pcre/doc/pcre2jit.3

.TH PCRE2JIT 3 "29 September 2014" "PCRE2 10.00"
.SH NAME
PCRE2 - Perl-compatible regular expressions (revised API)
.SH "PCRE2 JUST-IN-TIME COMPILER SUPPORT"
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FIXME: This needs checking over once JIT support is implemented.
.P
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 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 is available for all of the 8-bit, 16-bit and
32-bit PCRE2 libraries.
.P
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.
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.SH "AVAILABILITY OF JIT SUPPORT"
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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:
.sp
  ARM v5, v7, and Thumb2
  Intel x86 32-bit and 64-bit
  MIPS 32-bit
  Power PC 32-bit and 64-bit
  SPARC 32-bit (experimental)
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If --enable-jit is set on an unsupported platform, compilation fails.
.P
A program can tell if JIT support is available by calling \fBpcre2_config()\fP
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.
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.SH "SIMPLE USE OF JIT"
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To make use of the JIT support in the simplest way, all you have to do is to 
call \fBpcre2_jit_compile()\fP after successfully compiling a pattern with 
\fBpcre2_compile()\fP. This function has two arguments: the first is the 
compiled pattern pointer that was returned by \fBpcre2_compile()\fP, and the 
second is a set of option bits, which must include at least one of
PCRE2_JIT_COMPLETE, PCRE2_JIT_PARTIAL_HARD, or PCRE2_JIT_PARTIAL_SOFT.
.P
The returned value from \fBpcre2_jit_compile()\fP is FIXME FIXME.
.P
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 \fBpcre2_match()\fP, 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 \fBpcre2_match()\fP is called, the
appropriate code is run if it is available. Otherwise, the pattern is matched
using interpretive code.
.P
In some circumstances you may need to call additional functions. These are
described in the section entitled
.\" HTML <a href="#stackcontrol">
.\" </a>
"Controlling the JIT stack"
.\"
below.
.P
If JIT support is not available, a call to \fBpcre2_jit_comple()\fP does
nothing and returns FIXME. 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.
.P
There are some \fBpcre2_match()\fP 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
.\" HTML <a href="#stackcontrol">
.\" </a>
"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.
.P
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
\fBpcre2_pattern_info()\fP with the PCRE2_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 processed by \fBpcre2_jit_compile()\fP,
or the JIT compiler was not able to handle the pattern.
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.SH "UNSUPPORTED OPTIONS AND PATTERN ITEMS"
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The \fBpcre2_match()\fP 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.
.P
The only unsupported pattern items are \eC (match a single data unit) when
running in a UTF mode, and a callout immediately before an assertion condition
in a conditional group.
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.SH "RETURN VALUES FROM JIT MATCHING"
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When a pattern is matched using JIT matching, the return values are the same
as those given by the interpretive \fBpcre2_match()\fP 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
.\" HTML <a href="#stackcontrol">
.\" </a>
"Controlling the JIT stack"
.\"
below for a discussion of JIT stack usage.
.P
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_RECURSIONLIMIT error code is never returned
when JIT matching is used.
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.\" HTML <a name="stackcontrol"></a>
.SH "CONTROLLING THE JIT STACK"
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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
.\" HTML <a href="#stackcontrol">
.\" </a>
"JIT stack FAQ"
.\"
below.
.P
The \fBpcre2_jit_stack_alloc()\fP function creates a JIT stack. Its arguments
are a general context (for memory allocation functions, or NULL for standard
memory allocation), a starting size and a maximum size, and it returns a
pointer to an opaque structure of type \fBpcre2_jit_stack\fP, or NULL if there
is an error. The \fBpcre2_jit_stack_free()\fP 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.)  FIXME Is this right?
.P
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.
.P
The \fBpcre2_jit_stack_assign()\fP function specifies which stack JIT code
should use. Its arguments are as follows:
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  pcre2_code         *code
  pcre2_jit_callback  callback
  void               *data
.sp
The \fIcode\fP argument is a pointer to a compiled pattern, after it has been
processed by \fBpcre2_jit_compile()\fP. There are three cases for the values of
the other two options:
.sp
  (1) If \fIcallback\fP is NULL and \fIdata\fP is NULL, an internal 32K block
      on the machine stack is used.
