instruction sets at once. This
effectively doubles the amount of available registers, and on
chips with separate execution units for 387 and SSE the
execution resources too. Use this option with care, as it is
still experimental, because the GCC register allocator does
not model separate functional units well, resulting in
unstable performance.
-masm=dialect
Output assembly instructions using selected dialect. Also
affects which dialect is used for basic "asm" and extended "asm".
Supported choices (in dialect order) are att or intel. The
default is att. Darwin does not support intel.
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating-point
comparisons. These correctly handle the case where the result of
a comparison is unordered.
-m80387
-mhard-float
Generate output containing 80387 instructions for floating point.
-mno-80387
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not part of GCC. Normally
the facilities of the machine's usual C compiler are used, but
this cannot be done directly in cross-compilation. You must make
your own arrangements to provide suitable library functions for
cross-compilation.
On machines where a function returns floating-point results in
the 80387 register stack, some floating-point opcodes may be
emitted even if -msoft-float is used.
-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types
"float" and "double" in an FPU register, even if there is no FPU.
The idea is that the operating system should emulate an FPU.
The option -mno-fp-ret-in-387 causes such values to be returned
in ordinary CPU registers instead.
-mno-fancy-math-387
Some 387 emulators do not support the "sin", "cos" and "sqrt"
instructions for the 387. Specify this option to avoid
generating those instructions. This option is the default on
OpenBSD and NetBSD. This option is overridden when -march
indicates that the target CPU always has an FPU and so the
instruction does not need emulation. These instructions are not
generated unless you also use the -funsafe-math-optimizations
switch.
-malign-double
-mno-align-double
Control whether GCC aligns "double", "long double", and "long
long" variables on a two-word boundary or a one-word boundary.
Aligning "double" variables on a two-word boundary produces code
that runs somewhat faster on a Pentium at the expense of more
memory.
On x86-64, -malign-double is enabled by default.
Warning: if you use the -malign-double switch, structures
containing the above types are aligned differently than the
published application binary interface specifications for the
x86-32 and are not binary compatible with structures in code
compiled without that switch.
-m96bit-long-double
-m128bit-long-double
These switches control the size of "long double" type. The
x86-32 application binary interface specifies the size to be 96
bits, so -m96bit-long-double is the default in 32-bit mode.
Modern architectures (Pentium and newer) prefer "long double" to
be aligned to an 8- or 16-byte boundary. In arrays or structures
conforming to the ABI, this is not possible. So specifying
-m128bit-long-double aligns "long double" to a 16-byte boundary
by padding the "long double" with an additional 32-bit zero.
In the x86-64 compiler, -m128bit-long-double is the default
choice as its ABI specifies that "long double" is aligned on
16-byte boundary.
Notice that neither of these options enable any extra precision
over the x87 standard of 80 bits for a "long double".
Warning: if you override the default value for your target ABI,
this changes the size of structures and arrays containing "long
double" variables, as well as modifying the function calling
convention for functions taking "long double". Hence they are
not binary-compatible with code compiled without that switch.
-mlong-double-64
-mlong-double-80
-mlong-double-128
These switches control the size of "long double" type. A size of
64 bits makes the "long double" type equivalent to the "double"
type. This is the default for 32-bit Bionic C library. A size of
128 bits makes the "long double" type equivalent to the
"__float128" type. This is the default for 64-bit Bionic C
library.
Warning: if you override the default value for your target ABI,
this changes the size of structures and arrays containing "long
double" variables, as well as modifying the function calling
convention for functions taking "long double". Hence they are
not binary-compatible with code compiled without that switch.
-malign-data=type
Control how GCC aligns variables. Supported values for type are
compat uses increased alignment value compatible uses GCC 4.8 and
earlier, abi uses alignment value as specified by the psABI, and
cacheline uses increased alignment value to match the cache line
size. compat is the default.
-mlarge-data-threshold=threshold
When -mcmodel=medium is specified, data objects larger than
threshold are placed in the large data section. This value must
be the same across all objects linked into the binary, and
defaults to 65535.
-mrtd
Use a different function-calling convention, in which functions
that take a fixed number of arguments return with the "ret num"
instruction, which pops their arguments while returning. This
saves one instruction in the caller since there is no need to pop
the arguments there.
You can specify that an individual function is called with this
calling sequence with the function attribute "stdcall". You can
also override the -mrtd option by using the function attribute
"cdecl".
Warning: this calling convention is incompatible with the one
normally used on Unix, so you cannot use it if you need to call
libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including "printf");
otherwise incorrect code is generated for calls to those
functions.
In addition, seriously incorrect code results if you call a
function with too many arguments. (Normally, extra arguments are
harmlessly ignored.)
-mregparm=num
Control how many registers are used to pass integer arguments.
By default, no registers are used to pass arguments, and at most
3 registers can be used. You can control this behavior for a
specific function by using the function attribute "regparm".
Warning: if you use this switch, and num is nonzero, then you
must build all modules with the same value, including any
libraries. This includes the system libraries and startup
modules.
-msseregparm
Use SSE register passing conventions for float and double
arguments and return values. You can control this behavior for a
specific function by using the function attribute "sseregparm".
