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<title>pcrejit specification</title>
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<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
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<h1>pcrejit man page</h1>
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Return to the <a href="index.html">PCRE index page</a>.
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This page is part of the PCRE HTML documentation. It was generated automatically
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from the original man page. If there is any nonsense in it, please consult the
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man page, in case the conversion went wrong.
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<li><a name="TOC1" href="#SEC1">PCRE JUST-IN-TIME COMPILER SUPPORT</a>
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<li><a name="TOC2" href="#SEC2">8-BIT and 16-BIT SUPPORT</a>
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<li><a name="TOC3" href="#SEC3">AVAILABILITY OF JIT SUPPORT</a>
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<li><a name="TOC4" href="#SEC4">SIMPLE USE OF JIT</a>
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<li><a name="TOC5" href="#SEC5">UNSUPPORTED OPTIONS AND PATTERN ITEMS</a>
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<li><a name="TOC6" href="#SEC6">RETURN VALUES FROM JIT EXECUTION</a>
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<li><a name="TOC7" href="#SEC7">SAVING AND RESTORING COMPILED PATTERNS</a>
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<li><a name="TOC8" href="#SEC8">CONTROLLING THE JIT STACK</a>
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<li><a name="TOC9" href="#SEC9">JIT STACK FAQ</a>
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<li><a name="TOC10" href="#SEC10">EXAMPLE CODE</a>
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<li><a name="TOC11" href="#SEC11">SEE ALSO</a>
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<li><a name="TOC12" href="#SEC12">AUTHOR</a>
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<li><a name="TOC13" href="#SEC13">REVISION</a>
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<br><a name="SEC1" href="#TOC1">PCRE JUST-IN-TIME COMPILER SUPPORT</a><br>
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Just-in-time compiling is a heavyweight optimization that can greatly speed up
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pattern matching. However, it comes at the cost of extra processing before the
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match is performed. Therefore, it is of most benefit when the same pattern is
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going to be matched many times. This does not necessarily mean many calls of a
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matching function; if the pattern is not anchored, matching attempts may take
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place many times at various positions in the subject, even for a single call.
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Therefore, if the subject string is very long, it may still pay to use JIT for
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JIT support applies only to the traditional Perl-compatible matching function.
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It does not apply when the DFA matching function is being used. The code for
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this support was written by Zoltan Herczeg.
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<br><a name="SEC2" href="#TOC1">8-BIT and 16-BIT SUPPORT</a><br>
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JIT support is available for both the 8-bit and 16-bit PCRE libraries. To keep
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this documentation simple, only the 8-bit interface is described in what
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follows. If you are using the 16-bit library, substitute the 16-bit functions
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and 16-bit structures (for example, <i>pcre16_jit_stack</i> instead of
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<i>pcre_jit_stack</i>).
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<br><a name="SEC3" href="#TOC1">AVAILABILITY OF JIT SUPPORT</a><br>
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JIT support is an optional feature of PCRE. The "configure" option --enable-jit
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(or equivalent CMake option) must be set when PCRE is built if you want to use
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JIT. The support is limited to the following hardware platforms:
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ARM v5, v7, and Thumb2
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Intel x86 32-bit and 64-bit
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Power PC 32-bit and 64-bit
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The Power PC support is designated as experimental because it has not been
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fully tested. If --enable-jit is set on an unsupported platform, compilation
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A program that is linked with PCRE 8.20 or later can tell if JIT support is
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available by calling <b>pcre_config()</b> with the PCRE_CONFIG_JIT option. The
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result is 1 when JIT is available, and 0 otherwise. However, a simple program
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does not need to check this in order to use JIT. The API is implemented in a
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way that falls back to the ordinary PCRE code if JIT is not available.
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If your program may sometimes be linked with versions of PCRE that are older
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than 8.20, but you want to use JIT when it is available, you can test
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the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT macro such
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as PCRE_CONFIG_JIT, for compile-time control of your code.
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<br><a name="SEC4" href="#TOC1">SIMPLE USE OF JIT</a><br>
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You have to do two things to make use of the JIT support in the simplest way:
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(1) Call <b>pcre_study()</b> with the PCRE_STUDY_JIT_COMPILE option for
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each compiled pattern, and pass the resulting <b>pcre_extra</b> block to
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(2) Use <b>pcre_free_study()</b> to free the <b>pcre_extra</b> block when it is
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no longer needed instead of just freeing it yourself. This
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ensures that any JIT data is also freed.
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For a program that may be linked with pre-8.20 versions of PCRE, you can insert
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#ifndef PCRE_STUDY_JIT_COMPILE
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#define PCRE_STUDY_JIT_COMPILE 0
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so that no option is passed to <b>pcre_study()</b>, and then use something like
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this to free the study data:
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#ifdef PCRE_CONFIG_JIT
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pcre_free_study(study_ptr);
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pcre_free(study_ptr);
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In some circumstances you may need to call additional functions. These are
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described in the section entitled
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<a href="#stackcontrol">"Controlling the JIT stack"</a>
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If JIT support is not available, PCRE_STUDY_JIT_COMPILE is ignored, and no JIT
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data is set up. Otherwise, the compiled pattern is passed to the JIT compiler,
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which turns it into machine code that executes much faster than the normal
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interpretive code. When <b>pcre_exec()</b> is passed a <b>pcre_extra</b> block
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containing a pointer to JIT code, it obeys that instead of the normal code. The
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result is identical, but the code runs much faster.
