PCRE2JIT(3) Introduction to Library Functions PCRE2JIT(3)

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 JIT 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 (v7, and Thumb2)
ARM 64-bit
IBM s390x 64 bit
Intel x86 32-bit and 64-bit
LoongArch 64 bit
MIPS 32-bit and 64-bit
Power PC 32-bit and 64-bit
RISC-V 32-bit and 64-bit

If --enable-jit is set on an unsupported platform, compilation fails.

A client program can tell if JIT support has been compiled by calling
pcre2_config() with the PCRE2_CONFIG_JIT option. The result is one if
PCRE2 was built with JIT support, and zero otherwise. However, having
the JIT code available does not guarantee that it will be used for
any particular match. One reason for this is that there are a number
of options and pattern items that are not supported by JIT (see
below). Another reason is that in some environments JIT is unable to
get executable memory in which to build its compiled code. The only
guarantee from pcre2_config() is that if it returns zero, JIT will
definitely not be used.

As of release 10.45 there is a more informative way to test for JIT
support. If pcre2_compile_jit() is called with the single option
PCRE2_JIT_TEST_ALLOC it returns zero if JIT is available and has a
working allocator. Otherwise it returns PCRE2_ERROR_NOMEMORY if JIT
is available but cannot allocate executable memory, or
PCRE2_ERROR_JIT_UNSUPPORTED if JIT support is not compiled. The code
argument is ignored, so it can be a NULL value.

A simple program does not need to check availability in order to use
JIT when possible. The API is implemented in a way that falls back to
the interpretive code if JIT is not available or cannot be used for a
given match. For programs that need the best possible performance,
there is 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 support has been compiled.

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 compilation was successful for a
compiled 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. Successful JIT compilation does not, however, guarantee the
use of JIT at match time because there are some match time options
that are not supported by JIT.

MATCHING SUBJECTS CONTAINING INVALID UTF

When a pattern is compiled with the PCRE2_UTF option, subject strings
are normally expected to be a valid sequence of UTF code units. By
default, this is checked at the start of matching and an error is
generated if invalid UTF is detected. The PCRE2_NO_UTF_CHECK option
can be passed to pcre2_match() to skip the check (for improved
performance) if you are sure that a subject string is valid. If this
option is used with an invalid string, the result is undefined. The
calling program may crash or loop or otherwise misbehave.

However, a way of running matches on strings that may contain invalid
UTF sequences is available. Calling pcre2_compile() with the
PCRE2_MATCH_INVALID_UTF option has two effects: it tells the
interpreter in pcre2_match() to support invalid UTF, and, if
pcre2_jit_compile() is subsequently called, the compiled JIT code
also supports invalid UTF. Details of how this support works, in
both the JIT and the interpretive cases, is given in the pcre2unicode
documentation.

There is also an obsolete option for pcre2_jit_compile() called
PCRE2_JIT_INVALID_UTF, which currently exists only for backward
compatibility. It is superseded by the pcre2_compile() option
PCRE2_MATCH_INVALID_UTF and should no longer be used. It may be
removed in future.

UNSUPPORTED OPTIONS AND PATTERN ITEMS

The pcre2_match() options that are supported for JIT matching are
PCRE2_COPY_MATCHED_SUBJECT, PCRE2_NOTBOL, PCRE2_NOTEOL,
PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART, PCRE2_NO_UTF_CHECK,
PCRE2_PARTIAL_HARD, and PCRE2_PARTIAL_SOFT. The PCRE2_ANCHORED and
PCRE2_ENDANCHORED options are 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, 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 32KiB 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. If its argument is NULL, this function returns
immediately, without doing anything. (For the technically minded: the
address space is allocated by mmap or VirtualAlloc.) A maximum stack
size of 512KiB to 1MiB 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. If this argument is NULL, the
function returns immediately, without doing anything. There are three
cases for the values of the other two options:

(1) If callback is NULL and data is NULL, an internal 32KiB 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 32KiB 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 initialization
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 1MiB address space, and use only a single memory
page (usually 4KiB) if that is enough. However, we can still grow up
to 1MiB 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 1MiB? Is
that 1MiB kept until the stack is freed?

Especially on embedded systems, 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(). However, the subject
string must be specified with a length; PCRE2_ZERO_TERMINATED is not
supported. Unsupported option bits (for example, PCRE2_ANCHORED and
PCRE2_ENDANCHORED) are ignored, as is the PCRE2_NO_JIT option. The
return values are also the same as for pcre2_match(), plus
PCRE2_ERROR_JIT_BADOPTION if a matching mode (partial or complete) is
requested that was not compiled.

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 but the length is non-zero,
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. If invalid
UTF data is passed when PCRE2_MATCH_INVALID_UTF was not set for
pcre2_compile(), the result is undefined. The program may crash or
loop or give wrong results. In the absence of PCRE2_MATCH_INVALID_UTF
you should call pcre2_jit_match() in UTF mode only if you are sure
the subject is valid.

Bypassing the sanity checks and the pcre2_match() wrapping can give
speedups of more than 10%.

SEE ALSO

pcre2api(3), pcre2unicode(3)

AUTHOR

Philip Hazel (FAQ by Zoltan Herczeg)
Retired from University Computing Service
Cambridge, England.

REVISION

Last updated: 22 August 2024
Copyright (c) 1997-2024 University of Cambridge.

PCRE2 10.45 22 August 2024 PCRE2JIT(3)