PCRE — Perl-compatible regular expressions
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.
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 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.
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.
You have to do two things to make use of the JIT support in the simplest way:
(1) Callpcre_study
() with the PCRE_STUDY_JIT_COMPILE option for each compiled pattern, and pass the resultingpcre_extra
block topcre_exec
().
(2) Usepcre_free_study
() to free thepcre_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 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.
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.
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.
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(3)
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.
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) Ifcallback
is NULL anddata
is NULL, an internal 32K block on the machine stack is used.
(2) Ifcallback
is NULL anddata
is not NULL,data
must be a valid JIT stack, the result of callingpcre_jit_stack_alloc
().
(3) Ifcallback
not NULL, it must point to a function that is called withdata
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 callingpcre_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.
(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 difficult. 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 memory 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 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 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 memory 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 memory (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.
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);
Philip Hazel (FAQ by Zoltan Herczeg) University Computing Service Cambridge CB2 3QH, England.
Last updated: 26 November 2011 Copyright (c) 1997-2011 University of Cambridge.
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This manual page is taken from the PCRE library, which is distributed under the BSD license. |