pcrejit


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  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
       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, substi-
       tute  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

       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 API is implemented in a way that falls  back  to  the  interpretive
       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  sim-

         #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

       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():

         PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
         PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE

       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"
       below.

       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
       PCRE_NO_UTF8_CHECK,  PCRE_NO_UTF16_CHECK,   PCRE_NOTBOL,   PCRE_NOTEOL,
       PCRE_NOTEMPTY,  PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PAR-
       TIAL_SOFT.

       The unsupported pattern items are:

         \C             match a single byte; not supported in UTF-8 mode
         (?Cn)          callouts
         (*PRUNE)       )
         (*SKIP)        ) backtracking control verbs
         (*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 "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-
       strings.

       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-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.

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.

       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
             pcre_jit_stack_alloc().

       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
       interpreter.

       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
           pcre_jit_stack_free(thread_local_var)

       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
       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 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
       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 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
       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: 04 May 2012
       Copyright (c) 1997-2012 University of Cambridge.



PCRE 8.31                         04 May 2012                       PCREJIT(3)
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