Warning: if you use this switch then you must build all modules
with the same value, including any libraries. This includes the
system libraries and startup modules.
-mvect8-ret-in-mem
Return 8-byte vectors in memory instead of MMX registers. This
is the default on Solaris@tie{}8 and 9 and VxWorks to match the
ABI of the Sun Studio compilers until version 12. Later compiler
versions (starting with Studio 12 Update@tie{}1) follow the ABI
used by other x86 targets, which is the default on
Solaris@tie{}10 and later. Only use this option if you need to
remain compatible with existing code produced by those previous
compiler versions or older versions of GCC.
-mpc32
-mpc64
-mpc80
Set 80387 floating-point precision to 32, 64 or 80 bits. When
-mpc32 is specified, the significands of results of floating-
point operations are rounded to 24 bits (single precision);
-mpc64 rounds the significands of results of floating-point
operations to 53 bits (double precision) and -mpc80 rounds the
significands of results of floating-point operations to 64 bits
(extended double precision), which is the default. When this
option is used, floating-point operations in higher precisions
are not available to the programmer without setting the FPU
control word explicitly.
Setting the rounding of floating-point operations to less than
the default 80 bits can speed some programs by 2% or more. Note
that some mathematical libraries assume that extended-precision
(80-bit) floating-point operations are enabled by default;
routines in such libraries could suffer significant loss of
accuracy, typically through so-called "catastrophic
cancellation", when this option is used to set the precision to
less than extended precision.
-mstackrealign
Realign the stack at entry. On the x86, the -mstackrealign
option generates an alternate prologue and epilogue that realigns
the run-time stack if necessary. This supports mixing legacy
codes that keep 4-byte stack alignment with modern codes that
keep 16-byte stack alignment for SSE compatibility. See also the
attribute "force_align_arg_pointer", applicable to individual
functions.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num
byte boundary. If -mpreferred-stack-boundary is not specified,
the default is 4 (16 bytes or 128 bits).
Warning: When generating code for the x86-64 architecture with
SSE extensions disabled, -mpreferred-stack-boundary=3 can be used
to keep the stack boundary aligned to 8 byte boundary. Since
x86-64 ABI require 16 byte stack alignment, this is ABI
incompatible and intended to be used in controlled environment
where stack space is important limitation. This option leads to
wrong code when functions compiled with 16 byte stack alignment
(such as functions from a standard library) are called with
misaligned stack. In this case, SSE instructions may lead to
misaligned memory access traps. In addition, variable arguments
are handled incorrectly for 16 byte aligned objects (including
x87 long double and __int128), leading to wrong results. You
must build all modules with -mpreferred-stack-boundary=3,
including any libraries. This includes the system libraries and
startup modules.
-mincoming-stack-boundary=num
Assume the incoming stack is aligned to a 2 raised to num byte
boundary. If -mincoming-stack-boundary is not specified, the one
specified by -mpreferred-stack-boundary is used.
On Pentium and Pentium Pro, "double" and "long double" values
should be aligned to an 8-byte boundary (see -malign-double) or
suffer significant run time performance penalties. On Pentium
III, the Streaming SIMD Extension (SSE) data type "__m128" may
not work properly if it is not 16-byte aligned.
To ensure proper alignment of this values on the stack, the stack
boundary must be as aligned as that required by any value stored
on the stack. Further, every function must be generated such
that it keeps the stack aligned. Thus calling a function
compiled with a higher preferred stack boundary from a function
compiled with a lower preferred stack boundary most likely
misaligns the stack. It is recommended that libraries that use
callbacks always use the default setting.
This extra alignment does consume extra stack space, and
generally increases code size. Code that is sensitive to stack
space usage, such as embedded systems and operating system
kernels, may want to reduce the preferred alignment to
-mpreferred-stack-boundary=2.
-mmmx
-msse
-msse2
-msse3
-mssse3
-msse4
-msse4a
-msse4.1
-msse4.2
-mavx
-mavx2
-mavx512f
-mavx512pf
-mavx512er
-mavx512cd
-mavx512vl
-mavx512bw
-mavx512dq
-mavx512ifma
-mavx512vbmi
-msha
-maes
-mpclmul
-mclfushopt
-mfsgsbase
-mrdrnd
-mf16c
-mfma
-mfma4
-mprefetchwt1
-mxop
-mlwp
-m3dnow
-m3dnowa
-mpopcnt
-mabm
-mbmi
-mbmi2
-mlzcnt
-mfxsr
-mxsave
-mxsaveopt
-mxsavec
-mxsaves
-mrtm
-mtbm
-mmpx
-mmwaitx
-mclzero
-mpku
These switches enable the use of instructions in the MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, AVX, AVX2, AVX512F, AVX512PF,
AVX512ER, AVX512CD, SHA, AES, PCLMUL, FSGSBASE, RDRND, F16C, FMA,
SSE4A, FMA4, XOP, LWP, ABM, AVX512VL, AVX512BW, AVX512DQ,
AVX512IFMA AVX512VBMI, BMI, BMI2, FXSR, XSAVE, XSAVEOPT, LZCNT,
RTM, MPX, MWAITX, PKU, 3DNow! or enhanced 3DNow! extended
instruction sets. Each has a corresponding -mno- option to
disable use of these instructions.
