PCRE2JIT(3)                Library Functions Manual                PCRE2JIT(3)

       PCRE2 - Perl-compatible regular expressions (revised API)


       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, so it is of most benefit when
       the same pattern is going to be matched many times. This does not  nec-
       essarily  mean many calls of a matching function; if the pattern is not
       anchored, matching attempts may take place many times at various  posi-
       tions in the subject, even for a single call. Therefore, if the subject
       string is very long, it may still pay  to  use  JIT  even  for  one-off
       matches.  JIT  support  is  available  for all of the 8-bit, 16-bit and
       32-bit PCRE2 libraries.

       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 an optional feature of  PCRE2.  The  "configure"  option
       --enable-jit  (or  equivalent  CMake  option) must be set when PCRE2 is
       built if you want to use JIT. The support is limited to  the  following
       hardware platforms:

         ARM 32-bit (v5, v7, and Thumb2)
         ARM 64-bit
         IBM s390x 64 bit
         Intel x86 32-bit and 64-bit
         MIPS 32-bit and 64-bit
         Power PC 32-bit and 64-bit
         SPARC 32-bit

       If --enable-jit is set on an unsupported platform, compilation fails.

       A  program  can  tell if JIT support is available by calling pcre2_con-
       fig() with the PCRE2_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 interpretive code if JIT is not available. For pro-
       grams that need the best possible performance, there is  also  a  "fast
       path" API that is JIT-specific.


       To  make use of the JIT support in the simplest way, all you have to do
       is to call pcre2_jit_compile() after successfully compiling  a  pattern
       with pcre2_compile(). This function has two arguments: the first is the
       compiled pattern pointer that was returned by pcre2_compile(), and  the
       second  is  zero  or  more of the following option bits: PCRE2_JIT_COM-

       If JIT support is not available, a  call  to  pcre2_jit_compile()  does
       nothing  and returns PCRE2_ERROR_JIT_BADOPTION. 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, but yields
       exactly the same results. The returned value  from  pcre2_jit_compile()
       is zero on success, or a negative error code.

       There  is  a limit to the size of pattern that JIT supports, imposed by
       the size of machine stack that it uses. The exact rules are  not  docu-
       mented because they may change at any time, in particular, when new op-
       timizations are introduced.  If  a  pattern  is  too  big,  a  call  to
       pcre2_jit_compile() returns PCRE2_ERROR_NOMEMORY.

       PCRE2_JIT_COMPLETE  requests the JIT compiler to generate code for com-
       plete matches. If you want to run partial matches using the  PCRE2_PAR-
       TIAL_HARD  or  PCRE2_PARTIAL_SOFT  options of pcre2_match(), you should
       set one or both of  the  other  options  as  well  as,  or  instead  of
       PCRE2_JIT_COMPLETE. The JIT compiler generates different optimized code
       for each of the three modes (normal, soft partial, hard partial).  When
       pcre2_match()  is  called,  the appropriate code is run if it is avail-
       able. Otherwise, the pattern is matched using interpretive code.

       You can call pcre2_jit_compile() multiple times for the  same  compiled
       pattern.  It does nothing if it has previously compiled code for any of
       the option bits. For example, you can call it once with  PCRE2_JIT_COM-
       PLETE  and  (perhaps  later,  when  you find you need partial matching)
       again with PCRE2_JIT_COMPLETE and PCRE2_JIT_PARTIAL_HARD. This time  it
       will ignore PCRE2_JIT_COMPLETE and just compile code for partial match-
       ing. If pcre2_jit_compile() is called with no option bits set, it imme-
       diately returns zero. This is an alternative way of testing whether JIT
       is available.

       At present, it is not possible to free JIT compiled  code  except  when
       the entire compiled pattern is freed by calling pcre2_code_free().

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

       There are some pcre2_match() options that are not supported by JIT, and
       there are also some pattern items that JIT cannot handle.  Details  are
       given  below.  In  both cases, matching 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 match-time 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 matching is available after compiling a
       pattern by calling pcre2_pattern_info() with the PCRE2_INFO_JITSIZE op-
       tion.  A  non-zero  result means that JIT compilation was successful. A
       result of 0 means that JIT support is not available, or the pattern was
       not  processed by pcre2_jit_compile(), or the JIT compiler was not able
       to handle the pattern.


