Go to main content

man pages section 3: Library Interfaces and Headers

Exit Print View

Updated: Thursday, June 13, 2019
 
 

pcre2jit (3)

Name

pcre2jit - compatible regular expressions (revised API)

Synopsis

Please see following description for synopsis

Description

PCRE2JIT(3)                Library Functions Manual                PCRE2JIT(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

PCRE2 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, 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.

       JIT support is not available on Solaris.

       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()
       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
       optimizations 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"
       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.

UNSUPPORTED OPTIONS AND PATTERN ITEMS

       The pcre2_match() options that  are  supported  for  JIT  matching  are
       PCRE2_NOTBOL,   PCRE2_NOTEOL,  PCRE2_NOTEMPTY,  PCRE2_NOTEMPTY_ATSTART,
       PCRE2_NO_UTF_CHECK,  PCRE2_PARTIAL_HARD,  and  PCRE2_PARTIAL_SOFT.  The
       PCRE2_ANCHORED option is 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

       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
             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?

       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
       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 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

       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-
       ment.

EXAMPLE CODE

       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(). The return values are also
       the same, plus PCRE2_ERROR_JIT_BADOPTION if a matching mode (partial or
       complete) is requested that was not compiled. Unsupported  option  bits
       (for  example,  PCRE2_ANCHORED)  are  ignored,  as  is the PCRE2_NO_JIT
       option.

       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%.


ATTRIBUTES
       See attributes(7) for descriptions of the following attributes:


       +---------------+------------------+
       |ATTRIBUTE TYPE | ATTRIBUTE VALUE  |
       +---------------+------------------+
       |Availability   | library/pcre2    |
       +---------------+------------------+
       |Stability      | Uncommitted      |
       +---------------+------------------+
SEE ALSO

       pcre2api(3)

AUTHOR

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

REVISION

       Last updated: 28 June 2018
       Copyright (c) 1997-2018 University of Cambridge.



NOTES
       This    software    was    built    from    source     available     at
       https://github.com/oracle/solaris-userland.    The  original  community
       source was downloaded  from   ftp://ftp.csx.cam.ac.uk/pub/software/pro-
       gramming/pcre/pcre2-10.32.tar.gz

       Further information about this software can be found on the open source
       community website at http://pcre.org/.



PCRE2 10.32                      28 June 2018                      PCRE2JIT(3)