These extensions are also available as built-in functions: see
x86 Built-in Functions, for details of the functions enabled and
disabled by these switches.
To generate SSE/SSE2 instructions automatically from floating-
point code (as opposed to 387 instructions), see -mfpmath=sse.
GCC depresses SSEx instructions when -mavx is used. Instead, it
generates new AVX instructions or AVX equivalence for all SSEx
instructions when needed.
These options enable GCC to use these extended instructions in
generated code, even without -mfpmath=sse. Applications that
perform run-time CPU detection must compile separate files for
each supported architecture, using the appropriate flags. In
particular, the file containing the CPU detection code should be
compiled without these options.
-mdump-tune-features
This option instructs GCC to dump the names of the x86
performance tuning features and default settings. The names can
be used in -mtune-ctrl=feature-list.
-mtune-ctrl=feature-list
This option is used to do fine grain control of x86 code
generation features. feature-list is a comma separated list of
feature names. See also -mdump-tune-features. When specified, the
feature is turned on if it is not preceded with ^, otherwise, it
is turned off. -mtune-ctrl=feature-list is intended to be used
by GCC developers. Using it may lead to code paths not covered by
testing and can potentially result in compiler ICEs or runtime
errors.
-mno-default
This option instructs GCC to turn off all tunable features. See
also -mtune-ctrl=feature-list and -mdump-tune-features.
-mcld
This option instructs GCC to emit a "cld" instruction in the
prologue of functions that use string instructions. String
instructions depend on the DF flag to select between
autoincrement or autodecrement mode. While the ABI specifies the
DF flag to be cleared on function entry, some operating systems
violate this specification by not clearing the DF flag in their
exception dispatchers. The exception handler can be invoked with
the DF flag set, which leads to wrong direction mode when string
instructions are used. This option can be enabled by default on
32-bit x86 targets by configuring GCC with the --enable-cld
configure option. Generation of "cld" instructions can be
suppressed with the -mno-cld compiler option in this case.
-mvzeroupper
This option instructs GCC to emit a "vzeroupper" instruction
before a transfer of control flow out of the function to minimize
the AVX to SSE transition penalty as well as remove unnecessary
"zeroupper" intrinsics.
-mprefer-avx128
This option instructs GCC to use 128-bit AVX instructions instead
of 256-bit AVX instructions in the auto-vectorizer.
-mcx16
This option enables GCC to generate "CMPXCHG16B" instructions.
"CMPXCHG16B" allows for atomic operations on 128-bit double
quadword (or oword) data types. This is useful for high-
resolution counters that can be updated by multiple processors
(or cores). This instruction is generated as part of atomic
built-in functions: see __sync Builtins or __atomic Builtins for
details.
-msahf
This option enables generation of "SAHF" instructions in 64-bit
code. Early Intel Pentium 4 CPUs with Intel 64 support, prior to
the introduction of Pentium 4 G1 step in December 2005, lacked
the "LAHF" and "SAHF" instructions which are supported by AMD64.
These are load and store instructions, respectively, for certain
status flags. In 64-bit mode, the "SAHF" instruction is used to
optimize "fmod", "drem", and "remainder" built-in functions; see
Other Builtins for details.
-mmovbe
This option enables use of the "movbe" instruction to implement
"__builtin_bswap32" and "__builtin_bswap64".
-mcrc32
This option enables built-in functions "__builtin_ia32_crc32qi",
"__builtin_ia32_crc32hi", "__builtin_ia32_crc32si" and
"__builtin_ia32_crc32di" to generate the "crc32" machine
instruction.
-mrecip
This option enables use of "RCPSS" and "RSQRTSS" instructions
(and their vectorized variants "RCPPS" and "RSQRTPS") with an
additional Newton-Raphson step to increase precision instead of
"DIVSS" and "SQRTSS" (and their vectorized variants) for single-
precision floating-point arguments. These instructions are
generated only when -funsafe-math-optimizations is enabled
together with -ffinite-math-only and -fno-trapping-math. Note
that while the throughput of the sequence is higher than the
throughput of the non-reciprocal instruction, the precision of
the sequence can be decreased by up to 2 ulp (i.e. the inverse of
1.0 equals 0.99999994).
Note that GCC implements "1.0f/sqrtf(x)" in terms of "RSQRTSS"
(or "RSQRTPS") already with -ffast-math (or the above option
combination), and doesn't need -mrecip.
Also note that GCC emits the above sequence with additional
Newton-Raphson step for vectorized single-float division and
vectorized "sqrtf(x)" already with -ffast-math (or the above
option combination), and doesn't need -mrecip.
-mrecip=opt
This option controls which reciprocal estimate instructions may
be used. opt is a comma-separated list of options, which may be
preceded by a ! to invert the option:
all Enable all estimate instructions.
default
Enable the default instructions, equivalent to -mrecip.
none
Disable all estimate instructions, equivalent to -mno-recip.
div Enable the approximation for scalar division.
vec-div
Enable the approximation for vectorized division.
sqrt
Enable the approximation for scalar square root.
vec-sqrt
Enable the approximation for vectorized square root.
