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If there is any nonsense in it, please consult the +man page, in case the conversion went wrong. +<br> +<ul> +<li><a name="TOC1" href="#SEC1">PCRE JUST-IN-TIME COMPILER SUPPORT</a> +<li><a name="TOC2" href="#SEC2">8-BIT, 16-BIT AND 32-BIT SUPPORT</a> +<li><a name="TOC3" href="#SEC3">AVAILABILITY OF JIT SUPPORT</a> +<li><a name="TOC4" href="#SEC4">SIMPLE USE OF JIT</a> +<li><a name="TOC5" href="#SEC5">UNSUPPORTED OPTIONS AND PATTERN ITEMS</a> +<li><a name="TOC6" href="#SEC6">RETURN VALUES FROM JIT EXECUTION</a> +<li><a name="TOC7" href="#SEC7">SAVING AND RESTORING COMPILED PATTERNS</a> +<li><a name="TOC8" href="#SEC8">CONTROLLING THE JIT STACK</a> +<li><a name="TOC9" href="#SEC9">JIT STACK FAQ</a> +<li><a name="TOC10" href="#SEC10">EXAMPLE CODE</a> +<li><a name="TOC11" href="#SEC11">JIT FAST PATH API</a> +<li><a name="TOC12" href="#SEC12">SEE ALSO</a> +<li><a name="TOC13" href="#SEC13">AUTHOR</a> +<li><a name="TOC14" href="#SEC14">REVISION</a> +</ul> +<br><a name="SEC1" href="#TOC1">PCRE JUST-IN-TIME COMPILER SUPPORT</a><br> +<P> +Just-in-time compiling is a heavyweight optimization that can greatly speed up +pattern matching. However, it comes at the cost of extra processing before the +match is performed. Therefore, it is of most benefit when the same pattern is +going to be matched many times. This does not necessarily mean many calls of a +matching function; if the pattern is not anchored, matching attempts may take +place many times at various positions in the subject, even for a single call. +Therefore, if the subject string is very long, it may still pay to use JIT for +one-off matches. +</P> +<P> +JIT support applies only to the traditional Perl-compatible matching function. +It does not apply when the DFA matching function is being used. The code for +this support was written by Zoltan Herczeg. +</P> +<br><a name="SEC2" href="#TOC1">8-BIT, 16-BIT AND 32-BIT SUPPORT</a><br> +<P> +JIT support is available for all of the 8-bit, 16-bit and 32-bit PCRE +libraries. To keep this documentation simple, only the 8-bit interface is +described in what follows. If you are using the 16-bit library, substitute the +16-bit functions and 16-bit structures (for example, <i>pcre16_jit_stack</i> +instead of <i>pcre_jit_stack</i>). If you are using the 32-bit library, +substitute the 32-bit functions and 32-bit structures (for example, +<i>pcre32_jit_stack</i> instead of <i>pcre_jit_stack</i>). +</P> +<br><a name="SEC3" href="#TOC1">AVAILABILITY OF JIT SUPPORT</a><br> +<P> +JIT support is an optional feature of PCRE. The "configure" option --enable-jit +(or equivalent CMake option) must be set when PCRE is built if you want to use +JIT. The support is limited to the following hardware platforms: +<pre> + ARM v5, v7, and Thumb2 + Intel x86 32-bit and 64-bit + MIPS 32-bit + Power PC 32-bit and 64-bit + SPARC 32-bit (experimental) +</pre> +If --enable-jit is set on an unsupported platform, compilation fails. +</P> +<P> +A program that is linked with PCRE 8.20 or later can tell if JIT support is +available by calling <b>pcre_config()</b> with the PCRE_CONFIG_JIT option. The +result is 1 when JIT is available, and 0 otherwise. However, a simple program +does not need to check this in order to use JIT. The normal API is implemented +in a way that falls back to the interpretive code if JIT is not available. For +programs that need the best possible performance, there is also a "fast path" +API that is JIT-specific. +</P> +<P> +If your program may sometimes be linked with versions of PCRE that are older +than 8.20, but you want to use JIT when it is available, you can test +the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT macro such +as PCRE_CONFIG_JIT, for compile-time control of your code. +</P> +<br><a name="SEC4" href="#TOC1">SIMPLE USE OF JIT</a><br> +<P> +You have to do two things to make use of the JIT support in the simplest way: +<pre> + (1) Call <b>pcre_study()</b> with the PCRE_STUDY_JIT_COMPILE option for + each compiled pattern, and pass the resulting <b>pcre_extra</b> block to + <b>pcre_exec()</b>. + + (2) Use <b>pcre_free_study()</b> to free the <b>pcre_extra</b> block when it is + no longer needed, instead of just freeing it yourself. This ensures that + any JIT data is also freed. +</pre> +For a program that may be linked with pre-8.20 versions of PCRE, you can insert +<pre> + #ifndef PCRE_STUDY_JIT_COMPILE + #define PCRE_STUDY_JIT_COMPILE 0 + #endif +</pre> +so that no option is passed to <b>pcre_study()</b>, and then use