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There are some <b>pcre_exec()</b> options that are not supported for JIT
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execution. There are also some pattern items that JIT cannot handle. Details
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are given below. In both cases, execution automatically falls back to the
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If the JIT compiler finds an unsupported item, no JIT data is generated. You
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can find out if JIT execution is available after studying a pattern by calling
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<b>pcre_fullinfo()</b> with the PCRE_INFO_JIT option. A result of 1 means that
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JIT compilation was successful. A result of 0 means that JIT support is not
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available, or the pattern was not studied with PCRE_STUDY_JIT_COMPILE, or the
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JIT compiler was not able to handle the pattern.
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Once a pattern has been studied, with or without JIT, it can be used as many
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times as you like for matching different subject strings.
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<br><a name="SEC5" href="#TOC1">UNSUPPORTED OPTIONS AND PATTERN ITEMS</a><br>
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The only <b>pcre_exec()</b> options that are supported for JIT execution are
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PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and
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PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not
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The unsupported pattern items are:
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\C match a single byte; not supported in UTF-8 mode
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(*PRUNE) ) the backtracking control verbs
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Support for some of these may be added in future.
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<br><a name="SEC6" href="#TOC1">RETURN VALUES FROM JIT EXECUTION</a><br>
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When a pattern is matched using JIT execution, the return values are the same
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as those given by the interpretive <b>pcre_exec()</b> code, with the addition of
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one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means that the memory used
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for the JIT stack was insufficient. See
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<a href="#stackcontrol">"Controlling the JIT stack"</a>
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below for a discussion of JIT stack usage. For compatibility with the
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interpretive <b>pcre_exec()</b> code, no more than two-thirds of the
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<i>ovector</i> argument is used for passing back captured substrings.
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The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if searching a
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very large pattern tree goes on for too long, as it is in the same circumstance
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when JIT is not used, but the details of exactly what is counted are not the
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same. The PCRE_ERROR_RECURSIONLIMIT error code is never returned by JIT
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<br><a name="SEC7" href="#TOC1">SAVING AND RESTORING COMPILED PATTERNS</a><br>
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The code that is generated by the JIT compiler is architecture-specific, and is
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also position dependent. For those reasons it cannot be saved (in a file or
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database) and restored later like the bytecode and other data of a compiled
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pattern. Saving and restoring compiled patterns is not something many people
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do. More detail about this facility is given in the
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<a href="pcreprecompile.html"><b>pcreprecompile</b></a>
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documentation. It should be possible to run <b>pcre_study()</b> on a saved and
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restored pattern, and thereby recreate the JIT data, but because JIT
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compilation uses significant resources, it is probably not worth doing this;
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you might as well recompile the original pattern.
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<a name="stackcontrol"></a></P>
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<br><a name="SEC8" href="#TOC1">CONTROLLING THE JIT STACK</a><br>
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When the compiled JIT code runs, it needs a block of memory to use as a stack.
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By default, it uses 32K on the machine stack. However, some large or
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complicated patterns need more than this. The error PCRE_ERROR_JIT_STACKLIMIT
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is given when there is not enough stack. Three functions are provided for
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managing blocks of memory for use as JIT stacks. There is further discussion
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about the use of JIT stacks in the section entitled
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<a href="#stackcontrol">"JIT stack FAQ"</a>
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The <b>pcre_jit_stack_alloc()</b> function creates a JIT stack. Its arguments
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are a starting size and a maximum size, and it returns a pointer to an opaque
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structure of type <b>pcre_jit_stack</b>, or NULL if there is an error. The
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<b>pcre_jit_stack_free()</b> function can be used to free a stack that is no
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longer needed. (For the technically minded: the address space is allocated by
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mmap or VirtualAlloc.)
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JIT uses far less memory for recursion than the interpretive code,
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and a maximum stack size of 512K to 1M should be more than enough for any
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The <b>pcre_assign_jit_stack()</b> function specifies which stack JIT code
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should use. Its arguments are as follows:
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pcre_jit_callback callback
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The <i>extra</i> argument must be the result of studying a pattern with
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PCRE_STUDY_JIT_COMPILE. There are three cases for the values of the other two
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(1) If <i>callback</i> is NULL and <i>data</i> is NULL, an internal 32K block
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on the machine stack is used.
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(2) If <i>callback</i> is NULL and <i>data</i> is not NULL, <i>data</i> must be
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a valid JIT stack, the result of calling <b>pcre_jit_stack_alloc()</b>.
