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pcrejit(3)                                                          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  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, 16-BIT AND 32-BIT SUPPORT


       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).


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
         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-
       cific.

       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-
       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
             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

       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
       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.


UNSUPPORTED OPTIONS AND PATTERN ITEMS


       The  only  pcre_exec() options that are supported for JIT execution are
       PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK, PCRE_NO_UTF32_CHECK, PCRE_NOT-
       BOL,   PCRE_NOTEOL,   PCRE_NOTEMPTY,  PCRE_NOTEMPTY_ATSTART,  PCRE_PAR-
       TIAL_HARD, and PCRE_PARTIAL_SOFT.

       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.


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.

       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 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)

         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.


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 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  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.


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);



JIT FAST PATH API


       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%.


SEE ALSO


       pcreapi(3)


AUTHOR


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


REVISION


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



PCRE 8.33                        17 March 2013                      pcrejit(3)

pcre 8.33 - Generated Tue May 28 08:52:17 CDT 2013
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