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 processing 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 positions 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 can tell if JIT support is available by calling pcre_config() with the PCRE_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 ordinary PCRE code if JIT is not available.

SIMPLE USE OF JIT

You have to do two things to make use of the JIT support in the simplest 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.

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 execution. 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 generated. 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
  (?(<name>)... conditional test on setting of a named subpattern
  (?(R)... conditional test on whole pattern recursion
  (?(Rn)... conditional test on recursion, by number
  (?(R&name)... conditional test on recursion, by name
  (*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 "Controlling the JIT stack" below for a discussion of JIT stack usage. For compatibility with the interpretive pcre_exec() code, no more than two-thirds of the ovector argument is used for passing back captured substrings.

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-specific, 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 patterns 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.

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 multithread 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 inefficient 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.

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
University Computing Service
Cambridge CB2 3QH, England.

REVISION

Last updated: 19 October 2011
Copyright (c) 1997-2011 University of Cambridge.