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(3) If <i>callback</i> not NULL, it must point to a function that is called
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with <i>data</i> as an argument at the start of matching, in order to
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set up a JIT stack. If the result is NULL, the internal 32K stack
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is used; otherwise the return value must be a valid JIT stack,
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the result of calling <b>pcre_jit_stack_alloc()</b>.
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You may safely assign the same JIT stack to more than one pattern, as long as
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they are all matched sequentially in the same thread. In a multithread
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application, each thread must use its own JIT stack.
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Strictly speaking, even more is allowed. You can assign the same stack to any
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number of patterns as long as they are not used for matching by multiple
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threads at the same time. For example, you can assign the same stack to all
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compiled patterns, and use a global mutex in the callback to wait until the
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stack is available for use. However, this is an inefficient solution, and
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This is a suggestion for how a typical multithreaded program might operate:
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During thread initalization
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thread_local_var = pcre_jit_stack_alloc(...)
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pcre_jit_stack_free(thread_local_var)
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Use a one-line callback function
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return thread_local_var
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All the functions described in this section do nothing if JIT is not available,
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and <b>pcre_assign_jit_stack()</b> does nothing unless the <b>extra</b> argument
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is non-NULL and points to a <b>pcre_extra</b> block that is the result of a
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successful study with PCRE_STUDY_JIT_COMPILE.
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<a name="stackfaq"></a></P>
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<br><a name="SEC9" href="#TOC1">JIT STACK FAQ</a><br>
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(1) Why do we need JIT stacks?
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PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack where
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the local data of the current node is pushed before checking its child nodes.
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Allocating real machine stack on some platforms is difficult. For example, the
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stack chain needs to be updated every time if we extend the stack on PowerPC.
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Although it is possible, its updating time overhead decreases performance. So
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we do the recursion in memory.
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(2) Why don't we simply allocate blocks of memory with <b>malloc()</b>?
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Modern operating systems have a nice feature: they can reserve an address space
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instead of allocating memory. We can safely allocate memory pages inside this
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address space, so the stack could grow without moving memory data (this is
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important because of pointers). Thus we can allocate 1M address space, and use
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only a single memory page (usually 4K) if that is enough. However, we can still
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grow up to 1M anytime if needed.
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(3) Who "owns" a JIT stack?
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The owner of the stack is the user program, not the JIT studied pattern or
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anything else. The user program must ensure that if a stack is used by
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<b>pcre_exec()</b>, (that is, it is assigned to the pattern currently running),
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that stack must not be used by any other threads (to avoid overwriting the same
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memory area). The best practice for multithreaded programs is to allocate a
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stack for each thread, and return this stack through the JIT callback function.
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(4) When should a JIT stack be freed?
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You can free a JIT stack at any time, as long as it will not be used by
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<b>pcre_exec()</b> again. When you assign the stack to a pattern, only a pointer
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is set. There is no reference counting or any other magic. You can free the
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patterns and stacks in any order, anytime. Just <i>do not</i> call
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<b>pcre_exec()</b> with a pattern pointing to an already freed stack, as that
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will cause SEGFAULT. (Also, do not free a stack currently used by
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<b>pcre_exec()</b> in another thread). You can also replace the stack for a
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pattern at any time. You can even free the previous stack before assigning a
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(5) Should I allocate/free a stack every time before/after calling
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No, because this is too costly in terms of resources. However, you could
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implement some clever idea which release the stack if it is not used in let's
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say two minutes. The JIT callback can help to achive this without keeping a
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list of the currently JIT studied patterns.
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(6) OK, the stack is for long term memory allocation. But what happens if a
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pattern causes stack overflow with a stack of 1M? Is that 1M kept until the
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Especially on embedded sytems, it might be a good idea to release
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memory sometimes without freeing the stack. There is no API for this at the
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moment. Probably a function call which returns with the currently allocated
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memory for any stack and another which allows releasing memory (shrinking the
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stack) would be a good idea if someone needs this.
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(7) This is too much of a headache. Isn't there any better solution for JIT
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No, thanks to Windows. If POSIX threads were used everywhere, we could throw
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out this complicated API.
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<br><a name="SEC10" href="#TOC1">EXAMPLE CODE</a><br>
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This is a single-threaded example that specifies a JIT stack without using a
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pcre_jit_stack *jit_stack;
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re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
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/* Check for errors */
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extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
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jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024);
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/* Check for error (NULL) */
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pcre_assign_jit_stack(extra, NULL, jit_stack);
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rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
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pcre_free_study(extra);
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pcre_jit_stack_free(jit_stack);
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<br><a name="SEC11" href="#TOC1">SEE ALSO</a><br>
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<br><a name="SEC12" href="#TOC1">AUTHOR</a><br>
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Philip Hazel (FAQ by Zoltan Herczeg)
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University Computing Service
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Cambridge CB2 3QH, England.
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<br><a name="SEC13" href="#TOC1">REVISION</a><br>
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Last updated: 08 January 2012
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Copyright © 1997-2012 University of Cambridge.
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Return to the <a href="index.html">PCRE index page</a>.
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