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PCREJIT(3)							    PCREJIT(3)

       PCRE - Perl-compatible regular expressions


       Just-in-time  compiling	is a heavyweight optimization that can greatly
       speed up pattern matching. However, it comes at the cost of extra  pro‐
       cessing 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

       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.


       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, sub‐
       stitute the  16-bit  functions  and  16-bit  structures	(for  example,
       pcre16_jit_stack	 instead  of  pcre_jit_stack).	If  you	 are using the
       32-bit library, substitute the 32-bit functions and  32-bit  structures
       (for example, pcre32_jit_stack instead of pcre_jit_stack).


       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
	 SPARC 32-bit (experimental)

       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  sup‐
       port  is	 available  by	calling pcre_config() with the PCRE_CONFIG_JIT
       option. The result is 1 when JIT is available, and  0  otherwise.  How‐
       ever, 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 interpre‐
       tive code if JIT is not available. For programs that need the best pos‐
       sible performance, there is also a "fast path"  API  that  is  JIT-spe‐

       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.


       You have to do two things to make use of the JIT support	 in  the  sim‐
       plest way:

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

	 (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


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


       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  pcre_exec(),  you
       should  set  one	 or  both  of the following options in addition to, or
       instead of, PCRE_STUDY_JIT_COMPILE when you call pcre_study():


       The JIT compiler generates different optimized code  for	 each  of  the
       three  modes  (normal, soft partial, hard partial). When pcre_exec() is
       called, the appropriate code is run if it is available. Otherwise,  the
       pattern is matched using interpretive code.

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

       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 exe‐
       cutes much faster than the normal interpretive code.  When  pcre_exec()
       is  passed  a  pcre_extra 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.

       There are some pcre_exec() options that are not supported for JIT  exe‐
       cution.	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
       "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.

       If  the	JIT  compiler finds an unsupported item, no JIT data is gener‐
       ated. 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 etc., 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.


       The only pcre_exec() options that are supported for JIT	execution  are

       The  only  unsupported  pattern items are \C (match a single data unit)
       when running in a UTF mode, and a callout immediately before an	asser‐
       tion condition in a conditional group.


       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  "Control‐
       ling the JIT stack" below for a discussion of JIT stack usage. For com‐
       patibility with the interpretive pcre_exec() code, no  more  than  two-
       thirds  of  the ovector argument is used for passing back captured sub‐

       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.


       The code that is generated by the  JIT  compiler	 is  architecture-spe‐
       cific,  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 pat‐
       terns 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.


       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

       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 etc. 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 is 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 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

       A  callback function is obeyed whenever JIT code is about to be run; it
       is not obeyed when pcre_exec() is called with options that  are	incom‐
       patible for JIT execution. A callback function can therefore be used to
       determine whether a match operation was	executed  by  JIT  or  by  the

       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.

       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.

       This  is a suggestion for how a multithreaded program that needs to set
       up non-default JIT stacks might operate:

	 During thread initalization
	   thread_local_var = pcre_jit_stack_alloc(...)

	 During thread exit

	 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 etc.


       (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 diffi‐
       cult. 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 mem‐
       ory 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

       (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  run‐
       ning), that stack must not be used by any other threads (to avoid over‐
       writing the same memory area). The best practice for multithreaded pro‐
       grams  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

       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.

       (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  mem‐
       ory  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 mem‐
       ory (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.


       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 */


       Because	the  API  described  above falls back to interpreted execution
       when JIT is not available, it is convenient for programs that are writ‐
       ten  for	 general  use  in  many environments. However, calling JIT via
       pcre_exec() 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 pcre_exec() (obviously only for
       patterns that have been successfully studied by JIT).

       The fast path function is called pcre_jit_exec(), and it takes  exactly
       the  same  arguments  as pcre_exec(), 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 pcre_exec().

       When  you  call	pcre_exec(), as well as testing for invalid options, a
       number of other sanity checks are performed on the arguments. For exam‐
       ple,  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.

       Bypassing  the  sanity  checks  and  the	 pcre_exec() wrapping can give
       speedups of more than 10%.




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


       Last updated: 17 March 2013
       Copyright (c) 1997-2013 University of Cambridge.

PCRE 8.33			 17 March 2013			    PCREJIT(3)

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