pcrejit(3) — Linux manual page


PCREJIT(3)              Library Functions Manual              PCREJIT(3)

NAME         top

       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

       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).


       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

       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

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
             each compiled pattern, and pass the resulting pcre_extra
       block to

         (2) Use pcre_free_study() to free the pcre_extra block when it
             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

       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_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():


       If using pcre_jit_exec() and supporting a pre-8.32 version of
       PCRE, you can insert:

          #if PCRE_MAJOR >= 8 && PCRE_MINOR >= 32

       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

       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


       The only pcre_exec() options that are supported for JIT execution

       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


       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"

       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

       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

         (3) If callback is not NULL, it must point to a function that
             called with data as an argument at the start of matching,
             order to set up a JIT stack. If the return from the
             function is NULL, the internal 32K stack is used; otherwise
             return value must be a valid JIT stack, the result of

       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

       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

         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

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

       (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

       (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 */

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%.

       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.

SEE ALSO         top


AUTHOR         top

       Philip Hazel (FAQ by Zoltan Herczeg)
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION         top

       Last updated: 05 July 2017
       Copyright (c) 1997-2017 University of Cambridge.

COLOPHON         top

       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
       manual page, see
       ⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩.  This page was
       obtained from the tarball pcre-8.45.tar.gz fetched from
       ⟨ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/⟩ on
       2021-08-27.  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

PCRE 8.41                     05 July 2017                    PCREJIT(3)