doc/html/pcrejit.html

<html>
<head>
<title>pcrejit specification</title>
</head>
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<h1>pcrejit man page</h1>
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
<p>
This page is part of the PCRE HTML documentation. It was generated automatically
from the original man page. If there is any nonsense in it, please consult the
man page, in case the conversion went wrong.
<br>
<ul>
<li><a name="TOC1" href="#SEC1">PCRE JUST-IN-TIME COMPILER SUPPORT</a>
<li><a name="TOC2" href="#SEC2">AVAILABILITY OF JIT SUPPORT</a>
<li><a name="TOC3" href="#SEC3">SIMPLE USE OF JIT</a>
<li><a name="TOC4" href="#SEC4">UNSUPPORTED OPTIONS AND PATTERN ITEMS</a>
<li><a name="TOC5" href="#SEC5">RETURN VALUES FROM JIT EXECUTION</a>
<li><a name="TOC6" href="#SEC6">SAVING AND RESTORING COMPILED PATTERNS</a>
<li><a name="TOC7" href="#SEC7">CONTROLLING THE JIT STACK</a>
<li><a name="TOC8" href="#SEC8">JIT STACK FAQ</a>
<li><a name="TOC9" href="#SEC9">EXAMPLE CODE</a>
<li><a name="TOC10" href="#SEC10">SEE ALSO</a>
<li><a name="TOC11" href="#SEC11">AUTHOR</a>
<li><a name="TOC12" href="#SEC12">REVISION</a>
</ul>
<br><a name="SEC1" href="#TOC1">PCRE JUST-IN-TIME COMPILER SUPPORT</a><br>
<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
\fPpcre_exec()\fP; if the pattern is not anchored, matching attempts may take
place many times at various positions in the subject, even for a single call to
<b>pcre_exec()</b>. If the subject string is very long, it may still pay to use
JIT for one-off matches.
</P>
<P>
JIT support applies only to the traditional matching function,
<b>pcre_exec()</b>. It does not apply when <b>pcre_dfa_exec()</b> is being used.
The code for this support was written by Zoltan Herczeg.
</P>
<br><a name="SEC2" href="#TOC1">AVAILABILITY OF JIT SUPPORT</a><br>
<P>
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:
<pre>
  ARM v5, v7, and Thumb2
  Intel x86 32-bit and 64-bit
  MIPS 32-bit
  Power PC 32-bit and 64-bit (experimental)
</pre>
The Power PC support is designated as experimental because it has not been
fully tested. If --enable-jit is set on an unsupported platform, compilation
fails.
</P>
<P>
A program that is linked with PCRE 8.20 or later can tell if JIT support is
available by calling <b>pcre_config()</b> with the PCRE_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 ordinary PCRE code if JIT is not available.
</P>
<P>
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.
</P>
<br><a name="SEC3" href="#TOC1">SIMPLE USE OF JIT</a><br>
<P>
You have to do two things to make use of the JIT support in the simplest way:
<pre>
  (1) Call <b>pcre_study()</b> with the PCRE_STUDY_JIT_COMPILE option for
      each compiled pattern, and pass the resulting <b>pcre_extra</b> block to
      <b>pcre_exec()</b>.

  (2) Use <b>pcre_free_study()</b> to free the <b>pcre_extra</b> block when it is
      no longer needed instead of just freeing it yourself. This
      ensures that any JIT data is also freed.
</pre>
For a program that may be linked with pre-8.20 versions of PCRE, you can insert
<pre>
  #ifndef PCRE_STUDY_JIT_COMPILE
  #define PCRE_STUDY_JIT_COMPILE 0
  #endif
</pre>
so that no option is passed to <b>pcre_study()</b>, and then use something like
this to free the study data:
<pre>
  #ifdef PCRE_CONFIG_JIT
      pcre_free_study(study_ptr);
  #else
      pcre_free(study_ptr);
  #endif
</pre>
In some circumstances you may need to call additional functions. These are
described in the section entitled
<a href="#stackcontrol">"Controlling the JIT stack"</a>
below.
