pcre2jit


       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.

AVAILABILITY OF JIT SUPPORT

       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
         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.

SIMPLE USE OF JIT

       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-
       PLETE, PCRE2_JIT_PARTIAL_HARD, or PCRE2_JIT_PARTIAL_SOFT.

       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()
       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"
       below.

       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
       option.  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.

MATCHING SUBJECTS CONTAINING INVALID UTF

       When a pattern is compiled with the PCRE2_UTF option, the  interpretive
       matching  function expects its subject string to be a valid sequence of
       UTF code units.  If it is not, the result is undefined.  This  is  also
       true   by   default  of  matching  via  JIT.  However,  if  the  option
       PCRE2_JIT_INVALID_UTF is passed to pcre2_jit_compile(), code  that  can
       process a subject containing invalid UTF is compiled.

       In  this  mode, an invalid code unit sequence never matches any pattern
       item. It does not match dot, it does not match  \p{Any},  it  does  not
       even match negative items such as [^X]. A lookbehind assertion fails if
       it encounters an invalid sequence while moving the current point  back-
       PCRE2_COPY_MATCHED_SUBJECT, PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY,
       PCRE2_NOTEMPTY_ATSTART,  PCRE2_NO_UTF_CHECK,  PCRE2_PARTIAL_HARD,   and
       PCRE2_PARTIAL_SOFT.  The  PCRE2_ANCHORED  and PCRE2_ENDANCHORED options
       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.

RETURN VALUES FROM JIT MATCHING

       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.

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 32KiB on the machine stack.  However,  some
       large   or   complicated  patterns  need  more  than  this.  The  error
       PCRE2_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  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

             pcre2_jit_stack_create().

         (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
             pcre2_jit_stack_create().

       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
       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
       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,
       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 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
       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 = pcre2_jit_stack_create(...)

         During thread exit
           pcre2_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.

JIT STACK FAQ

       (1) Why do we need JIT stacks?
       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 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
       pcre2_match()?

       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.

FREEING JIT SPECULATIVE MEMORY

       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 */

         pcre2_code_free(re);
         pcre2_match_data_free(match_data);
         pcre2_match_context_free(mcontext);
         pcre2_jit_stack_free(jit_stack);

JIT FAST PATH API

       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
       exactly  the  same  arguments  as  pcre2_match().  However, the subject
       string must be specified with a length;  PCRE2_ZERO_TERMINATED  is  not
       supported.   Unsupported  option  bits  (for  example,  PCRE2_ANCHORED,
       PCRE2_ENDANCHORED 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, an immediate 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%.

       Last updated: 06 March 2019
       Copyright (c) 1997-2019 University of Cambridge.

PCRE2 10.33                      06 March 2019                     PCRE2JIT(3)
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