something like +this to free the study data: +<pre> + #ifdef PCRE_CONFIG_JIT + pcre_free_study(study_ptr); + #else + pcre_free(study_ptr); + #endif +</pre> +PCRE_STUDY_JIT_COMPILE requests the JIT compiler to generate code for complete +matches. If you want to run partial matches using the PCRE_PARTIAL_HARD or +PCRE_PARTIAL_SOFT options of <b>pcre_exec()</b>, you should set one or both of +the following options in addition to, or instead of, PCRE_STUDY_JIT_COMPILE +when you call <b>pcre_study()</b>: +<pre> + PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE + PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE +</pre> +The JIT compiler generates different optimized code for each of the three +modes (normal, soft partial, hard partial). When <b>pcre_exec()</b> is called, +the appropriate code is run if it is available. Otherwise, the pattern is +matched using interpretive code. +</P> +<P> +In some circumstances you may need to call additional functions. These are +described in the section entitled +<a href="#stackcontrol">"Controlling the JIT stack"</a> +below. +</P> +<P> +If JIT support is not available, PCRE_STUDY_JIT_COMPILE etc. are ignored, and +no JIT data is created. Otherwise, the compiled pattern is passed to the JIT +compiler, which turns it into machine code that executes much faster than the +normal interpretive code. When <b>pcre_exec()</b> is passed a <b>pcre_extra</b> +block containing a pointer to JIT code of the appropriate mode (normal or +hard/soft partial), it obeys that code instead of running the interpreter. The +result is identical, but the compiled JIT code runs much faster. +</P> +<P> +There are some <b>pcre_exec()</b> options that are not supported for JIT +execution. There are also some pattern items that JIT cannot handle. Details +are given below. In both cases, execution automatically falls back to the +interpretive code. If you want to know whether JIT was actually used for a +particular match, you should arrange for a JIT callback function to be set up +as described in the section entitled +<a href="#stackcontrol">"Controlling the JIT stack"</a> +below, even if you do not need to supply a non-default JIT stack. Such a +callback function is called whenever JIT code is about to be obeyed. If the +execution options are not right for JIT execution, the callback function is not +obeyed. +</P> +<P> +If the JIT compiler finds an unsupported item, no JIT data is generated. You +can find out if JIT execution is available after studying a pattern by calling +<b>pcre_fullinfo()</b> with the PCRE_INFO_JIT option. A result of 1 means that +JIT compilation was successful. A result of 0 means that JIT support is not +available, or the pattern was not studied with PCRE_STUDY_JIT_COMPILE etc., or +the JIT compiler was not able to handle the pattern. +</P> +<P> +Once a pattern has been studied, with or without JIT, it can be used as many +times as you like for matching different subject strings. +</P> +<br><a name="SEC5" href="#TOC1">UNSUPPORTED OPTIONS AND PATTERN ITEMS</a><br> +<P> +The only <b>pcre_exec()</b> options that are supported for JIT execution are +PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK, PCRE_NO_UTF32_CHECK, PCRE_NOTBOL, +PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and +PCRE_PARTIAL_SOFT. +</P> +<P> +The only unsupported pattern items are \C (match a single data unit) when +running in a UTF mode, and a callout immediately before an assertion condition +in a conditional group. +</P> +<br><a name="SEC6" href="#TOC1">RETURN VALUES FROM JIT EXECUTION</a><br> +<P> +When a pattern is matched using JIT execution, the return values are the same +as those given by the interpretive <b>pcre_exec()</b> code, with the addition of +one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means that the memory used +for the JIT stack was insufficient. See +<a href="#stackcontrol">"Controlling the JIT stack"</a> +below for a discussion of JIT stack usage. For compatibility with the +interpretive <b>pcre_exec()</b> code, no more than two-thirds of the +<i>ovector</i> argument is used for passing back captured substrings. +</P> +<P> +The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if searching a +very large pattern tree goes on for too long, as it is in the same circumstance +when JIT is not used, but the details of exactly what is counted are not the +same. The PCRE_ERROR_RECURSIONLIMIT error code is never returned by JIT +execution. +</P> +<br><a name="SEC7" href="#TOC1">SAVING AND RESTORING COMPILED PATTERNS</a><br> +<P> +The