So, for example, -mrecip=all,!sqrt enables all of the reciprocal
approximations, except for square root.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an
external library. Supported values for type are svml for the
Intel short vector math library and acml for the AMD math core
library. To use this option, both -ftree-vectorize and
-funsafe-math-optimizations have to be enabled, and an SVML or
ACML ABI-compatible library must be specified at link time.
GCC currently emits calls to "vmldExp2", "vmldLn2", "vmldLog102",
"vmldLog102", "vmldPow2", "vmldTanh2", "vmldTan2", "vmldAtan2",
"vmldAtanh2", "vmldCbrt2", "vmldSinh2", "vmldSin2", "vmldAsinh2",
"vmldAsin2", "vmldCosh2", "vmldCos2", "vmldAcosh2", "vmldAcos2",
"vmlsExp4", "vmlsLn4", "vmlsLog104", "vmlsLog104", "vmlsPow4",
"vmlsTanh4", "vmlsTan4", "vmlsAtan4", "vmlsAtanh4", "vmlsCbrt4",
"vmlsSinh4", "vmlsSin4", "vmlsAsinh4", "vmlsAsin4", "vmlsCosh4",
"vmlsCos4", "vmlsAcosh4" and "vmlsAcos4" for corresponding
function type when -mveclibabi=svml is used, and "__vrd2_sin",
"__vrd2_cos", "__vrd2_exp", "__vrd2_log", "__vrd2_log2",
"__vrd2_log10", "__vrs4_sinf", "__vrs4_cosf", "__vrs4_expf",
"__vrs4_logf", "__vrs4_log2f", "__vrs4_log10f" and "__vrs4_powf"
for the corresponding function type when -mveclibabi=acml is
used.
-mabi=name
Generate code for the specified calling convention. Permissible
values are sysv for the ABI used on GNU/Linux and other systems,
and ms for the Microsoft ABI. The default is to use the
Microsoft ABI when targeting Microsoft Windows and the SysV ABI
on all other systems. You can control this behavior for specific
functions by using the function attributes "ms_abi" and
"sysv_abi".
-mtls-dialect=type
Generate code to access thread-local storage using the gnu or
gnu2 conventions. gnu is the conservative default; gnu2 is more
efficient, but it may add compile- and run-time requirements that
cannot be satisfied on all systems.
-mpush-args
-mno-push-args
Use PUSH operations to store outgoing parameters. This method is
shorter and usually equally fast as method using SUB/MOV
operations and is enabled by default. In some cases disabling it
may improve performance because of improved scheduling and
reduced dependencies.
-maccumulate-outgoing-args
If enabled, the maximum amount of space required for outgoing
arguments is computed in the function prologue. This is faster
on most modern CPUs because of reduced dependencies, improved
scheduling and reduced stack usage when the preferred stack
boundary is not equal to 2. The drawback is a notable increase
in code size. This switch implies -mno-push-args.
-mthreads
Support thread-safe exception handling on MinGW. Programs that
rely on thread-safe exception handling must compile and link all
code with the -mthreads option. When compiling, -mthreads
defines -D_MT; when linking, it links in a special thread helper
library -lmingwthrd which cleans up per-thread exception-handling
data.
-mms-bitfields
-mno-ms-bitfields
Enable/disable bit-field layout compatible with the native
Microsoft Windows compiler.
If "packed" is used on a structure, or if bit-fields are used, it
may be that the Microsoft ABI lays out the structure differently
than the way GCC normally does. Particularly when moving packed
data between functions compiled with GCC and the native Microsoft
compiler (either via function call or as data in a file), it may
be necessary to access either format.
This option is enabled by default for Microsoft Windows targets.
This behavior can also be controlled locally by use of variable
or type attributes. For more information, see x86 Variable
Attributes and x86 Type Attributes.
The Microsoft structure layout algorithm is fairly simple with
the exception of the bit-field packing. The padding and
alignment of members of structures and whether a bit-field can
straddle a storage-unit boundary are determine by these rules:
1. Structure members are stored sequentially in the order in
which they are
declared: the first member has the lowest memory address and
the last member the highest.
2. Every data object has an alignment requirement. The alignment
requirement
for all data except structures, unions, and arrays is either
the size of the object or the current packing size (specified
with either the "aligned" attribute or the "pack" pragma),
whichever is less. For structures, unions, and arrays, the
alignment requirement is the largest alignment requirement of
its members. Every object is allocated an offset so that:
offset % alignment_requirement == 0
3. Adjacent bit-fields are packed into the same 1-, 2-, or 4-byte
allocation
unit if the integral types are the same size and if the next
bit-field fits into the current allocation unit without
crossing the boundary imposed by the common alignment
requirements of the bit-fields.
MSVC interprets zero-length bit-fields in the following ways:
1. If a zero-length bit-field is inserted between two bit-fields
that
are normally coalesced, the bit-fields are not coalesced.
For example:
struct
{
unsigned long bf_1 : 12;
unsigned long : 0;
unsigned long bf_2 : 12;
} t1;
The size of "t1" is 8 bytes with the zero-length bit-field.
If the zero-length bit-field were removed, "t1"'s size would
be 4 bytes.
2. If a zero-length bit-field is inserted after a bit-field,
"foo", and the
alignment of the zero-length bit-field is greater than the
member that follows it, "bar", "bar" is aligned as the type
of the zero-length bit-field.
