PCREJIT(3)                OpenBSD Programmer's Manual               PCREJIT(3)



NAME
       PCRE - Perl-compatible regular expressions

PCRE JUST-IN-TIME COMPILER SUPPORT

       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.

       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.

8-BIT and 16-BIT SUPPORT

       JIT support is available for both the 8-bit and 16-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,
       pcre16_jit_stack instead of pcre_jit_stack).

AVAILABILITY OF JIT SUPPORT

       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:

         ARM v5, v7, and Thumb2
         Intel x86 32-bit and 64-bit
         MIPS 32-bit
         Power PC 32-bit and 64-bit

       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.

       A program that is linked with PCRE 8.20 or later can tell if JIT
       support is available by calling pcre_config() 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.

       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.

SIMPLE USE OF JIT

       You have to do two things to make use of the JIT support in the
       simplest way:

         (1) Call pcre_study() with the PCRE_STUDY_JIT_COMPILE option for
             each compiled pattern, and pass the resulting pcre_extra block to
             pcre_exec().

         (2) Use pcre_free_study() to free the pcre_extra block when it is
             no longer needed instead of just freeing it yourself. This
             ensures that any JIT data is also freed.

       For a program that may be linked with pre-8.20 versions of PCRE, you
       can insert

         #ifndef PCRE_STUDY_JIT_COMPILE
         #define PCRE_STUDY_JIT_COMPILE 0
         #endif

       so that no option is passed to pcre_study(), and then use something
       like this to free the study data:

         #ifdef PCRE_CONFIG_JIT
             pcre_free_study(study_ptr);
         #else
             pcre_free(study_ptr);
         #endif

       In some circumstances you may need to call additional functions. These
       are described in the section entitled "Controlling the JIT stack"
       below.

       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 pcre_exec() is passed a
       pcre_extra 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.

       There are some pcre_exec() 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 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 pcre_fullinfo() 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.

       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.

UNSUPPORTED OPTIONS AND PATTERN ITEMS

       The only pcre_exec() 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.

       The unsupported pattern items are:

         \C             match a single byte; not supported in UTF-8 mode
         (?Cn)          callouts
         (*COMMIT)      )
         (*MARK)        )
         (*PRUNE)       ) the backtracking control verbs
         (*SKIP)        )
         (*THEN)        )

       Support for some of these may be added in future.

RETURN VALUES FROM JIT EXECUTION

       When a pattern is matched using JIT execution, the return values are
       the same as those given by the interpretive pcre_exec() 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
       "Controlling the JIT stack" below for a discussion of JIT stack usage.
       For compatibility with the interpretive pcre_exec() code, no more than
       two-thirds of the ovector argument is used for passing back captured
       substrings.

       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.

SAVING AND RESTORING COMPILED PATTERNS

       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 pcreprecompile documentation. It should be
       possible to run pcre_study() 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.

CONTROLLING THE JIT STACK

       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 "JIT stack FAQ" below.

       The pcre_jit_stack_alloc() 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 pcre_jit_stack, or NULL if there is an error.
       The pcre_jit_stack_free() 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.)

       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.

       The pcre_assign_jit_stack() function specifies which stack JIT code
       should use. Its arguments are as follows:

         pcre_extra         *extra
         pcre_jit_callback  callback
         void               *data

       The extra 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:

         (1) If callback is NULL and data is NULL, an internal 32K block
             on the machine stack is used.

         (2) If callback is NULL and data is not NULL, data must be
             a valid JIT stack, the result of calling pcre_jit_stack_alloc().

         (3) If callback not NULL, it must point to a function that is called
             with data 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 pcre_jit_stack_alloc().

       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.

       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.

       This is a suggestion for how a typical multithreaded program might
       operate:

         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

       All the functions described in this section do nothing if JIT is not
       available, and pcre_assign_jit_stack() does nothing unless the extra
       argument is non-NULL and points to a pcre_extra block that is the
       result of a successful study with PCRE_STUDY_JIT_COMPILE.

JIT STACK FAQ

       (1) Why do we need JIT stacks?

       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.

       (2) Why don't we simply allocate blocks of memory with malloc()?

       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.

       (3) Who "owns" a JIT stack?

       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 pcre_exec(), (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.

       (4) When should a JIT stack be freed?

       You can free a JIT stack at any time, as long as it will not be used by
       pcre_exec() 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 do not
       call pcre_exec() with a pattern pointing to an already freed stack, as
       that will cause SEGFAULT. (Also, do not free a stack currently used by
       pcre_exec() 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.

       (5) Should I allocate/free a stack every time before/after calling
       pcre_exec()?

       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.

       (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?

       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.

       (7) This is too much of a headache. Isn't there any better solution for
       JIT stack handling?

       No, thanks to Windows. If POSIX threads were used everywhere, we could
       throw out this complicated API.

EXAMPLE CODE

       This is a single-threaded example that specifies a JIT stack without
       using a callback.

         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);


SEE ALSO

       pcreapi(3)

AUTHOR

       Philip Hazel (FAQ by Zoltan Herczeg)
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

       Last updated: 08 January 2012
       Copyright (c) 1997-2012 University of Cambridge.



                                                                    PCREJIT(3)
