PCREJIT(3) Library Functions Manual PCREJIT(3)
PCRE - Perl-compatible regular expressions
PCRE JUST-IN-TIME COMPILER SUPPORT top
Just-in-time compiling is a heavyweight optimization that can greatly
speed up pattern matching. However, it comes at the cost of extra
processing before the match is performed. Therefore, it is of most
benefit when the same pattern is going to be matched many times. This
does not necessarily mean many calls of a matching function; if the
pattern is not anchored, matching attempts may take place many times
at various positions in the subject, even for a single call.
Therefore, if the subject string is very long, it may still pay to
use JIT for one-off matches.
JIT support applies only to the traditional Perl-compatible matching
function. It does not apply when the DFA matching function is being
used. The code for this support was written by Zoltan Herczeg.
8-BIT, 16-BIT AND 32-BIT SUPPORT top
JIT support is available for all of the 8-bit, 16-bit and 32-bit PCRE
libraries. To keep this documentation simple, only the 8-bit
interface is described in what follows. If you are using the 16-bit
library, substitute the 16-bit functions and 16-bit structures (for
example, pcre16_jit_stack instead of pcre_jit_stack). If you are
using the 32-bit library, substitute the 32-bit functions and 32-bit
structures (for example, pcre32_jit_stack instead of pcre_jit_stack).
AVAILABILITY OF JIT SUPPORT top
JIT support is an optional feature of PCRE. The "configure" option
--enable-jit (or equivalent CMake option) must be set when PCRE is
built if you want to use JIT. The support is limited to the following
hardware platforms:
ARM v5, v7, and Thumb2
Intel x86 32-bit and 64-bit
MIPS 32-bit
Power PC 32-bit and 64-bit
SPARC 32-bit (experimental)
If --enable-jit is set on an unsupported platform, compilation fails.
A program that is linked with PCRE 8.20 or later can tell if JIT
support is available by calling pcre_config() with the
PCRE_CONFIG_JIT option. The result is 1 when JIT is available, and 0
otherwise. However, a simple program does not need to check this in
order to use JIT. The normal API is implemented in a way that falls
back to the interpretive code if JIT is not available. For programs
that need the best possible performance, there is also a "fast path"
API that is JIT-specific.
If your program may sometimes be linked with versions of PCRE that
are older than 8.20, but you want to use JIT when it is available,
you can test the values of PCRE_MAJOR and PCRE_MINOR, or the
existence of a JIT macro such as PCRE_CONFIG_JIT, for compile-time
control of your code.
SIMPLE USE OF JIT top
You have to do two things to make use of the JIT support in the
simplest way:
(1) Call pcre_study() with the PCRE_STUDY_JIT_COMPILE option for
each compiled pattern, and pass the resulting pcre_extra block
to
pcre_exec().
(2) Use pcre_free_study() to free the pcre_extra block when it is
no longer needed, instead of just freeing it yourself. This
ensures that
any JIT data is also freed.
For a program that may be linked with pre-8.20 versions of PCRE, you
can insert
#ifndef PCRE_STUDY_JIT_COMPILE
#define PCRE_STUDY_JIT_COMPILE 0
#endif
so that no option is passed to pcre_study(), and then use something
like this to free the study data:
#ifdef PCRE_CONFIG_JIT
pcre_free_study(study_ptr);
#else
pcre_free(study_ptr);
#endif
PCRE_STUDY_JIT_COMPILE requests the JIT compiler to generate code for
complete matches. If you want to run partial matches using the
PCRE_PARTIAL_HARD or PCRE_PARTIAL_SOFT options of pcre_exec(), you
should set one or both of the following options in addition to, or
instead of, PCRE_STUDY_JIT_COMPILE when you call pcre_study():
PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
The JIT compiler generates different optimized code for each of the
three modes (normal, soft partial, hard partial). When pcre_exec() is
called, the appropriate code is run if it is available. Otherwise,
the pattern is matched using interpretive code.
In some circumstances you may need to call additional functions.
These are described in the section entitled "Controlling the JIT
stack" below.
If JIT support is not available, PCRE_STUDY_JIT_COMPILE etc. are
ignored, and no JIT data is created. Otherwise, the compiled pattern
is passed to the JIT compiler, which turns it into machine code that
executes much faster than the normal interpretive code. When
pcre_exec() is passed a pcre_extra block containing a pointer to JIT
code of the appropriate mode (normal or hard/soft partial), it obeys
that code instead of running the interpreter. The result is
identical, but the compiled JIT code runs much faster.
There are some pcre_exec() options that are not supported for JIT
execution. There are also some pattern items that JIT cannot handle.
Details are given below. In both cases, execution automatically falls
back to the interpretive code. If you want to know whether JIT was
actually used for a particular match, you should arrange for a JIT
callback function to be set up as described in the section entitled
"Controlling the JIT stack" below, even if you do not need to supply
a non-default JIT stack. Such a callback function is called whenever
JIT code is about to be obeyed. If the execution options are not
right for JIT execution, the callback function is not obeyed.
