402 lines
17 KiB
HTML
402 lines
17 KiB
HTML
<html>
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<head>
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<title>pcre2jit 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>pcre2jit man page</h1>
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<p>
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Return to the <a href="index.html">PCRE2 index page</a>.
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</p>
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<p>
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This page is part of the PCRE2 HTML documentation. It was generated
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automatically from the original man page. If there is any nonsense in it,
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please consult the 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">PCRE2 JUST-IN-TIME COMPILER SUPPORT</a>
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<li><a name="TOC2" href="#SEC2">AVAILABILITY OF JIT SUPPORT</a>
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<li><a name="TOC3" href="#SEC3">SIMPLE USE OF JIT</a>
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<li><a name="TOC4" href="#SEC4">UNSUPPORTED OPTIONS AND PATTERN ITEMS</a>
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<li><a name="TOC5" href="#SEC5">RETURN VALUES FROM JIT MATCHING</a>
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<li><a name="TOC6" href="#SEC6">CONTROLLING THE JIT STACK</a>
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<li><a name="TOC7" href="#SEC7">JIT STACK FAQ</a>
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<li><a name="TOC8" href="#SEC8">EXAMPLE CODE</a>
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<li><a name="TOC9" href="#SEC9">JIT FAST PATH API</a>
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<li><a name="TOC10" href="#SEC10">SEE ALSO</a>
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<li><a name="TOC11" href="#SEC11">AUTHOR</a>
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<li><a name="TOC12" href="#SEC12">REVISION</a>
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</ul>
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<br><a name="SEC1" href="#TOC1">PCRE2 JUST-IN-TIME COMPILER SUPPORT</a><br>
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<P>
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FIXME: This needs checking over once JIT support is implemented.
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</P>
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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. JIT support is available for all of the 8-bit, 16-bit and
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32-bit PCRE2 libraries.
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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">AVAILABILITY OF JIT SUPPORT</a><br>
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<P>
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JIT support is an optional feature of PCRE2. The "configure" option
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--enable-jit (or equivalent CMake option) must be set when PCRE2 is built if
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you want to use JIT. The support is limited to the following hardware
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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 can tell if JIT support is available by calling <b>pcre2_config()</b>
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with the PCRE2_CONFIG_JIT option. The result is 1 when JIT is available, and 0
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otherwise. However, a simple program does not need to check this in order to
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use JIT. The API is implemented in a way that falls back to the interpretive
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code if JIT is not available. For programs that need the best possible
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performance, there is also a "fast path" API that is JIT-specific.
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</P>
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<br><a name="SEC3" href="#TOC1">SIMPLE USE OF JIT</a><br>
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<P>
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To make use of the JIT support in the simplest way, all you have to do is to
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call <b>pcre2_jit_compile()</b> after successfully compiling a pattern with
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<b>pcre2_compile()</b>. This function has two arguments: the first is the
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compiled pattern pointer that was returned by <b>pcre2_compile()</b>, and the
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second is a set of option bits, which must include at least one of
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PCRE2_JIT_COMPLETE, PCRE2_JIT_PARTIAL_HARD, or PCRE2_JIT_PARTIAL_SOFT.
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</P>
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<P>
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The returned value from <b>pcre2_jit_compile()</b> is FIXME FIXME.
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</P>
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<P>
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PCRE2_JIT_COMPLETE requests the JIT compiler to generate code for complete
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matches. If you want to run partial matches using the PCRE2_PARTIAL_HARD or
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PCRE2_PARTIAL_SOFT options of <b>pcre2_match()</b>, you should set one or both
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of the other options as well as, or instead of PCRE2_JIT_COMPLETE. The JIT
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compiler generates different optimized code for each of the three modes
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(normal, soft partial, hard partial). When <b>pcre2_match()</b> is called, the
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appropriate code is run if it is available. Otherwise, the pattern is matched
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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, a call to <b>pcre2_jit_comple()</b> does
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nothing and returns FIXME. 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, but yields exactly the same results.
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</P>
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<P>
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There are some <b>pcre2_match()</b> options that are not supported by JIT, and
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there are also some pattern items that JIT cannot handle. Details are given
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below. In both cases, matching automatically falls back to the interpretive
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code. If you want to know whether JIT was actually used for a particular match,
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you should arrange for a JIT callback function to be set up as described in the
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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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match-time options are not right for JIT execution, the callback function is
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not 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 matching is available after compiling a pattern by calling
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<b>pcre2_pattern_info()</b> with the PCRE2_INFO_JIT option. A result of 1 means
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that JIT compilation was successful. A result of 0 means that JIT support is
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not available, or the pattern was not processed by <b>pcre2_jit_compile()</b>,
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or the JIT compiler was not able to handle the pattern.
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</P>
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<br><a name="SEC4" href="#TOC1">UNSUPPORTED OPTIONS AND PATTERN ITEMS</a><br>
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<P>
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The <b>pcre2_match()</b> options that are supported for JIT matching are
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PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART,
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PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, and PCRE2_PARTIAL_SOFT. The options
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that are not supported at match time are PCRE2_ANCHORED and
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PCRE2_NO_START_OPTIMIZE, though they are supported if given at compile time.
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</P>
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<P>
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The only unsupported pattern items are \C (match a single data unit) when
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running in a UTF mode, and a callout immediately before an assertion condition
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in a conditional group.
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</P>
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<br><a name="SEC5" href="#TOC1">RETURN VALUES FROM JIT MATCHING</a><br>
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<P>
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When a pattern is matched using JIT matching, the return values are the same
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as those given by the interpretive <b>pcre2_match()</b> code, with the addition
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of one new error code: PCRE2_ERROR_JIT_STACKLIMIT. This means that the memory
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used 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.
