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 a matching function; if the pattern is
not anchored, matching attempts may take place many times at various
positions in the subject, even for a single call. Therefore, 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 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.
8-BIT and 16-BIT SUPPORT
JIT support is available for both the 8-bit and 16-bit PCRE libraries.
To keep this documentation simple, only the 8-bit interface is
described in what follows. If you are using the 16-bit library, substi‐
tute the 16-bit functions and 16-bit structures (for example,
pcre16_jit_stack instead of pcre_jit_stack).
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
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 interpretive
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
PCRE_STUDY_JIT_COMPILE requests the JIT compiler to generate code for
complete matches. If you want to run partial matches using the
PCRE_PARTIAL_HARD or PCRE_PARTIAL_SOFT options of pcre_exec(), you
should set one or both of the following options in addition to, or
instead of, PCRE_STUDY_JIT_COMPILE when you call pcre_study():
PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
The JIT compiler generates different optimized code for each of the
three modes (normal, soft partial, hard partial). When pcre_exec() is
called, the appropriate code is run if it is available. Otherwise, the
pattern is matched using interpretive code.
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 etc. are
ignored, and no JIT data is created. Otherwise, the compiled pattern is
passed to the JIT compiler, which turns it into machine code that exe‐
cutes much faster than the normal interpretive code. When pcre_exec()
is passed a pcre_extra block containing a pointer to JIT code of the
appropriate mode (normal or hard/soft partial), it obeys that code
instead of running the interpreter. The result is identical, but the
compiled JIT 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 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 execution 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 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 etc., 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_NO_UTF16_CHECK, PCRE_NOTBOL, PCRE_NOTEOL,
PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PAR‐
TIAL_SOFT.
The unsupported pattern items are:
\C match a single byte; not supported in UTF-8 mode
(?Cn) callouts
(*PRUNE) )
(*SKIP) ) backtracking control verbs
(*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 etc. 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 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 32K stack is used; otherwise the
return value must be a valid JIT stack, the result of calling
pcre_jit_stack_alloc().
A callback function is obeyed whenever JIT code is about to be run; it
is not obeyed when pcre_exec() is called with options that are incom‐
patible for JIT execution. 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 all
matched sequentially in the same thread. 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 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 multithreaded program that needs to set
up non-default JIT stacks 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 etc.
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 ALSOpcreapi(3)AUTHOR
Philip Hazel (FAQ by Zoltan Herczeg)
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 04 May 2012
Copyright (c) 1997-2012 University of Cambridge.
PCRE 8.31 04 May 2012 PCREJIT(3)