If the JIT compiler finds an unsupported item, no JIT data is
generated. You can find out if JIT execution is available after
studying a pattern by calling pcre_fullinfo() with the PCRE_INFO_JIT
option. A result of 1 means that JIT compilation was successful. A
result of 0 means that JIT support is not available, or the pattern
was not studied with PCRE_STUDY_JIT_COMPILE etc., or the JIT compiler
was not able to handle the pattern.
Once a pattern has been studied, with or without JIT, it can be used
as many times as you like for matching different subject strings.
UNSUPPORTED OPTIONS AND PATTERN ITEMS top
The only pcre_exec() options that are supported for JIT execution are
PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK, PCRE_NO_UTF32_CHECK,
PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT.
The only unsupported pattern items are \C (match a single data unit)
when running in a UTF mode, and a callout immediately before an
assertion condition in a conditional group.
RETURN VALUES FROM JIT EXECUTION top
When a pattern is matched using JIT execution, the return values are
the same as those given by the interpretive pcre_exec() code, with
the addition of one new error code: PCRE_ERROR_JIT_STACKLIMIT. This
means that the memory used for the JIT stack was insufficient. See
"Controlling the JIT stack" below for a discussion of JIT stack
usage. For compatibility with the interpretive pcre_exec() code, no
more than two-thirds of the ovector argument is used for passing back
captured substrings.
The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if
searching a very large pattern tree goes on for too long, as it is in
the same circumstance when JIT is not used, but the details of
exactly what is counted are not the same. The
PCRE_ERROR_RECURSIONLIMIT error code is never returned by JIT
execution.
SAVING AND RESTORING COMPILED PATTERNS top
The code that is generated by the JIT compiler is architecture-
specific, and is also position dependent. For those reasons it cannot
be saved (in a file or database) and restored later like the bytecode
and other data of a compiled pattern. Saving and restoring compiled
patterns is not something many people do. More detail about this
facility is given in the pcreprecompile documentation. It should be
possible to run pcre_study() on a saved and restored pattern, and
thereby recreate the JIT data, but because JIT compilation uses
significant resources, it is probably not worth doing this; you might
as well recompile the original pattern.
CONTROLLING THE JIT STACK top
When the compiled JIT code runs, it needs a block of memory to use as
a stack. By default, it uses 32K on the machine stack. However, some
large or complicated patterns need more than this. The error
PCRE_ERROR_JIT_STACKLIMIT is given when there is not enough stack.
Three functions are provided for managing blocks of memory for use as
JIT stacks. There is further discussion about the use of JIT stacks
in the section entitled "JIT stack FAQ" below.
The pcre_jit_stack_alloc() function creates a JIT stack. Its
arguments are a starting size and a maximum size, and it returns a
pointer to an opaque structure of type pcre_jit_stack, or NULL if
there is an error. The pcre_jit_stack_free() function can be used to
free a stack that is no longer needed. (For the technically minded:
the address space is allocated by mmap or VirtualAlloc.)
JIT uses far less memory for recursion than the interpretive code,
and a maximum stack size of 512K to 1M should be more than enough for
any pattern.
The pcre_assign_jit_stack() function specifies which stack JIT code
should use. Its arguments are as follows:
pcre_extra *extra
pcre_jit_callback callback
void *data
The extra argument must be the result of studying a pattern with
PCRE_STUDY_JIT_COMPILE etc. There are three cases for the values of
the other two options:
(1) If callback is NULL and data is NULL, an internal 32K block
on the machine stack is used.
(2) If callback is NULL and data is not NULL, data must be
a valid JIT stack, the result of calling
pcre_jit_stack_alloc().
(3) If callback is not NULL, it must point to a function that is
called with data as an argument at the start of matching, in
order to set up a JIT stack. If the return from the callback
function is NULL, the internal 32K stack is used; otherwise the
return value must be a valid JIT stack, the result of calling
pcre_jit_stack_alloc().
A callback function is obeyed whenever JIT code is about to be run;
it is not obeyed when pcre_exec() is called with options that are
incompatible for JIT execution. A callback function can therefore be
used to determine whether a match operation was executed by JIT or by
the interpreter.
You may safely use the same JIT stack for more than one pattern
(either by assigning directly or by callback), as long as the
patterns are all matched sequentially in the same thread. In a
multithread application, if you do not specify a JIT stack, or if you
assign or pass back NULL from a callback, that is thread-safe,
because each thread has its own machine stack. However, if you assign
or pass back a non-NULL JIT stack, this must be a different stack for
each thread so that the application is thread-safe.
Strictly speaking, even more is allowed. You can assign the same non-
NULL stack to any number of patterns as long as they are not used for
matching by multiple threads at the same time. For example, you can
assign the same stack to all compiled patterns, and use a global
mutex in the callback to wait until the stack is available for use.
However, this is an inefficient solution, and not recommended.
This is a suggestion for how a multithreaded program that needs to
set up non-default JIT stacks might operate:
During thread initalization
thread_local_var = pcre_jit_stack_alloc(...)