</P>
<P>
If JIT support is not available, PCRE_STUDY_JIT_COMPILE is ignored, and no JIT
data is set up. 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. When <b>pcre_exec()</b> is passed a <b>pcre_extra</b> block
containing a pointer to JIT code, it obeys that instead of the normal code. The
result is identical, but the code runs much faster.
</P>
<P>
There are some <b>pcre_exec()</b> options that are not supported for JIT
execution. 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.
</P>
<P>
If the JIT compiler finds an unsupported item, no JIT data is generated. You
can find out if JIT execution is available after studying a pattern by calling
<b>pcre_fullinfo()</b> 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, or the
JIT compiler was not able to handle the pattern.
</P>
<P>
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.
</P>
<br><a name="SEC4" href="#TOC1">UNSUPPORTED OPTIONS AND PATTERN ITEMS</a><br>
<P>
The only <b>pcre_exec()</b> options that are supported for JIT execution are
PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and
PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not
supported.
</P>
<P>
The unsupported pattern items are:
<pre>
  \C             match a single byte; not supported in UTF-8 mode
  (?Cn)          callouts
  (*COMMIT)      )
  (*MARK)        )
  (*PRUNE)       ) the backtracking control verbs
  (*SKIP)        )
  (*THEN)        )
</pre>
Support for some of these may be added in future.
</P>
<br><a name="SEC5" href="#TOC1">RETURN VALUES FROM JIT EXECUTION</a><br>
<P>
When a pattern is matched using JIT execution, the return values are the same
as those given by the interpretive <b>pcre_exec()</b> 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
<a href="#stackcontrol">"Controlling the JIT stack"</a>
below for a discussion of JIT stack usage. For compatibility with the
interpretive <b>pcre_exec()</b> code, no more than two-thirds of the
<i>ovector</i> argument is used for passing back captured substrings.
</P>
<P>
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.
</P>
<br><a name="SEC6" href="#TOC1">SAVING AND RESTORING COMPILED PATTERNS</a><br>
<P>
The code that is generated by the JIT compiler is architecture-specific, 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 patterns is not something many people
do. More detail about this facility is given in the
<a href="pcreprecompile.html"><b>pcreprecompile</b></a>
documentation. It should be possible to run <b>pcre_study()</b> 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.
<a name="stackcontrol"></a></P>
<br><a name="SEC7" href="#TOC1">CONTROLLING THE JIT STACK</a><br>
<P>
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
<a href="#stackcontrol">"JIT stack FAQ"</a>
below.
</P>
<P>
The <b>pcre_jit_stack_alloc()</b> 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 <b>pcre_jit_stack</b>, or NULL if there is an error. The
<b>pcre_jit_stack_free()</b> 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.)
</P>
<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>
<P>
The <b>pcre_assign_jit_stack()</b> function specifies which stack JIT code
should use. Its arguments are as follows:
<pre>
  pcre_extra         *extra
  pcre_jit_callback  callback
  void               *data
</pre>
The <i>extra</i> argument must be the result of studying a pattern with
PCRE_STUDY_JIT_COMPILE. There are three cases for the values of the other two
options:
<pre>
  (1) If <i>callback</i> is NULL and <i>data</i> is NULL, an internal 32K block
      on the machine stack is used.

  (2) If <i>callback</i> is NULL and <i>data</i> is not NULL, <i>data</i> must be
      a valid JIT stack, the result of calling <b>pcre_jit_stack_alloc()</b>.

  (3) If <i>callback</i> not NULL, it must point to a function that is called
      with <i>data</i> as an argument at the start of matching, in order to
      set up a JIT stack. If the result is NULL, the internal 32K stack
      is used; otherwise the return value must be a valid JIT stack,
      the result of calling <b>pcre_jit_stack_alloc()</b>.
</pre>
You may safely assign the same JIT stack to more than one pattern, as long as
they are all matched sequentially in the same thread. In a multithread
application, each thread must use its own JIT stack.
</P>
<P>
Strictly speaking, even more is allowed. You can assign the same 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>
<P>
This is a suggestion for how a typical multithreaded program might operate:
<pre>
  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
</pre>
All the functions described in this section do nothing if JIT is not available,
and <b>pcre_assign_jit_stack()</b> does nothing unless the <b>extra</b> argument
is non-NULL and points to a <b>pcre_extra</b> block that is the result of a
successful study with PCRE_STUDY_JIT_COMPILE.
