| .TH PCREJIT 3 |
| .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 |
| \fPpcre_exec()\fP; if the pattern is not anchored, matching attempts may take |
| place many times at various positions in the subject, even for a single call to |
| \fBpcre_exec()\fP. 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 matching function, |
| \fBpcre_exec()\fP. It does not apply when \fBpcre_dfa_exec()\fP is being used. |
| The code for this support was written by Zoltan Herczeg. |
| . |
| . |
| .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 (experimental) |
| .sp |
| The Power PC support is designated as experimental because it has not been |
| fully tested. 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 API is implemented in a |
| way that falls back to the ordinary PCRE code if JIT is not available. |
| .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 |
| 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 is ignored, and no JIT |
| data is set up. 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, it obeys that instead of the normal code. The |
| result is identical, but the 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. |
| .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, 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_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and |
| PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not |
| supported. |
| .P |
| The unsupported pattern items are: |
| .sp |
| \eC match a single byte; not supported in UTF-8 mode |
| (?Cn) callouts |
| (*COMMIT) ) |
| (*MARK) ) |
| (*PRUNE) ) the backtracking control verbs |
| (*SKIP) ) |
| (*THEN) ) |
| .sp |
| Support for some of these may be added in future. |
| . |
| . |
| .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. 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 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 result 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 |
| You may safely assign the same JIT stack to more than one pattern, as long as |
| they are all matched sequentially in the same thread. In a multithread |
| application, each thread must use its own JIT stack. |
| .P |
| Strictly speaking, even more is allowed. You can assign the same 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 typical multithreaded program 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. |
| . |
| . |
| .\" 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 achive 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 "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: 26 November 2011 |
| Copyright (c) 1997-2011 University of Cambridge. |
| .fi |