       When a pattern is compiled with the PCRE2_UTF option,  subject  strings
       are  normally expected to be a valid sequence of UTF code units. By de-
       fault, this is checked at the start of matching and an error is  gener-
       ated  if  invalid UTF is detected. The PCRE2_NO_UTF_CHECK option can be
       passed to pcre2_match() to skip the check (for improved performance) if
       you  are  sure  that  a subject string is valid. If this option is used
       with an invalid string, the result is undefined.

       However, a way of running matches on strings that may  contain  invalid
       UTF   sequences   is   available.   Calling  pcre2_compile()  with  the
       PCRE2_MATCH_INVALID_UTF option has two effects:  it  tells  the  inter-
       preter  in pcre2_match() to support invalid UTF, and, if pcre2_jit_com-
       pile() is called, the compiled JIT code also supports invalid UTF.  De-
       tails  of  how this support works, in both the JIT and the interpretive
       cases, is given in the pcre2unicode documentation.

       There  is  also  an  obsolete  option  for  pcre2_jit_compile()  called
       PCRE2_JIT_INVALID_UTF, which currently exists only for backward compat-
       ibility.    It   is   superseded   by   the   pcre2_compile()    option
       PCRE2_MATCH_INVALID_UTF and should no longer be used. It may be removed
       in future.


       The pcre2_match() options that  are  supported  for  JIT  matching  are
       are not supported at match time.

       If the PCRE2_NO_JIT option is passed to pcre2_match() it  disables  the
       use of JIT, forcing matching by the interpreter code.

       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 matching, the return values are the
       same as those given by the interpretive pcre2_match()  code,  with  the
       addition  of one new error code: PCRE2_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.

       The  error  code  PCRE2_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 PCRE2_ERROR_DEPTHLIMIT error code
       is never returned when JIT matching is used.


       When the compiled JIT code runs, it needs a block of memory to use as a
       stack.  By default, it uses 32KiB on the machine stack.  However,  some
       large  or complicated patterns need more than this. The error PCRE2_ER-
       ROR_JIT_STACKLIMIT is given when there is not enough stack. Three func-
       tions 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  pcre2_jit_stack_create()  function  creates a JIT stack. Its argu-
       ments are a starting size, a maximum size, and a general  context  (for
       memory  allocation  functions, or NULL for standard memory allocation).
       It returns a pointer to an opaque structure of type pcre2_jit_stack, or
       NULL  if there is an error. The pcre2_jit_stack_free() function is used
       to free a stack that is no longer needed. If its argument is NULL, this
       function  returns immediately, without doing anything. (For the techni-
       cally minded: the address space is allocated by mmap or  VirtualAlloc.)
       A  maximum  stack size of 512KiB to 1MiB should be more than enough for
       any pattern.

       The pcre2_jit_stack_assign() function specifies which  stack  JIT  code
       should use. Its arguments are as follows:

         pcre2_match_context  *mcontext
         pcre2_jit_callback    callback
         void                 *data

       The first argument is a pointer to a match context. When this is subse-
       quently passed to a matching function, its information determines which
       JIT stack is used. If this argument is NULL, the function returns imme-
       diately, without doing anything. There are three cases for  the  values
       of the other two options:

         (1) If callback is NULL and data is NULL, an internal 32KiB block
             on the machine stack is used. This is the default when a match
             context is created.

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

         (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 32KiB 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 pcre2_match() is called with options that are incom-
       patible  for JIT matching. A callback function can therefore be used to
       determine whether a match operation was executed by JIT or by  the  in-

       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
       matched sequentially in the same thread. Currently, the only way to set
       up non-sequential matches in one thread is to use callouts: if a  call-
       out  function starts another match, that match must use a different JIT
       stack to the one used for currently suspended match(es).