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</P>
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<P>
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The error code PCRE2_ERROR_MATCHLIMIT is returned by the JIT code if searching
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a very large pattern tree goes on for too long, as it is in the same
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circumstance when JIT is not used, but the details of exactly what is counted
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are not the same. The PCRE2_ERROR_RECURSIONLIMIT error code is never returned
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when JIT matching is used.
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<a name="stackcontrol"></a></P>
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<br><a name="SEC6" 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 PCRE2_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>pcre2_jit_stack_alloc()</b> function creates a JIT stack. Its arguments
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are a general context (for memory allocation functions, or NULL for standard
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memory allocation), a starting size and a maximum size, and it returns a
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pointer to an opaque structure of type <b>pcre2_jit_stack</b>, or NULL if there
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is an error. The <b>pcre2_jit_stack_free()</b> function is used to free a stack
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that is no longer needed. (For the technically minded: the address space is
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allocated by mmap or VirtualAlloc.) FIXME Is this right?
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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>pcre2_jit_stack_assign()</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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pcre2_code *code
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pcre2_jit_callback callback
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void *data
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</pre>
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The <i>code</i> argument is a pointer to a compiled pattern, after it has been
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processed by <b>pcre2_jit_compile()</b>. There are three cases for the values of
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the other 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>pcre2_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>pcre2_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>pcre2_match()</b> is called with options that are incompatible
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for JIT matching. A callback function can therefore be used to determine
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whether a 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 = pcre2_jit_stack_alloc(...)
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During thread exit
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pcre2_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>pcre2_jit_stack_assign()</b> does nothing unless the <b>code</b> argument
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is non-NULL and points to a <b>pcre2_code</b> block that has been successfully
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processed by <b>pcre2_jit_compile()</b>.
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<a name="stackfaq"></a></P>
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<br><a name="SEC7" 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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PCRE2 (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>pcre2_match()</b>, (that is, it is assigned to the pattern currently
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running), that stack must not be used by any other threads (to avoid
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overwriting the same memory area). The best practice for multithreaded programs
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is to allocate a stack for each thread, and return this stack through the JIT
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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>pcre2_match()</b> again. When you assign the stack to a pattern, only a
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pointer is set. There is no reference counting or any other magic. You can free
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the patterns and stacks in any order, anytime. Just <i>do not</i> call
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<b>pcre2_match()</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>pcre2_match()</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>pcre2_match()</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="SEC8" 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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pcre2_code *re;
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pcre2_match_data *match_data;
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pcre2_jit_stack *jit_stack;
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re = pcre2_compile(pattern, PCRE2_ZERO_TERMINATED, 0,
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&errornumber, &erroffset, NULL);
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/* Check for errors */
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rc = pcre2_jit_compile(re, PCRE2_JIT_COMPLETE);
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/* Check for errors */
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jit_stack = pcre2_jit_stack_alloc(NULL, 32*1024, 512*1024);
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/* Check for error (NULL) */
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pcre2_jit_stack_assign(re, NULL, jit_stack);
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match_data = pcre2_match_data_create(re, 10);
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rc = pcre2_match(re, subject, length, 0, 0, match_data, NULL);
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/* Check results */
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pcre2_free(re);
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pcre2_jit_stack_free(jit_stack);
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</PRE>
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</P>
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<br><a name="SEC9" href="#TOC1">JIT FAST PATH API</a><br>
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<P>
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Because the API described above falls back to interpreted matching when JIT is
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not available, it is convenient for programs that are written for general use
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in many environments. However, calling JIT via <b>pcre2_match()</b> does have a
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performance impact. Programs that are written for use where JIT is known to be
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available, and which need the best possible performance, can instead use a
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"fast path" API to call JIT matching directly instead of calling
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<b>pcre2_match()</b> (obviously only for patterns that have been successfully
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processed by <b>pcre2_jit_compile()</b>).
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</P>
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<P>
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The fast path function is called <b>pcre2_jit_match()</b>, and it takes exactly
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the same arguments as <b>pcre2_match()</b>, plus one additional argument that
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must point to a JIT stack. The JIT stack arrangements described above do not
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apply. The return values are the same as for <b>pcre2_match()</b>.
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</P>
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<P>
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When you call <b>pcre2_match()</b>, as well as testing for invalid options, a
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number of other sanity checks are performed on the arguments. For example, if
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the subject pointer is NULL, an immediate error is given. Also, unless
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PCRE2_NO_UTF_CHECK is set, a UTF subject string is tested for validity. In the
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interests of speed, these checks do not happen on the JIT fast path, and if
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invalid data is passed, the result is undefined.
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</P>
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<P>
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Bypassing the sanity checks and the <b>pcre2_match()</b> wrapping can give
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speedups of more than 10%.
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</P>
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<br><a name="SEC10" href="#TOC1">SEE ALSO</a><br>
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<P>
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<b>pcre2api</b>(3)
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</P>
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<br><a name="SEC11" href="#TOC1">AUTHOR</a><br>
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<P>
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Philip Hazel (FAQ by Zoltan Herczeg)
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<br>
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University Computing Service
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<br>
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Cambridge CB2 3QH, England.
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<br>
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</P>
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<br><a name="SEC12" href="#TOC1">REVISION</a><br>
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<P>
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Last updated: 29 September 2014
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<br>
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Copyright © 1997-2014 University of Cambridge.
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<br>
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<p>
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Return to the <a href="index.html">PCRE2 index page</a>.
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</p>
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