<a name="stackfaq"></a></P>
<br><a name="SEC8" href="#TOC1">JIT STACK FAQ</a><br>
<P>
(1) Why do we need JIT stacks?
<br>
<br>
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 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>
<P>
(2) Why don't we simply allocate blocks of memory with <b>malloc()</b>?
<br>
<br>
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>
<P>
(3) Who "owns" a JIT stack?
<br>
<br>
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
<b>pcre_exec()</b>, (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>
<P>
(4) When should a JIT stack be freed?
<br>
<br>
You can free a JIT stack at any time, as long as it will not be used by
<b>pcre_exec()</b> 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 <i>do not</i> call
<b>pcre_exec()</b> with a pattern pointing to an already freed stack, as that
will cause SEGFAULT. (Also, do not free a stack currently used by
<b>pcre_exec()</b> 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>
<P>
(5) Should I allocate/free a stack every time before/after calling
<b>pcre_exec()</b>?
<br>
<br>
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.
</P>
<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?
<br>
<br>
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>
<P>
(7) This is too much of a headache. Isn't there any better solution for JIT
stack handling?
<br>
<br>
No, thanks to Windows. If POSIX threads were used everywhere, we could throw
out this complicated API.
</P>
<br><a name="SEC9" href="#TOC1">EXAMPLE CODE</a><br>
<P>
This is a single-threaded example that specifies a JIT stack without using a
callback.
<pre>
  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);

</PRE>
</P>
<br><a name="SEC10" href="#TOC1">SEE ALSO</a><br>
<P>
<b>pcreapi</b>(3)
</P>
<br><a name="SEC11" href="#TOC1">AUTHOR</a><br>
<P>
Philip Hazel (FAQ by Zoltan Herczeg)
<br>
University Computing Service
<br>
Cambridge CB2 3QH, England.
<br>
</P>
<br><a name="SEC12" href="#TOC1">REVISION</a><br>
<P>
Last updated: 26 November 2011
<br>
Copyright &copy; 1997-2011 University of Cambridge.
<br>
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
1	ph10	678	<html>
2			<head>
3			<title>pcrejit specification</title>
4			</head>
5			<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6			<h1>pcrejit man page</h1>
7			<p>
8			Return to the <a href="index.html">PCRE index page</a>.
9			</p>
10			<p>
11			This page is part of the PCRE HTML documentation. It was generated automatically
12			from the original man page. If there is any nonsense in it, please consult the
13			man page, in case the conversion went wrong.
14			<br>
15			<ul>
16	ph10	691	<li><a name="TOC1" href="#SEC1">PCRE JUST-IN-TIME COMPILER SUPPORT</a>
17			<li><a name="TOC2" href="#SEC2">AVAILABILITY OF JIT SUPPORT</a>
18			<li><a name="TOC3" href="#SEC3">SIMPLE USE OF JIT</a>
19			<li><a name="TOC4" href="#SEC4">UNSUPPORTED OPTIONS AND PATTERN ITEMS</a>
20			<li><a name="TOC5" href="#SEC5">RETURN VALUES FROM JIT EXECUTION</a>
21			<li><a name="TOC6" href="#SEC6">SAVING AND RESTORING COMPILED PATTERNS</a>
22			<li><a name="TOC7" href="#SEC7">CONTROLLING THE JIT STACK</a>
23	ph10	836	<li><a name="TOC8" href="#SEC8">JIT STACK FAQ</a>
24			<li><a name="TOC9" href="#SEC9">EXAMPLE CODE</a>
25			<li><a name="TOC10" href="#SEC10">SEE ALSO</a>
26			<li><a name="TOC11" href="#SEC11">AUTHOR</a>
27			<li><a name="TOC12" href="#SEC12">REVISION</a>
28	ph10	678	</ul>
29	ph10	691	<br><a name="SEC1" href="#TOC1">PCRE JUST-IN-TIME COMPILER SUPPORT</a><br>
30			<P>
31			Just-in-time compiling is a heavyweight optimization that can greatly speed up
32			pattern matching. However, it comes at the cost of extra processing before the
33			match is performed. Therefore, it is of most benefit when the same pattern is
34			going to be matched many times. This does not necessarily mean many calls of
35			\fPpcre_exec()\fP; if the pattern is not anchored, matching attempts may take
36			place many times at various positions in the subject, even for a single call to
37			<b>pcre_exec()</b>. If the subject string is very long, it may still pay to use
