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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  pcre_exec();	 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 to pcre_exec(). 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  matching  function,
       pcre_exec(). It does not apply when pcre_dfa_exec() is being used.  The
       code for this support was written by Zoltan Herczeg.

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 (experimental)

       The Power PC support is designated as experimental because it  has  not
       been  fully  tested. 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 API is implemented in a way that falls back to  the	ordinary  PCRE
       code if JIT is not available.

       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

       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 is ignored, and
       no JIT data is set up. 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. When pcre_exec() is passed  a
       pcre_extra  block  containing  a	 pointer  to  JIT  code, it obeys that
       instead of the normal code. The result is identical, but the 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 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, 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_NOTBOL,   PCRE_NOTEOL,	  PCRE_NOTEMPTY,   and
       PCRE_NOTEMPTY_ATSTART.  Note in particular that partial matching is not
       supported.

       The unsupported pattern items are:

	 \C		match a single byte; not supported in UTF-8 mode
	 (?Cn)		callouts
	 (*COMMIT)	)
	 (*MARK)	)
	 (*PRUNE)	) the backtracking control verbs
	 (*SKIP)	)
	 (*THEN)	)

       Support for some of these may be added in future.

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.	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 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 result 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().

       You may safely assign the same JIT stack to more than one  pattern,  as
       long as they are all matched sequentially in the same thread. In a mul‐
       tithread application, each thread must use its own JIT stack.

       Strictly speaking, even more is allowed. You can assign the same	 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 inef‐
       ficient solution, and not recommended.

       This  is	 a  suggestion	for  how a typical multithreaded program 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.

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 achive 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);

SEE ALSO

       pcreapi(3)

AUTHOR

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

REVISION

       Last updated: 26 November 2011
       Copyright (c) 1997-2011 University of Cambridge.

								    PCREJIT(3)
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