code that is generated by the JIT compiler is architecture-specific, and is +also position dependent. For those reasons it cannot be saved (in a file or +database) and restored later like the bytecode and other data of a compiled +pattern. Saving and restoring compiled patterns is not something many people +do. More detail about this facility is given in the +<a href="pcreprecompile.html"><b>pcreprecompile</b></a> +documentation. It should be possible to run <b>pcre_study()</b> on a saved and +restored pattern, and thereby recreate the JIT data, but because JIT +compilation uses significant resources, it is probably not worth doing this; +you might as well recompile the original pattern. +<a name="stackcontrol"></a></P> +<br><a name="SEC8" href="#TOC1">CONTROLLING THE JIT STACK</a><br> +<P> +When the compiled JIT code runs, it needs a block of memory to use as a stack. +By default, it uses 32K on the machine stack. However, some large or +complicated patterns need more than this. The error PCRE_ERROR_JIT_STACKLIMIT +is given when there is not enough stack. Three functions are provided for +managing blocks of memory for use as JIT stacks. There is further discussion +about the use of JIT stacks in the section entitled +<a href="#stackcontrol">"JIT stack FAQ"</a> +below. +</P> +<P> +The <b>pcre_jit_stack_alloc()</b> function creates a JIT stack. Its arguments +are a starting size and a maximum size, and it returns a pointer to an opaque +structure of type <b>pcre_jit_stack</b>, or NULL if there is an error. The +<b>pcre_jit_stack_free()</b> function can be used to free a stack that is no +longer needed. (For the technically minded: the address space is allocated by +mmap or VirtualAlloc.) +</P> +<P> +JIT uses far less memory for recursion than the interpretive code, +and a maximum stack size of 512K to 1M should be more than enough for any +pattern. +</P> +<P> +The <b>pcre_assign_jit_stack()</b> function specifies which stack JIT code +should use. Its arguments are as follows: +<pre> + pcre_extra *extra + pcre_jit_callback callback + void *data +</pre> +The <i>extra</i> argument must be the result of studying a pattern with +PCRE_STUDY_JIT_COMPILE etc. There are three cases for the values of the other +two options: +<pre> + (1) If <i>callback</i> is NULL and <i>data</i> is NULL, an internal 32K block + on the machine stack is used. + + (2) If <i>callback</i> is NULL and <i>data</i> is not NULL, <i>data</i> must be + a valid JIT stack, the result of calling <b>pcre_jit_stack_alloc()</b>. + + (3) If <i>callback</i> is not NULL, it must point to a function that is + called with <i>data</i> as an argument at the start of matching, in + order to set up a JIT stack. If the return from the callback + function is NULL, the internal 32K stack is used; otherwise the + return value must be a valid JIT stack, the result of calling + <b>pcre_jit_stack_alloc()</b>. +</pre> +A callback function is obeyed whenever JIT code is about to be run; it is not +obeyed when <b>pcre_exec()</b> is called with options that are incompatible for +JIT execution. A callback function can therefore be used to determine whether a +match operation was executed by JIT or by the interpreter. +</P> +<P> +You may safely use the same JIT stack for more than one pattern (either by +assigning directly or by callback), as long as the patterns are all matched +sequentially in the same thread. In a multithread application, if you do not +specify a JIT stack, or if you assign or pass back NULL from a callback, that +is thread-safe, because each thread has its own machine stack. However, if you +assign or pass back a non-NULL JIT stack, this must be a different stack for +each thread so that the application is thread-safe. +</P> +<P> +Strictly speaking, even more is allowed. You can assign the same non-NULL stack +to any number of patterns as long as they are not used for matching by multiple +threads at the same time. For example, you can assign the same stack to all +compiled patterns, and use a global mutex in the callback to wait until the +stack is available for use. However, this is an inefficient solution, and not +recommended. +</P> +<P> +This is a suggestion for how a multithreaded program that needs to set up +non-default JIT stacks might operate: +<pre> + During thread initalization + thread_local_var = pcre_jit_stack_alloc(...) + + During thread exit + pcre_jit_stack_free(thread_local_var) + + Use a one-line callback function + return thread_local_var +</pre> +All the functions described in this section do nothing if JIT