| .TH PCREJIT 3 "17 March 2013" "PCRE 8.33" |
| .SH NAME |
| PCRE - Perl-compatible regular expressions |
| .SH "PCRE JUST-IN-TIME COMPILER SUPPORT" |
| .rs |
| .sp |
| 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 |
| 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. |
| . |
| . |
| .SH "8-BIT, 16-BIT AND 32-BIT SUPPORT" |
| .rs |
| .sp |
| 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, \fIpcre16_jit_stack\fP |
| instead of \fIpcre_jit_stack\fP). If you are using the 32-bit library, |
| substitute the 32-bit functions and 32-bit structures (for example, |
| \fIpcre32_jit_stack\fP instead of \fIpcre_jit_stack\fP). |
| . |
| . |
| .SH "AVAILABILITY OF JIT SUPPORT" |
| .rs |
| .sp |
| 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: |
| .sp |
| 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) |
| .sp |
| If --enable-jit is set on an unsupported platform, compilation fails. |
| .P |
| A program that is linked with PCRE 8.20 or later can tell if JIT support is |
| available by calling \fBpcre_config()\fP 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 |
| 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. |
| . |
| . |
| .SH "SIMPLE USE OF JIT" |
| .rs |
| .sp |
| You have to do two things to make use of the JIT support in the simplest way: |
| .sp |
| (1) Call \fBpcre_study()\fP with the PCRE_STUDY_JIT_COMPILE option for |
| each compiled pattern, and pass the resulting \fBpcre_extra\fP block to |
| \fBpcre_exec()\fP. |
| .sp |
| (2) Use \fBpcre_free_study()\fP to free the \fBpcre_extra\fP block when it is |
| no longer needed, instead of just freeing it yourself. This ensures that |
| any JIT data is also freed. |
| .sp |
| For a program that may be linked with pre-8.20 versions of PCRE, you can insert |
| .sp |
| #ifndef PCRE_STUDY_JIT_COMPILE |
| #define PCRE_STUDY_JIT_COMPILE 0 |
| #endif |
| .sp |
| so that no option is passed to \fBpcre_study()\fP, and then use something like |
| this to free the study data: |
| .sp |
| #ifdef PCRE_CONFIG_JIT |
| pcre_free_study(study_ptr); |
| #else |
| pcre_free(study_ptr); |
| #endif |
| .sp |
| 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 \fBpcre_exec()\fP, you should set one or both of |
| the following options in addition to, or instead of, PCRE_STUDY_JIT_COMPILE |
| when you call \fBpcre_study()\fP: |
| .sp |
| PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE |
| PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE |
| .sp |
| The JIT compiler generates different optimized code for each of the three |
| modes (normal, soft partial, hard partial). When \fBpcre_exec()\fP is called, |
| the appropriate code is run if it is available. Otherwise, the pattern is |
| matched using interpretive code. |
| .P |
| In some circumstances you may need to call additional functions. These are |
| described in the section entitled |
| .\" HTML <a href="#stackcontrol"> |
| .\" </a> |
| "Controlling the JIT stack" |
| .\" |
| below. |
| .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 \fBpcre_exec()\fP is passed a \fBpcre_extra\fP |
| 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 |
| There are some \fBpcre_exec()\fP 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 |
| .\" HTML <a href="#stackcontrol"> |
| .\" </a> |
| "Controlling the JIT stack" |
| .\" |
| below, even if you do not need to supply a non-default JIT stack. Such a |
| callback function is called whenever JIT code is about to be obeyed. If the |
| execution options are not right for JIT execution, the callback function is not |
| obeyed. |
| .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 |
| \fBpcre_fullinfo()\fP 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 |
| 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. |
| . |
| . |
| .SH "UNSUPPORTED OPTIONS AND PATTERN ITEMS" |
| .rs |
| .sp |
| The only \fBpcre_exec()\fP 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 |
| The only unsupported pattern items are \eC (match a single data unit) when |
| running in a UTF mode, and a callout immediately before an assertion condition |
| in a conditional group. |
| . |
| . |
| .SH "RETURN VALUES FROM JIT EXECUTION" |
| .rs |
| .sp |
| When a pattern is matched using JIT execution, the return values are the same |
| as those given by the interpretive \fBpcre_exec()\fP 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 |
| .\" HTML <a href="#stackcontrol"> |
| .\" </a> |
| "Controlling the JIT stack" |
| .\" |
| below for a discussion of JIT stack usage. For compatibility with the |
| interpretive \fBpcre_exec()\fP code, no more than two-thirds of the |
| \fIovector\fP argument is used for passing back captured substrings. |
| .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. |
| . |
| . |
| .SH "SAVING AND RESTORING COMPILED PATTERNS" |
| .rs |
| .sp |
| 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 |
| .\" HREF |
| \fBpcreprecompile\fP |
| .\" |
| documentation. It should be possible to run \fBpcre_study()\fP 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. |
| . |
| . |
| .\" HTML <a name="stackcontrol"></a> |
