Provided by: libpcre2-dev_10.31-2_amd64 
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 processing 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 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 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.
A program can tell if JIT support is available by calling pcre2_config() 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 programs 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_COMPLETE, 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 documented 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 complete matches. If you want to run
partial matches using the PCRE2_PARTIAL_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 available. 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_COMPLETE 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 matching. If pcre2_jit_compile() is called with no option bits set, it
immediately 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 generated. 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 assertion 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 "Controlling 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 32K
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 arguments 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. (For the technically
minded: the address space is allocated by mmap or VirtualAlloc.) A maximum stack size of 512K to 1M
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 subsequently passed to a matching
function, its information determines which JIT stack is used. 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. 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 32K 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 incompatible 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 callout 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 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 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, anytime. 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 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.
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 possible. 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 context, for custom memory management, or NULL
for standard memory management.
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 example, 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%.
SEE ALSO
pcre2api(3)
AUTHOR
Philip Hazel (FAQ by Zoltan Herczeg)
University Computing Service
Cambridge, England.
REVISION
Last updated: 31 March 2017
Copyright (c) 1997-2017 University of Cambridge.