38			JIT for one-off matches.
39			</P>
40			<P>
41			JIT support applies only to the traditional matching function,
42			<b>pcre_exec()</b>. It does not apply when <b>pcre_dfa_exec()</b> is being used.
43			The code for this support was written by Zoltan Herczeg.
44			</P>
45			<br><a name="SEC2" href="#TOC1">AVAILABILITY OF JIT SUPPORT</a><br>
46			<P>
47			JIT support is an optional feature of PCRE. The "configure" option --enable-jit
48			(or equivalent CMake option) must be set when PCRE is built if you want to use
49			JIT. The support is limited to the following hardware platforms:
50			<pre>
51			ARM v5, v7, and Thumb2
52			Intel x86 32-bit and 64-bit
53			MIPS 32-bit
54	ph10	733	Power PC 32-bit and 64-bit (experimental)
55	ph10	691	</pre>
56	ph10	733	The Power PC support is designated as experimental because it has not been
57			fully tested. If --enable-jit is set on an unsupported platform, compilation
58			fails.
59	ph10	691	</P>
60			<P>
61	ph10	836	A program that is linked with PCRE 8.20 or later can tell if JIT support is
62			available by calling <b>pcre_config()</b> with the PCRE_CONFIG_JIT option. The
63			result is 1 when JIT is available, and 0 otherwise. However, a simple program
64			does not need to check this in order to use JIT. The API is implemented in a
65			way that falls back to the ordinary PCRE code if JIT is not available.
66	ph10	691	</P>
67	ph10	836	<P>
68			If your program may sometimes be linked with versions of PCRE that are older
69			than 8.20, but you want to use JIT when it is available, you can test
70			the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT macro such
71			as PCRE_CONFIG_JIT, for compile-time control of your code.
72			</P>
73	ph10	691	<br><a name="SEC3" href="#TOC1">SIMPLE USE OF JIT</a><br>
74			<P>
75			You have to do two things to make use of the JIT support in the simplest way:
76			<pre>
77			(1) Call <b>pcre_study()</b> with the PCRE_STUDY_JIT_COMPILE option for
78			each compiled pattern, and pass the resulting <b>pcre_extra</b> block to
79			<b>pcre_exec()</b>.
80
81			(2) Use <b>pcre_free_study()</b> to free the <b>pcre_extra</b> block when it is
82			no longer needed instead of just freeing it yourself. This
83			ensures that any JIT data is also freed.
84			</pre>
85	ph10	836	For a program that may be linked with pre-8.20 versions of PCRE, you can insert
86			<pre>
87			#ifndef PCRE_STUDY_JIT_COMPILE
88			#define PCRE_STUDY_JIT_COMPILE 0
89			#endif
90			</pre>
91			so that no option is passed to <b>pcre_study()</b>, and then use something like
92			this to free the study data:
93			<pre>
94			#ifdef PCRE_CONFIG_JIT
95			pcre_free_study(study_ptr);
96			#else
97			pcre_free(study_ptr);
98			#endif
99			</pre>
100	ph10	691	In some circumstances you may need to call additional functions. These are
101			described in the section entitled
102			<a href="#stackcontrol">"Controlling the JIT stack"</a>
103			below.
104			</P>
105			<P>
106			If JIT support is not available, PCRE_STUDY_JIT_COMPILE is ignored, and no JIT
107			data is set up. Otherwise, the compiled pattern is passed to the JIT compiler,
108			which turns it into machine code that executes much faster than the normal
109			interpretive code. When <b>pcre_exec()</b> is passed a <b>pcre_extra</b> block
110			containing a pointer to JIT code, it obeys that instead of the normal code. The
111			result is identical, but the code runs much faster.
