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           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
           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
           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_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
           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.
             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
           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
             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%.





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