| .SH "CONTROLLING THE JIT STACK" |
| .rs |
| .sp |
| 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 |
| .\" HTML <a href="#stackcontrol"> |
| .\" </a> |
| "JIT stack FAQ" |
| .\" |
| below. |
| .P |
| The \fBpcre_jit_stack_alloc()\fP 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 \fBpcre_jit_stack\fP, or NULL if there is an error. The |
| \fBpcre_jit_stack_free()\fP 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 |
| 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 |
| The \fBpcre_assign_jit_stack()\fP function specifies which stack JIT code |
| should use. Its arguments are as follows: |
| .sp |
| pcre_extra *extra |
| pcre_jit_callback callback |
| void *data |
| .sp |
| The \fIextra\fP 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: |
| .sp |
| (1) If \fIcallback\fP is NULL and \fIdata\fP is NULL, an internal 32K block |
| on the machine stack is used. |
| .sp |
| (2) If \fIcallback\fP is NULL and \fIdata\fP is not NULL, \fIdata\fP must be |
| a valid JIT stack, the result of calling \fBpcre_jit_stack_alloc()\fP. |
| .sp |
| (3) If \fIcallback\fP is not NULL, it must point to a function that is |
| called with \fIdata\fP 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 |
| \fBpcre_jit_stack_alloc()\fP. |
| .sp |
| A callback function is obeyed whenever JIT code is about to be run; it is not |
| obeyed when \fBpcre_exec()\fP 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 |
| 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 |
| 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 |
| This is a suggestion for how a multithreaded program that needs to set up |
| non-default JIT stacks might operate: |
| .sp |
| During thread initalization |
| thread_local_var = pcre_jit_stack_alloc(...) |
| .sp |
| During thread exit |
| pcre_jit_stack_free(thread_local_var) |
| .sp |
| Use a one-line callback function |
| return thread_local_var |
| .sp |
| All the functions described in this section do nothing if JIT is not available, |
| and \fBpcre_assign_jit_stack()\fP does nothing unless the \fBextra\fP argument |
| is non-NULL and points to a \fBpcre_extra\fP block that is the result of a |
| successful study with PCRE_STUDY_JIT_COMPILE etc. |
| . |
| . |
| .\" HTML <a name="stackfaq"></a> |
| .SH "JIT STACK FAQ" |
| .rs |
| .sp |
| (1) Why do we need JIT stacks? |
| .sp |
| 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 |
| (2) Why don't we simply allocate blocks of memory with \fBmalloc()\fP? |
| .sp |
| 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 |
| (3) Who "owns" a JIT stack? |
| .sp |
| 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 |
| \fBpcre_exec()\fP, (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 |
| (4) When should a JIT stack be freed? |
| .sp |
| You can free a JIT stack at any time, as long as it will not be used by |
| \fBpcre_exec()\fP 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 \fIdo not\fP call |
| \fBpcre_exec()\fP with a pattern pointing to an already freed stack, as that |
| will cause SEGFAULT. (Also, do not free a stack currently used by |
| \fBpcre_exec()\fP 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 |
| (5) Should I allocate/free a stack every time before/after calling |
| \fBpcre_exec()\fP? |
| .sp |
| 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 |
| (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? |
| .sp |
| 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 |
| (7) This is too much of a headache. Isn't there any better solution for JIT |
| stack handling? |
| .sp |
| No, thanks to Windows. If POSIX threads were used everywhere, we could throw |
| out this complicated API. |
| . |
| . |
| .SH "EXAMPLE CODE" |
| .rs |
| .sp |
| This is a single-threaded example that specifies a JIT stack without using a |
| callback. |
| .sp |
| int rc; |
| int ovector[30]; |
| pcre *re; |
| pcre_extra *extra; |
| pcre_jit_stack *jit_stack; |
| .sp |
| 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); |
| .sp |
| . |
| . |
| .SH "JIT FAST PATH API" |
| .rs |
| .sp |
| 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 \fBpcre_exec()\fP 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 |
| \fBpcre_exec()\fP (obviously only for patterns that have been successfully |
| studied by JIT). |
| .P |
| The fast path function is called \fBpcre_jit_exec()\fP, and it takes exactly |
| the same arguments as \fBpcre_exec()\fP, 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 \fBpcre_exec()\fP. |
| .P |
| When you call \fBpcre_exec()\fP, 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 |
| Bypassing the sanity checks and the \fBpcre_exec()\fP wrapping can give |
| speedups of more than 10%. |
| . |
| . |
| .SH "SEE ALSO" |
| .rs |
| .sp |
| \fBpcreapi\fP(3) |
| . |
| . |
| .SH AUTHOR |
| .rs |
| .sp |
| .nf |
| Philip Hazel (FAQ by Zoltan Herczeg) |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| .fi |
| . |
| . |
| .SH REVISION |
| .rs |
| .sp |
| .nf |
| Last updated: 17 March 2013 |
| Copyright (c) 1997-2013 University of Cambridge. |
| .fi |