During thread exit
pcre_jit_stack_free(thread_local_var)
Use a one-line callback function
return thread_local_var
All the functions described in this section do nothing if JIT is not
available, and pcre_assign_jit_stack() does nothing unless the extra
argument is non-NULL and points to a pcre_extra block that is the
result of a successful study with PCRE_STUDY_JIT_COMPILE etc.
JIT STACK FAQ top
(1) Why do we need JIT stacks?
PCRE (and JIT) is a recursive, depth-first engine, so it needs a
stack where the local data of the current node is pushed before
checking its child nodes. Allocating real machine stack on some
platforms is difficult. For example, the stack chain needs to be
updated every time if we extend the stack on PowerPC. Although it is
possible, its updating time overhead decreases performance. So we do
the recursion in memory.
(2) Why don't we simply allocate blocks of memory with malloc()?
Modern operating systems have a nice feature: they can reserve an
address space instead of allocating memory. We can safely allocate
memory pages inside this address space, so the stack could grow
without moving memory data (this is important because of pointers).
Thus we can allocate 1M address space, and use only a single memory
page (usually 4K) if that is enough. However, we can still grow up to
1M anytime if needed.
(3) Who "owns" a JIT stack?
The owner of the stack is the user program, not the JIT studied
pattern or anything else. The user program must ensure that if a
stack is used by pcre_exec(), (that is, it is assigned to the pattern
currently running), that stack must not be used by any other threads
(to avoid overwriting the same memory area). The best practice for
multithreaded programs is to allocate a stack for each thread, and
return this stack through the JIT callback function.
(4) When should a JIT stack be freed?
You can free a JIT stack at any time, as long as it will not be used
by pcre_exec() again. When you assign the stack to a pattern, only a
pointer is set. There is no reference counting or any other magic.
You can free the patterns and stacks in any order, anytime. Just do
not call pcre_exec() with a pattern pointing to an already freed
stack, as that will cause SEGFAULT. (Also, do not free a stack
currently used by pcre_exec() in another thread). You can also
replace the stack for a pattern at any time. You can even free the
previous stack before assigning a replacement.
(5) Should I allocate/free a stack every time before/after calling
pcre_exec()?
No, because this is too costly in terms of resources. However, you
could implement some clever idea which release the stack if it is not
used in let's say two minutes. The JIT callback can help to achieve
this without keeping a list of the currently JIT studied patterns.
(6) OK, the stack is for long term memory allocation. But what
happens if a pattern causes stack overflow with a stack of 1M? Is
that 1M kept until the stack is freed?
Especially on embedded sytems, it might be a good idea to release
memory sometimes without freeing the stack. There is no API for this
at the moment. Probably a function call which returns with the
currently allocated memory for any stack and another which allows
releasing memory (shrinking the stack) would be a good idea if
someone needs this.
(7) This is too much of a headache. Isn't there any better solution
for JIT stack handling?
No, thanks to Windows. If POSIX threads were used everywhere, we
could throw out this complicated API.
EXAMPLE CODE top
This is a single-threaded example that specifies a JIT stack without
using a callback.
int rc;
int ovector[30];
pcre *re;
pcre_extra *extra;
pcre_jit_stack *jit_stack;
re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
/* Check for errors */
extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024);
/* Check for error (NULL) */
pcre_assign_jit_stack(extra, NULL, jit_stack);
rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
/* Check results */
pcre_free(re);
pcre_free_study(extra);
pcre_jit_stack_free(jit_stack);
JIT FAST PATH API top
Because the API described above falls back to interpreted execution
when JIT is not available, it is convenient for programs that are
written for general use in many environments. However, calling JIT
via pcre_exec() does have a performance impact. Programs that are
written for use where JIT is known to be available, and which need
the best possible performance, can instead use a "fast path" API to
call JIT execution directly instead of calling pcre_exec() (obviously
only for patterns that have been successfully studied by JIT).
The fast path function is called pcre_jit_exec(), and it takes
exactly the same arguments as pcre_exec(), plus one additional
argument that must point to a JIT stack. The JIT stack arrangements
described above do not apply. The return values are the same as for
pcre_exec().
When you call pcre_exec(), as well as testing for invalid options, a
number of other sanity checks are performed on the arguments. For
example, if the subject pointer is NULL, or its length is negative,
an immediate error is given. Also, unless PCRE_NO_UTF[8|16|32] is
set, a UTF subject string is tested for validity. In the interests of
speed, these checks do not happen on the JIT fast path, and if
invalid data is passed, the result is undefined.
Bypassing the sanity checks and the pcre_exec() wrapping can give
speedups of more than 10%.
SEE ALSO top
pcreapi(3)
AUTHOR top
Philip Hazel (FAQ by Zoltan Herczeg)
University Computing Service
Cambridge CB2 3QH, England.
REVISION top
Last updated: 17 March 2013
Copyright (c) 1997-2013 University of Cambridge.
COLOPHON top
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