For example:
struct
{
char foo : 4;
short : 0;
char bar;
} t2;
struct
{
char foo : 4;
short : 0;
double bar;
} t3;
For "t2", "bar" is placed at offset 2, rather than offset 1.
Accordingly, the size of "t2" is 4. For "t3", the zero-
length bit-field does not affect the alignment of "bar" or,
as a result, the size of the structure.
Taking this into account, it is important to note the
following:
1. If a zero-length bit-field follows a normal bit-field, the
type of the
zero-length bit-field may affect the alignment of the
structure as whole. For example, "t2" has a size of 4
bytes, since the zero-length bit-field follows a normal
bit-field, and is of type short.
2. Even if a zero-length bit-field is not followed by a
normal bit-field, it may
still affect the alignment of the structure:
struct
{
char foo : 6;
long : 0;
} t4;
Here, "t4" takes up 4 bytes.
3. Zero-length bit-fields following non-bit-field members are
ignored:
struct
{
char foo;
long : 0;
char bar;
} t5;
Here, "t5" takes up 2 bytes.
-mno-align-stringops
Do not align the destination of inlined string operations. This
switch reduces code size and improves performance in case the
destination is already aligned, but GCC doesn't know about it.
-minline-all-stringops
By default GCC inlines string operations only when the
destination is known to be aligned to least a 4-byte boundary.
This enables more inlining and increases code size, but may
improve performance of code that depends on fast "memcpy",
"strlen", and "memset" for short lengths.
-minline-stringops-dynamically
For string operations of unknown size, use run-time checks with
inline code for small blocks and a library call for large blocks.
-mstringop-strategy=alg
Override the internal decision heuristic for the particular
algorithm to use for inlining string operations. The allowed
values for alg are:
rep_byte
rep_4byte
rep_8byte
Expand using i386 "rep" prefix of the specified size.
byte_loop
loop
unrolled_loop
Expand into an inline loop.
libcall
Always use a library call.
-mmemcpy-strategy=strategy
Override the internal decision heuristic to decide if
"__builtin_memcpy" should be inlined and what inline algorithm to
use when the expected size of the copy operation is known.
strategy is a comma-separated list of alg:max_size:dest_align
triplets. alg is specified in -mstringop-strategy, max_size
specifies the max byte size with which inline algorithm alg is
allowed. For the last triplet, the max_size must be "-1". The
max_size of the triplets in the list must be specified in
increasing order. The minimal byte size for alg is 0 for the
first triplet and "max_size + 1" of the preceding range.
-mmemset-strategy=strategy
The option is similar to -mmemcpy-strategy= except that it is to
control "__builtin_memset" expansion.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions.
This avoids the instructions to save, set up, and restore frame
pointers and makes an extra register available in leaf functions.
The option -fomit-leaf-frame-pointer removes the frame pointer
for leaf functions, which might make debugging harder.
-mtls-direct-seg-refs
-mno-tls-direct-seg-refs
Controls whether TLS variables may be accessed with offsets from
the TLS segment register (%gs for 32-bit, %fs for 64-bit), or
whether the thread base pointer must be added. Whether or not
this is valid depends on the operating system, and whether it
maps the segment to cover the entire TLS area.
For systems that use the GNU C Library, the default is on.
-msse2avx
-mno-sse2avx
Specify that the assembler should encode SSE instructions with
VEX prefix. The option -mavx turns this on by default.
-mfentry
-mno-fentry
If profiling is active (-pg), put the profiling counter call
before the prologue. Note: On x86 architectures the attribute
"ms_hook_prologue" isn't possible at the moment for -mfentry and
-pg.
-mrecord-mcount
-mno-record-mcount
If profiling is active (-pg), generate a __mcount_loc section
that contains pointers to each profiling call. This is useful for
automatically patching and out calls.
-mnop-mcount
-mno-nop-mcount
If profiling is active (-pg), generate the calls to the profiling
functions as NOPs. This is useful when they should be patched in
later dynamically. This is likely only useful together with
-mrecord-mcount.
-mskip-rax-setup
-mno-skip-rax-setup
When generating code for the x86-64 architecture with SSE
extensions disabled, -mskip-rax-setup can be used to skip setting
up RAX register when there are no variable arguments passed in
vector registers.
Warning: Since RAX register is used to avoid unnecessarily saving
vector registers on stack when passing variable arguments, the
impacts of this option are callees may waste some stack space,
misbehave or jump to a random location. GCC 4.4 or newer don't
have those issues, regardless the RAX register value.
-m8bit-idiv
-mno-8bit-idiv
On some processors, like Intel Atom, 8-bit unsigned integer
divide is much faster than 32-bit/64-bit integer divide. This
option generates a run-time check. If both dividend and divisor
are within range of 0 to 255, 8-bit unsigned integer divide is
used instead of 32-bit/64-bit integer divide.
-mavx256-split-unaligned-load
-mavx256-split-unaligned-store
Split 32-byte AVX unaligned load and store.
-mstack-protector-guard=guard
Generate stack protection code using canary at guard. Supported
locations are global for global canary or tls for per-thread
canary in the TLS block (the default). This option has effect
only when -fstack-protector or -fstack-protector-all is
specified.
