|
HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
PCRE2JIT(3) Library Functions Manual PCRE2JIT(3)
PCRE2 - Perl-compatible regular expressions (revised API)
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.
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 is available 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 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.
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.
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 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.
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.
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.
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.
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.
(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.
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.
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);
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%.
pcre2api(3), pcre2unicode(3)
Philip Hazel (FAQ by Zoltan Herczeg)
Retired from University Computing Service
Cambridge, England.
Last updated: 21 February 2024
Copyright (c) 1997-2024 University of Cambridge.
This page is part of the PCRE (Perl Compatible Regular
Expressions) project. Information about the project can be found
at ⟨http://www.pcre.org/⟩. If you have a bug report for this
manual page, see
⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩. This page was
obtained from the tarball fetched from
⟨https://github.com/PhilipHazel/pcre2.git⟩ on 2024-06-14. If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to man-pages@man7.org
PCRE2 10.43 21 February 2024 PCRE2JIT(3)
|
HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|