PCREJIT(3) Library Functions Manual PCREJIT(3)
PCRE - Perl-compatible regular expressions
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.
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).
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. Also beware that the
pcre_jit_exec() function was not available at all before 8.32,
and may not be available at all if PCRE isn't compiled with
--enable-jit. See the "JIT FAST PATH API" section below for
details.
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
If using pcre_jit_exec() and supporting a pre-8.32 version of
PCRE, you can insert:
#if PCRE_MAJOR >= 8 && PCRE_MINOR >= 32
pcre_jit_exec(...);
#else
pcre_exec(...)
#endif
but as described in the "JIT FAST PATH API" section below this
assumes version 8.32 and later are compiled with --enable-jit,
which may break.
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.
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.
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.
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.
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 initialization
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.
(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.
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);
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%.
Note that the pcre_jit_exec() function is not available in
versions of PCRE before 8.32 (released in November 2012). If you
need to support versions that old you must either use the slower
pcre_exec(), or switch between the two codepaths by checking the
values of PCRE_MAJOR and PCRE_MINOR.
Due to an unfortunate implementation oversight, even in versions
8.32 and later there will be no pcre_jit_exec() stub function
defined when PCRE is compiled with --disable-jit, which is the
default, and there's no way to detect whether PCRE was compiled
with --enable-jit via a macro.
If you need to support versions older than 8.32, or versions that
may not build with --enable-jit, you must either use the slower
pcre_exec(), or switch between the two codepaths by checking the
values of PCRE_MAJOR and PCRE_MINOR.
Switching between the two by checking the version assumes that
all the versions being targeted are built with --enable-jit. To
also support builds that may use --disable-jit either pcre_exec()
must be used, or a compile-time check for JIT via pcre_config()
(which assumes the runtime environment will be the same), or as
the Git project decided to do, simply assume that pcre_jit_exec()
is present in 8.32 or later unless a compile-time flag is
provided, see the "grep: un-break building with PCRE >= 8.32
without --enable-jit" commit in git.git for an example of that.
pcreapi(3)
Philip Hazel (FAQ by Zoltan Herczeg)
University Computing Service
Cambridge CB2 3QH, England.
Last updated: 05 July 2017
Copyright (c) 1997-2017 University of Cambridge.
This page is part of the PCRE (Perl Compatible Regular
Expressions) project. Information about the project can be found
at ⟨http://www.pcre.org/⟩. If you have a bug report for this
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PCRE 8.41 05 July 2017 PCREJIT(3)
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