       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, be-
       cause 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 a match context that is used by any number of patterns,
       as long as they are not used for matching by multiple  threads  at  the
       same  time.  For  example, you could use the same stack in all compiled
       patterns, with a global mutex in the callback to wait until  the  stack
       is available for use. However, this is an inefficient solution, and not

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

         During thread initialization
           thread_local_var = pcre2_jit_stack_create(...)

         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


       (1) Why do we need JIT stacks?

       PCRE2 (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 ad-
       dress 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  allo-
       cate  1MiB  address  space,  and use only a single memory page (usually
       4KiB) if that is enough. However, we can still grow up to 1MiB  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 being
       used by pcre2_match(), (that is, it is assigned to a match context that
       is passed 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
       pcre2_match() again. When you assign the stack to a match context, only
       a  pointer  is  set. There is no reference counting or any other magic.
       You can free compiled patterns, contexts, and stacks in any order, any-
       time.   Just do not call pcre2_match() with a match context pointing to
       an already freed stack, as that will cause SEGFAULT. (Also, do not free
       a  stack  currently  used  by pcre2_match() in another thread). You can
       also replace the stack in a context at any time when it is not in  use.
       You should 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 patterns.

       (6)  OK, the stack is for long term memory allocation. But what happens
       if a pattern causes stack overflow with a stack of 1MiB? Is  that  1MiB
       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.


       void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);

       The JIT executable allocator does not free all memory when it is possi-
       ble. It expects new allocations, and keeps some free memory  around  to
       improve  allocation  speed. However, in low memory conditions, it might
       be better to free all possible memory. You can cause this to happen  by
       calling  pcre2_jit_free_unused_memory(). Its argument is a general con-
       text, for custom memory management, or NULL for standard memory manage-


       This  is  a  single-threaded example that specifies a JIT stack without
       using a callback. A real program should include  error  checking  after
       all the function calls.

         int rc;
         pcre2_code *re;
         pcre2_match_data *match_data;
         pcre2_match_context *mcontext;
         pcre2_jit_stack *jit_stack;

         re = pcre2_compile(pattern, PCRE2_ZERO_TERMINATED, 0,
           &errornumber, &erroffset, NULL);
         rc = pcre2_jit_compile(re, PCRE2_JIT_COMPLETE);
         mcontext = pcre2_match_context_create(NULL);
         jit_stack = pcre2_jit_stack_create(32*1024, 512*1024, NULL);
         pcre2_jit_stack_assign(mcontext, NULL, jit_stack);
         match_data = pcre2_match_data_create(re, 10);
         rc = pcre2_match(re, subject, length, 0, 0, match_data, mcontext);
         /* Process result */



       Because the API described above falls back to interpreted matching when
       JIT is not available, it is convenient for programs  that  are  written
       for  general  use  in  many  environments.  However,  calling  JIT  via
       pcre2_match() 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
       matching  directly instead of calling pcre2_match() (obviously only for
       patterns that have been successfully processed by pcre2_jit_compile()).

       The fast path function is called pcre2_jit_match(), and  it  takes  ex-
       actly  the same arguments as pcre2_match(). However, the subject string
       must be specified with a  length;  PCRE2_ZERO_TERMINATED  is  not  sup-
       ported. Unsupported option bits (for example, PCRE2_ANCHORED, PCRE2_EN-
       DANCHORED  and  PCRE2_COPY_MATCHED_SUBJECT)  are  ignored,  as  is  the
       PCRE2_NO_JIT  option.  The  return  values  are  also  the  same as for
       pcre2_match(), plus PCRE2_ERROR_JIT_BADOPTION if a matching mode  (par-
       tial or complete) is requested that was not compiled.

       When  you call pcre2_match(), 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 but the length is non-zero, an im-
       mediate error is given. Also, unless PCRE2_NO_UTF_CHECK 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 pcre2_match() wrapping can give
       speedups of more than 10%.




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


       Last updated: 30 November 2021
       Copyright (c) 1997-2021 University of Cambridge.

PCRE2 10.40                    30 November 2021                    PCRE2JIT(3)
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