-mmitigate-rop
Try to avoid generating code sequences that contain unintended
return opcodes, to mitigate against certain forms of attack. At
the moment, this option is limited in what it can do and should
not be relied on to provide serious protection.
-mgeneral-regs-only
Generate code that uses only the general-purpose registers. This
prevents the compiler from using floating-point, vector, mask and
bound registers.
These -m switches are supported in addition to the above on x86-64
processors in 64-bit environments.
-m32
-m64
-mx32
-m16
-miamcu
Generate code for a 16-bit, 32-bit or 64-bit environment. The
-m32 option sets "int", "long", and pointer types to 32 bits, and
generates code that runs on any i386 system.
The -m64 option sets "int" to 32 bits and "long" and pointer
types to 64 bits, and generates code for the x86-64 architecture.
For Darwin only the -m64 option also turns off the -fno-pic and
-mdynamic-no-pic options.
The -mx32 option sets "int", "long", and pointer types to 32
bits, and generates code for the x86-64 architecture.
The -m16 option is the same as -m32, except for that it outputs
the ".code16gcc" assembly directive at the beginning of the
assembly output so that the binary can run in 16-bit mode.
The -miamcu option generates code which conforms to Intel MCU
psABI. It requires the -m32 option to be turned on.
-mno-red-zone
Do not use a so-called "red zone" for x86-64 code. The red zone
is mandated by the x86-64 ABI; it is a 128-byte area beyond the
location of the stack pointer that is not modified by signal or
interrupt handlers and therefore can be used for temporary data
without adjusting the stack pointer. The flag -mno-red-zone
disables this red zone.
-mcmodel=small
Generate code for the small code model: the program and its
symbols must be linked in the lower 2 GB of the address space.
Pointers are 64 bits. Programs can be statically or dynamically
linked. This is the default code model.
-mcmodel=kernel
Generate code for the kernel code model. The kernel runs in the
negative 2 GB of the address space. This model has to be used
for Linux kernel code.
-mcmodel=medium
Generate code for the medium model: the program is linked in the
lower 2 GB of the address space. Small symbols are also placed
there. Symbols with sizes larger than -mlarge-data-threshold are
put into large data or BSS sections and can be located above 2GB.
Programs can be statically or dynamically linked.
-mcmodel=large
Generate code for the large model. This model makes no
assumptions about addresses and sizes of sections.
-maddress-mode=long
Generate code for long address mode. This is only supported for
64-bit and x32 environments. It is the default address mode for
64-bit environments.
-maddress-mode=short
Generate code for short address mode. This is only supported for
32-bit and x32 environments. It is the default address mode for
32-bit and x32 environments.
x86 Windows Options
These additional options are available for Microsoft Windows targets:
-mconsole
This option specifies that a console application is to be
generated, by instructing the linker to set the PE header
subsystem type required for console applications. This option is
available for Cygwin and MinGW targets and is enabled by default
on those targets.
-mdll
This option is available for Cygwin and MinGW targets. It
specifies that a DLL---a dynamic link library---is to be
generated, enabling the selection of the required runtime startup
object and entry point.
-mnop-fun-dllimport
This option is available for Cygwin and MinGW targets. It
specifies that the "dllimport" attribute should be ignored.
-mthread
This option is available for MinGW targets. It specifies that
MinGW-specific thread support is to be used.
-municode
This option is available for MinGW-w64 targets. It causes the
"UNICODE" preprocessor macro to be predefined, and chooses
Unicode-capable runtime startup code.
-mwin32
This option is available for Cygwin and MinGW targets. It
specifies that the typical Microsoft Windows predefined macros
are to be set in the pre-processor, but does not influence the
choice of runtime library/startup code.
-mwindows
This option is available for Cygwin and MinGW targets. It
specifies that a GUI application is to be generated by
instructing the linker to set the PE header subsystem type
appropriately.
-fno-set-stack-executable
This option is available for MinGW targets. It specifies that the
executable flag for the stack used by nested functions isn't set.
This is necessary for binaries running in kernel mode of
Microsoft Windows, as there the User32 API, which is used to set
executable privileges, isn't available.
-fwritable-relocated-rdata
This option is available for MinGW and Cygwin targets. It
specifies that relocated-data in read-only section is put into
the ".data" section. This is a necessary for older runtimes not
supporting modification of ".rdata" sections for pseudo-
relocation.
-mpe-aligned-commons
This option is available for Cygwin and MinGW targets. It
specifies that the GNU extension to the PE file format that
permits the correct alignment of COMMON variables should be used
when generating code. It is enabled by default if GCC detects
that the target assembler found during configuration supports the
feature.
See also under x86 Options for standard options.
Xstormy16 Options
These options are defined for Xstormy16:
-msim
Choose startup files and linker script suitable for the
simulator.
Xtensa Options
These options are supported for Xtensa targets:
-mconst16
-mno-const16
Enable or disable use of "CONST16" instructions for loading
constant values. The "CONST16" instruction is currently not a
standard option from Tensilica. When enabled, "CONST16"
instructions are always used in place of the standard "L32R"
instructions. The use of "CONST16" is enabled by default only if
the "L32R" instruction is not available.