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  (2) If \fIcallback\fP is NULL and \fIdata\fP is not NULL, \fIdata\fP must be
      a valid JIT stack, the result of calling \fBpcre2_jit_stack_alloc()\fP.
.sp
  (3) If \fIcallback\fP is not NULL, it must point to a function that is
      called with \fIdata\fP 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
      \fBpcre2_jit_stack_alloc()\fP.
.sp
A callback function is obeyed whenever JIT code is about to be run; it is not
obeyed when \fBpcre2_match()\fP 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.
.P
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.
.P
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.
.P
This is a suggestion for how a multithreaded program that needs to set up
non-default JIT stacks might operate:
.sp
  During thread initalization
    thread_local_var = pcre2_jit_stack_alloc(...)
.sp
  During thread exit
    pcre2_jit_stack_free(thread_local_var)
.sp
  Use a one-line callback function
    return thread_local_var
.sp
All the functions described in this section do nothing if JIT is not available,
and \fBpcre2_jit_stack_assign()\fP does nothing unless the \fBcode\fP argument
is non-NULL and points to a \fBpcre2_code\fP block that has been successfully 
processed by \fBpcre2_jit_compile()\fP.
.
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.\" HTML <a name="stackfaq"></a>
.SH "JIT STACK FAQ"
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(1) Why do we need JIT stacks?
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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.
.P
(2) Why don't we simply allocate blocks of memory with \fBmalloc()\fP?
.sp
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.
.P
(3) Who "owns" a JIT stack?
.sp
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
\fBpcre2_match()\fP, (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.
.P
(4) When should a JIT stack be freed?
.sp
You can free a JIT stack at any time, as long as it will not be used by
\fBpcre2_match()\fP 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 \fIdo not\fP call
\fBpcre2_match()\fP with a pattern pointing to an already freed stack, as that
will cause SEGFAULT. (Also, do not free a stack currently used by
\fBpcre2_match()\fP 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.
.P
(5) Should I allocate/free a stack every time before/after calling
\fBpcre2_match()\fP?
.sp
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 the currently JIT studied patterns.
.P
(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?
.sp
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.
.P
(7) This is too much of a headache. Isn't there any better solution for JIT
stack handling?
.sp
No, thanks to Windows. If POSIX threads were used everywhere, we could throw
out this complicated API.
.
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.SH "EXAMPLE CODE"
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This is a single-threaded example that specifies a JIT stack without using a
callback.
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  int rc;
  pcre2_code *re;
  pcre2_match_data *match_data; 
  pcre2_jit_stack *jit_stack;
.sp
  re = pcre2_compile(pattern, PCRE2_ZERO_TERMINATED, 0, 
    &errornumber, &erroffset, NULL);
  /* Check for errors */
  rc = pcre2_jit_compile(re, PCRE2_JIT_COMPLETE);
  /* Check for errors */ 
  jit_stack = pcre2_jit_stack_alloc(NULL, 32*1024, 512*1024);
  /* Check for error (NULL) */
  pcre2_jit_stack_assign(re, NULL, jit_stack);
  match_data = pcre2_match_data_create(re, 10); 
  rc = pcre2_match(re, subject, length, 0, 0, match_data, NULL);
  /* Check results */
  pcre2_free(re);
  pcre2_jit_stack_free(jit_stack);
.sp
.
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.SH "JIT FAST PATH API"
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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 \fBpcre2_match()\fP 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
\fBpcre2_match()\fP (obviously only for patterns that have been successfully
processed by \fBpcre2_jit_compile()\fP).
.P
The fast path function is called \fBpcre2_jit_match()\fP, and it takes exactly
the same arguments as \fBpcre2_match()\fP, plus one additional argument that
must point to a JIT stack. The JIT stack arrangements described above do not
apply. The return values are the same as for \fBpcre2_match()\fP.
.P
When you call \fBpcre2_match()\fP, 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.
.P
Bypassing the sanity checks and the \fBpcre2_match()\fP wrapping can give
speedups of more than 10%.
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.SH "SEE ALSO"
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\fBpcre2api\fP(3)
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.SH AUTHOR
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Philip Hazel (FAQ by Zoltan Herczeg)
University Computing Service
Cambridge CB2 3QH, England.
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.SH REVISION
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Last updated: 29 September 2014
Copyright (c) 1997-2014 University of Cambridge.
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