459 lines
20 KiB
HTML
459 lines
20 KiB
HTML
<html>
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<head>
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<title>pcrejit specification</title>
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</head>
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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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<p>
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Return to the <a href="index.html">PCRE index page</a>.
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</p>
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<p>
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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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<br>
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<ul>
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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, 16-BIT AND 32-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">JIT FAST PATH API</a>
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<li><a name="TOC12" href="#SEC12">SEE ALSO</a>
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<li><a name="TOC13" href="#SEC13">AUTHOR</a>
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<li><a name="TOC14" href="#SEC14">REVISION</a>
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</ul>
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<br><a name="SEC1" href="#TOC1">PCRE JUST-IN-TIME COMPILER SUPPORT</a><br>
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<P>
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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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one-off matches.
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</P>
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<P>
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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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</P>
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<br><a name="SEC2" href="#TOC1">8-BIT, 16-BIT AND 32-BIT SUPPORT</a><br>
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<P>
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JIT support is available for all of the 8-bit, 16-bit and 32-bit PCRE
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libraries. To keep this documentation simple, only the 8-bit interface is
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described in what follows. If you are using the 16-bit library, substitute the
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16-bit functions and 16-bit structures (for example, <i>pcre16_jit_stack</i>
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instead of <i>pcre_jit_stack</i>). If you are using the 32-bit library,
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substitute the 32-bit functions and 32-bit structures (for example,
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<i>pcre32_jit_stack</i> instead of <i>pcre_jit_stack</i>).
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</P>
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<br><a name="SEC3" href="#TOC1">AVAILABILITY OF JIT SUPPORT</a><br>
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<P>
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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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<pre>
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ARM v5, v7, and Thumb2
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Intel x86 32-bit and 64-bit
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MIPS 32-bit
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Power PC 32-bit and 64-bit
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SPARC 32-bit (experimental)
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</pre>
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If --enable-jit is set on an unsupported platform, compilation fails.
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</P>
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<P>
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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 normal API is implemented
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in a way that falls back to the interpretive code if JIT is not available. For
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programs that need the best possible performance, there is also a "fast path"
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API that is JIT-specific.
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</P>
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<P>
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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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</P>
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<br><a name="SEC4" href="#TOC1">SIMPLE USE OF JIT</a><br>
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<P>
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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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<pre>
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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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<b>pcre_exec()</b>.
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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 ensures that
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any JIT data is also freed.
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</pre>
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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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<pre>
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#ifndef PCRE_STUDY_JIT_COMPILE
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#define PCRE_STUDY_JIT_COMPILE 0
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#endif
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</pre>
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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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<pre>
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#ifdef PCRE_CONFIG_JIT
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pcre_free_study(study_ptr);
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#else
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pcre_free(study_ptr);
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#endif
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</pre>
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PCRE_STUDY_JIT_COMPILE requests the JIT compiler to generate code for complete
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matches. If you want to run partial matches using the PCRE_PARTIAL_HARD or
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PCRE_PARTIAL_SOFT options of <b>pcre_exec()</b>, you should set one or both of
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the following options in addition to, or instead of, PCRE_STUDY_JIT_COMPILE
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when you call <b>pcre_study()</b>:
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<pre>
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PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
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PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
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</pre>
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The JIT compiler generates different optimized code for each of the three
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modes (normal, soft partial, hard partial). When <b>pcre_exec()</b> is called,
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the appropriate code is run if it is available. Otherwise, the pattern is
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matched using interpretive code.
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</P>
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<P>
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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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below.
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</P>
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<P>
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If JIT support is not available, PCRE_STUDY_JIT_COMPILE etc. are ignored, and
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no JIT data is created. Otherwise, the compiled pattern is passed to the JIT
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compiler, which turns it into machine code that executes much faster than the
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normal interpretive code. When <b>pcre_exec()</b> is passed a <b>pcre_extra</b>
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block containing a pointer to JIT code of the appropriate mode (normal or
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hard/soft partial), it obeys that code instead of running the interpreter. The
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result is identical, but the compiled JIT code runs much faster.
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</P>
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<P>
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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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interpretive code. If you want to know whether JIT was actually used for a
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particular match, you should arrange for a JIT callback function to be set up
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as described in the section entitled
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<a href="#stackcontrol">"Controlling the JIT stack"</a>
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below, even if you do not need to supply a non-default JIT stack. Such a
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callback function is called whenever JIT code is about to be obeyed. If the
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execution options are not right for JIT execution, the callback function is not
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obeyed.
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</P>
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<P>
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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 etc., or
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the JIT compiler was not able to handle the pattern.
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</P>
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<P>
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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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</P>
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<br><a name="SEC5" href="#TOC1">UNSUPPORTED OPTIONS AND PATTERN ITEMS</a><br>
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<P>
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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_NO_UTF16_CHECK, PCRE_NO_UTF32_CHECK, PCRE_NOTBOL,
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PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and
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PCRE_PARTIAL_SOFT.
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</P>
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<P>
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The unsupported pattern items are:
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<pre>
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\C match a single byte; not supported in UTF-8 mode
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(?Cn) callouts
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(*PRUNE) )
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(*SKIP) ) backtracking control verbs
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(*THEN) )
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</pre>
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Support for some of these may be added in future.