-mfused-madd
-mno-fused-madd
Enable or disable use of fused multiply/add and multiply/subtract
instructions in the floating-point option. This has no effect if
the floating-point option is not also enabled. Disabling fused
multiply/add and multiply/subtract instructions forces the
compiler to use separate instructions for the multiply and
add/subtract operations. This may be desirable in some cases
where strict IEEE 754-compliant results are required: the fused
multiply add/subtract instructions do not round the intermediate
result, thereby producing results with more bits of precision
than specified by the IEEE standard. Disabling fused multiply
add/subtract instructions also ensures that the program output is
not sensitive to the compiler's ability to combine multiply and
add/subtract operations.
-mserialize-volatile
-mno-serialize-volatile
When this option is enabled, GCC inserts "MEMW" instructions
before "volatile" memory references to guarantee sequential
consistency. The default is -mserialize-volatile. Use
-mno-serialize-volatile to omit the "MEMW" instructions.
-mforce-no-pic
For targets, like GNU/Linux, where all user-mode Xtensa code must
be position-independent code (PIC), this option disables PIC for
compiling kernel code.
-mtext-section-literals
-mno-text-section-literals
These options control the treatment of literal pools. The
default is -mno-text-section-literals, which places literals in a
separate section in the output file. This allows the literal
pool to be placed in a data RAM/ROM, and it also allows the
linker to combine literal pools from separate object files to
remove redundant literals and improve code size. With
-mtext-section-literals, the literals are interspersed in the
text section in order to keep them as close as possible to their
references. This may be necessary for large assembly files.
Literals for each function are placed right before that function.
-mauto-litpools
-mno-auto-litpools
These options control the treatment of literal pools. The
default is -mno-auto-litpools, which places literals in a
separate section in the output file unless
-mtext-section-literals is used. With -mauto-litpools the
literals are interspersed in the text section by the assembler.
Compiler does not produce explicit ".literal" directives and
loads literals into registers with "MOVI" instructions instead of
"L32R" to let the assembler do relaxation and place literals as
necessary. This option allows assembler to create several
literal pools per function and assemble very big functions, which
may not be possible with -mtext-section-literals.
-mtarget-align
-mno-target-align
When this option is enabled, GCC instructs the assembler to
automatically align instructions to reduce branch penalties at
the expense of some code density. The assembler attempts to
widen density instructions to align branch targets and the
instructions following call instructions. If there are not
enough preceding safe density instructions to align a target, no
widening is performed. The default is -mtarget-align. These
options do not affect the treatment of auto-aligned instructions
like "LOOP", which the assembler always aligns, either by
widening density instructions or by inserting NOP instructions.
-mlongcalls
-mno-longcalls
When this option is enabled, GCC instructs the assembler to
translate direct calls to indirect calls unless it can determine
that the target of a direct call is in the range allowed by the
call instruction. This translation typically occurs for calls to
functions in other source files. Specifically, the assembler
translates a direct "CALL" instruction into an "L32R" followed by
a "CALLX" instruction. The default is -mno-longcalls. This
option should be used in programs where the call target can
potentially be out of range. This option is implemented in the
assembler, not the compiler, so the assembly code generated by
GCC still shows direct call instructions---look at the
disassembled object code to see the actual instructions. Note
that the assembler uses an indirect call for every cross-file
call, not just those that really are out of range.
zSeries Options
These are listed under
This section describes several environment variables that affect how
GCC operates. Some of them work by specifying directories or
prefixes to use when searching for various kinds of files. Some are
used to specify other aspects of the compilation environment.
Note that you can also specify places to search using options such as
-B, -I and -L. These take precedence over places specified using
environment variables, which in turn take precedence over those
specified by the configuration of GCC.
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
These environment variables control the way that GCC uses
localization information which allows GCC to work with different
national conventions. GCC inspects the locale categories
LC_CTYPE and LC_MESSAGES if it has been configured to do so.
These locale categories can be set to any value supported by your
installation. A typical value is en_GB.UTF-8 for English in the
United Kingdom encoded in UTF-8.
The LC_CTYPE environment variable specifies character
classification. GCC uses it to determine the character
boundaries in a string; this is needed for some multibyte
encodings that contain quote and escape characters that are
otherwise interpreted as a string end or escape.
The LC_MESSAGES environment variable specifies the language to
use in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value
of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE and LC_MESSAGES
default to the value of the LANG environment variable. If none
of these variables are set, GCC defaults to traditional C English
behavior.
TMPDIR
If TMPDIR is set, it specifies the directory to use for temporary
files. GCC uses temporary files to hold the output of one stage
of compilation which is to be used as input to the next stage:
for example, the output of the preprocessor, which is the input
to the compiler proper.
GCC_COMPARE_DEBUG
Setting GCC_COMPARE_DEBUG is nearly equivalent to passing
-fcompare-debug to the compiler driver. See the documentation of
this option for more details.
GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the
names of the subprograms executed by the compiler. No slash is
added when this prefix is combined with the name of a subprogram,
but you can specify a prefix that ends with a slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC attempts to figure out an
appropriate prefix to use based on the pathname it is invoked
with.
If GCC cannot find the subprogram using the specified prefix, it
tries looking in the usual places for the subprogram.