is not available, +and <b>pcre_assign_jit_stack()</b> does nothing unless the <b>extra</b> argument +is non-NULL and points to a <b>pcre_extra</b> block that is the result of a +successful study with PCRE_STUDY_JIT_COMPILE etc. +<a name="stackfaq"></a></P> +<br><a name="SEC9" href="#TOC1">JIT STACK FAQ</a><br> +<P> +(1) Why do we need JIT stacks? +<br> +<br> +PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack where +the local data of the current node is pushed before checking its child nodes. +Allocating real machine stack on some platforms is difficult. For example, the +stack chain needs to be updated every time if we extend the stack on PowerPC. +Although it is possible, its updating time overhead decreases performance. So +we do the recursion in memory. +</P> +<P> +(2) Why don't we simply allocate blocks of memory with <b>malloc()</b>? +<br> +<br> +Modern operating systems have a nice feature: they can reserve an address space +instead of allocating memory. We can safely allocate memory pages inside this +address space, so the stack could grow without moving memory data (this is +important because of pointers). Thus we can allocate 1M address space, and use +only a single memory page (usually 4K) if that is enough. However, we can still +grow up to 1M anytime if needed. +</P> +<P> +(3) Who "owns" a JIT stack? +<br> +<br> +The owner of the stack is the user program, not the JIT studied pattern or +anything else. The user program must ensure that if a stack is used by +<b>pcre_exec()</b>, (that is, it is assigned to the pattern currently running), +that stack must not be used by any other threads (to avoid overwriting the same +memory area). The best practice for multithreaded programs is to allocate a +stack for each thread, and return this stack through the JIT callback function. +</P> +<P> +(4) When should a JIT stack be freed? +<br> +<br> +You can free a JIT stack at any time, as long as it will not be used by +<b>pcre_exec()</b> again. When you assign the stack to a pattern, only a pointer +is set. There is no reference counting or any other magic. You can free the +patterns and stacks in any order, anytime. Just <i>do not</i> call +<b>pcre_exec()</b> with a pattern pointing to an already freed stack, as that +will cause SEGFAULT. (Also, do not free a stack currently used by +<b>pcre_exec()</b> in another thread). You can also replace the stack for a +pattern at any time. You can even free the previous stack before assigning a +replacement. +</P> +<P> +(5) Should I allocate/free a stack every time before/after calling +<b>pcre_exec()</b>? +<br> +<br> +No, because this is too costly in terms of resources. However, you could +implement some clever idea which release the stack if it is not used in let's +say two minutes. The JIT callback can help to achieve this without keeping a +list of the currently JIT studied patterns. +</P> +<P> +(6) OK, the stack is for long term memory allocation. But what happens if a +pattern causes stack overflow with a stack of 1M? Is that 1M kept until the +stack is freed? +<br> +<br> +Especially on embedded sytems, it might be a good idea to release memory +sometimes without freeing the stack. There is no API for this at the moment. +Probably a function call which returns with the currently allocated memory for +any stack and another which allows releasing memory (shrinking the stack) would +be a good idea if someone needs this. +</P> +<P> +(7) This is too much of a headache. Isn't there any better solution for JIT +stack handling? +<br> +<br> +No, thanks to Windows. If POSIX threads were used everywhere, we could throw +out this complicated API. +</P> +<br><a name="SEC10" href="#TOC1">EXAMPLE CODE</a><br> +<P> +This is a single-threaded example that specifies a JIT stack without using a +callback. +<pre> + int rc; + int ovector[30]; + pcre *re; + pcre_extra *extra; + pcre_jit_stack *jit_stack; + + re = pcre_compile(pattern, 0, &error, &erroffset, NULL); + /* Check for errors */ + extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error); + jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024); + /* Check for error (NULL) */ + pcre_assign_jit_stack(extra, NULL, jit_stack); + rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30); + /* Check results */ + pcre_free(re); + pcre_free_study(extra); + pcre_jit_stack_free(jit_stack); + +</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: 17 March 2013 +<br> +Copyright © 1997-2013 University of Cambridge. +<br> +<p> +Return to the <a href="index.html">PCRE index page</a>. +</p> |