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</P>
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<br><a name="SEC6" href="#TOC1">RETURN VALUES FROM JIT EXECUTION</a><br>
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<P>
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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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</P>
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<P>
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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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execution.
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</P>
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<br><a name="SEC7" href="#TOC1">SAVING AND RESTORING COMPILED PATTERNS</a><br>
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<P>
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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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<P>
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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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below.
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</P>
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<P>
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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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</P>
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<P>
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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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pattern.
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</P>
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<P>
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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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<pre>
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pcre_extra *extra
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pcre_jit_callback callback
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void *data
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</pre>
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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 etc. There are three cases for the values of the other
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two options:
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<pre>
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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> is not NULL, it must point to a function that is
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called with <i>data</i> as an argument at the start of matching, in
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order to set up a JIT stack. If the return from the callback
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function is NULL, the internal 32K stack is used; otherwise the
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return value must be a valid JIT stack, the result of calling
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<b>pcre_jit_stack_alloc()</b>.
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</pre>
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A callback function is obeyed whenever JIT code is about to be run; it is not
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obeyed when <b>pcre_exec()</b> is called with options that are incompatible for
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JIT execution. A callback function can therefore be used to determine whether a
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match operation was executed by JIT or by the interpreter.
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</P>
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<P>
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You may safely use the same JIT stack for more than one pattern (either by
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assigning directly or by callback), as long as the patterns are all matched
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sequentially in the same thread. In a multithread application, if you do not
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specify a JIT stack, or if you assign or pass back NULL from a callback, that
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is thread-safe, because each thread has its own machine stack. However, if you
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assign or pass back a non-NULL JIT stack, this must be a different stack for
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each thread so that the application is thread-safe.
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</P>
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<P>
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Strictly speaking, even more is allowed. You can assign the same non-NULL stack
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to any 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 not
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recommended.
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</P>
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<P>
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This is a suggestion for how a multithreaded program that needs to set up
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non-default JIT stacks might operate:
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<pre>
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During thread initalization
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thread_local_var = pcre_jit_stack_alloc(...)
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During thread exit
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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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</pre>
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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 etc.
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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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<P>
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(1) Why do we need JIT stacks?
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<br>
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<br>
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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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</P>
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<P>
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(2) Why don't we simply allocate blocks of memory with <b>malloc()</b>?
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<br>
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<br>
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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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</P>
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<P>
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(3) Who "owns" a JIT stack?
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<br>
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<br>
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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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</P>
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<P>
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(4) When should a JIT stack be freed?
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<br>
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<br>
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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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replacement.
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</P>
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<P>
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(5) Should I allocate/free a stack every time before/after calling
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<b>pcre_exec()</b>?
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<br>
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<br>
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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 achieve this without keeping a
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list of the currently JIT studied patterns.
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</P>
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<P>
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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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stack is freed?
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<br>
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<br>
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Especially on embedded sytems, it might be a good idea to release memory
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sometimes without freeing the stack. There is no API for this at the moment.
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Probably a function call which returns with the currently allocated memory for
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any stack and another which allows releasing memory (shrinking the stack) would
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be a good idea if someone needs this.
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</P>
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<P>
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(7) This is too much of a headache. Isn't there any better solution for JIT
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stack handling?
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<br>
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<br>
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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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</P>
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<br><a name="SEC10" href="#TOC1">EXAMPLE CODE</a><br>
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<P>
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This is a single-threaded example that specifies a JIT stack without using a
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callback.
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<pre>
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int rc;
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int ovector[30];
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pcre *re;
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pcre_extra *extra;
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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);
|
|
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);
|
|
|
|
</PRE>
|
|
</P>
|
|
<br><a name="SEC11" href="#TOC1">JIT FAST PATH API</a><br>
|
|
<P>
|
|
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 <b>pcre_exec()</b> 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
|
|
<b>pcre_exec()</b> (obviously only for patterns that have been successfully
|
|
studied by JIT).
|
|
</P>
|
|
<P>
|
|
The fast path function is called <b>pcre_jit_exec()</b>, and it takes exactly
|
|
the same arguments as <b>pcre_exec()</b>, 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 <b>pcre_exec()</b>.
|
|
</P>
|
|
<P>
|
|
When you call <b>pcre_exec()</b>, 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.
|
|
</P>
|
|
<P>
|
|
Bypassing the sanity checks and the <b>pcre_exec()</b> wrapping can give
|
|
speedups of more than 10%.
|
|
</P>
|
|
<br><a name="SEC12" href="#TOC1">SEE ALSO</a><br>
|
|
<P>
|
|
<b>pcreapi</b>(3)
|
|
</P>
|
|
<br><a name="SEC13" href="#TOC1">AUTHOR</a><br>
|
|
<P>
|
|
Philip Hazel (FAQ by Zoltan Herczeg)
|
|
<br>
|
|
University Computing Service
|
|
<br>
|
|
Cambridge CB2 3QH, England.
|
|
<br>
|
|
</P>
|
|
<br><a name="SEC14" href="#TOC1">REVISION</a><br>
|
|
<P>
|
|
Last updated: 31 October 2012
|
|
<br>
|
|
Copyright © 1997-2012 University of Cambridge.
|
|
<br>
|
|
<p>
|
|
Return to the <a href="index.html">PCRE index page</a>.
|
|
</p>
|