The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where
prefix is the prefix to the installed compiler. In many cases
prefix is the value of "prefix" when you ran the configure
script.
Other prefixes specified with -B take precedence over this
prefix.
This prefix is also used for finding files such as crt0.o that
are used for linking.
In addition, the prefix is used in an unusual way in finding the
directories to search for header files. For each of the standard
directories whose name normally begins with /usr/local/lib/gcc
(more precisely, with the value of GCC_INCLUDE_DIR), GCC tries
replacing that beginning with the specified prefix to produce an
alternate directory name. Thus, with -Bfoo/, GCC searches
foo/bar just before it searches the standard directory
/usr/local/lib/bar. If a standard directory begins with the
configured prefix then the value of prefix is replaced by
GCC_EXEC_PREFIX when looking for header files.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of
directories, much like PATH. GCC tries the directories thus
specified when searching for subprograms, if it cannot find the
subprograms using GCC_EXEC_PREFIX.
LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of
directories, much like PATH. When configured as a native
compiler, GCC tries the directories thus specified when searching
for special linker files, if it cannot find them using
GCC_EXEC_PREFIX. Linking using GCC also uses these directories
when searching for ordinary libraries for the -l option (but
directories specified with -L come first).
LANG
This variable is used to pass locale information to the compiler.
One way in which this information is used is to determine the
character set to be used when character literals, string literals
and comments are parsed in C and C++. When the compiler is
configured to allow multibyte characters, the following values
for LANG are recognized:
C-JIS
Recognize JIS characters.
C-SJIS
Recognize SJIS characters.
C-EUCJP
Recognize EUCJP characters.
If LANG is not defined, or if it has some other value, then the
compiler uses "mblen" and "mbtowc" as defined by the default
locale to recognize and translate multibyte characters.
Some additional environment variables affect the behavior of the
preprocessor.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable's value is a list of directories separated by a
special character, much like PATH, in which to look for header
files. The special character, "PATH_SEPARATOR", is target-
dependent and determined at GCC build time. For Microsoft
Windows-based targets it is a semicolon, and for almost all other
targets it is a colon.
CPATH specifies a list of directories to be searched as if
specified with -I, but after any paths given with -I options on
the command line. This environment variable is used regardless
of which language is being preprocessed.
The remaining environment variables apply only when preprocessing
the particular language indicated. Each specifies a list of
directories to be searched as if specified with -isystem, but
after any paths given with -isystem options on the command line.
In all these variables, an empty element instructs the compiler
to search its current working directory. Empty elements can
appear at the beginning or end of a path. For instance, if the
value of CPATH is ":/special/include", that has the same effect
as -I. -I/special/include.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output
dependencies for Make based on the non-system header files
processed by the compiler. System header files are ignored in
the dependency output.
The value of DEPENDENCIES_OUTPUT can be just a file name, in
which case the Make rules are written to that file, guessing the
target name from the source file name. Or the value can have the
form file target, in which case the rules are written to file
file using target as the target name.
In other words, this environment variable is equivalent to
combining the options -MM and -MF, with an optional -MT switch
too.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above),
except that system header files are not ignored, so it implies -M
rather than -MM. However, the dependence on the main input file
is omitted.
SOURCE_DATE_EPOCH
If this variable is set, its value specifies a UNIX timestamp to
be used in replacement of the current date and time in the
"__DATE__" and "__TIME__" macros, so that the embedded timestamps
become reproducible.
The value of SOURCE_DATE_EPOCH must be a UNIX timestamp, defined
as the number of seconds (excluding leap seconds) since 01 Jan
1970 00:00:00 represented in ASCII; identical to the output of
@command{date +%s} on GNU/Linux and other systems that support
the %s extension in the "date" command.
The value should be a known timestamp such as the last
modification time of the source or package and it should be set
by the build process.
For instructions on reporting bugs, see <https://gcc.gnu.org/bugs/ >.
1. On some systems, gcc -shared needs to build supplementary stub
code for constructors to work. On multi-libbed systems, gcc
-shared must select the correct support libraries to link
against. Failing to supply the correct flags may lead to subtle
defects. Supplying them in cases where they are not necessary is
innocuous.
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1),
gdb(1), adb(1), dbx(1), sdb(1) and the Info entries for gcc, cpp, as,
ld, binutils and gdb.
See the Info entry for gcc, or
<http://gcc.gnu.org/onlinedocs/gcc/Contributors.html >, for
contributors to GCC.
Copyright (c) 1988-2017 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "GNU General Public License" and "Funding
Free Software", the Front-Cover texts being (a) (see below), and with
the Back-Cover Texts being (b) (see below). A copy of the license is
included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
This page is part of the gcc (GNU Compiler Collection) project.
Information about the project can be found at ⟨http://gcc.gnu.org/⟩.
If you have a bug report for this manual page, see
⟨http://gcc.gnu.org/bugs/⟩. This page was obtained from the tarball
gcc-7.1.0.tar.gz fetched from
⟨ftp://ftp.fu-berlin.de/unix/languages/gcc/releases/⟩ on 2017-07-05.
If you discover any rendering problems in this HTML version of the
page, or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the information
in this COLOPHON (which is not part of the original manual page),
send a mail to man-pages@man7.org
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