112			</P>
113			<P>
114			There are some <b>pcre_exec()</b> options that are not supported for JIT
115			execution. There are also some pattern items that JIT cannot handle. Details
116			are given below. In both cases, execution automatically falls back to the
117			interpretive code.
118			</P>
119			<P>
120			If the JIT compiler finds an unsupported item, no JIT data is generated. You
121			can find out if JIT execution is available after studying a pattern by calling
122			<b>pcre_fullinfo()</b> with the PCRE_INFO_JIT option. A result of 1 means that
123	ph10	708	JIT compilation was successful. A result of 0 means that JIT support is not
124	ph10	691	available, or the pattern was not studied with PCRE_STUDY_JIT_COMPILE, or the
125			JIT compiler was not able to handle the pattern.
126			</P>
127	ph10	708	<P>
128			Once a pattern has been studied, with or without JIT, it can be used as many
129			times as you like for matching different subject strings.
130			</P>
131	ph10	691	<br><a name="SEC4" href="#TOC1">UNSUPPORTED OPTIONS AND PATTERN ITEMS</a><br>
132			<P>
133			The only <b>pcre_exec()</b> options that are supported for JIT execution are
134			PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and
135			PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not
136			supported.
137			</P>
138			<P>
139			The unsupported pattern items are:
140			<pre>
141	ph10	836	\C match a single byte; not supported in UTF-8 mode
142	ph10	691	(?Cn) callouts
143			(*COMMIT) )
144			(*MARK) )
145			(*PRUNE) ) the backtracking control verbs
146			(*SKIP) )
147			(*THEN) )
148			</pre>
149			Support for some of these may be added in future.
150			</P>
151			<br><a name="SEC5" href="#TOC1">RETURN VALUES FROM JIT EXECUTION</a><br>
152			<P>
153			When a pattern is matched using JIT execution, the return values are the same
154			as those given by the interpretive <b>pcre_exec()</b> code, with the addition of
155			one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means that the memory used
156			for the JIT stack was insufficient. See
157			<a href="#stackcontrol">"Controlling the JIT stack"</a>
158			below for a discussion of JIT stack usage. For compatibility with the
159			interpretive <b>pcre_exec()</b> code, no more than two-thirds of the
160			<i>ovector</i> argument is used for passing back captured substrings.
161			</P>
162			<P>
163			The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if searching a
164			very large pattern tree goes on for too long, as it is in the same circumstance
165			when JIT is not used, but the details of exactly what is counted are not the
166			same. The PCRE_ERROR_RECURSIONLIMIT error code is never returned by JIT
167			execution.
168			</P>
169			<br><a name="SEC6" href="#TOC1">SAVING AND RESTORING COMPILED PATTERNS</a><br>
170			<P>
171			The code that is generated by the JIT compiler is architecture-specific, and is
172	ph10	708	also position dependent. For those reasons it cannot be saved (in a file or
173			database) and restored later like the bytecode and other data of a compiled
174			pattern. Saving and restoring compiled patterns is not something many people
175			do. More detail about this facility is given in the
176			<a href="pcreprecompile.html"><b>pcreprecompile</b></a>
177			documentation. It should be possible to run <b>pcre_study()</b> on a saved and
178			restored pattern, and thereby recreate the JIT data, but because JIT
179			compilation uses significant resources, it is probably not worth doing this;
180			you might as well recompile the original pattern.
181	ph10	691	<a name="stackcontrol"></a></P>
182			<br><a name="SEC7" href="#TOC1">CONTROLLING THE JIT STACK</a><br>
183			<P>
184			When the compiled JIT code runs, it needs a block of memory to use as a stack.
185			By default, it uses 32K on the machine stack. However, some large or
186			complicated patterns need more than this. The error PCRE_ERROR_JIT_STACKLIMIT
187			is given when there is not enough stack. Three functions are provided for
188	ph10	836	managing blocks of memory for use as JIT stacks. There is further discussion
189			about the use of JIT stacks in the section entitled
190			<a href="#stackcontrol">"JIT stack FAQ"</a>
191			below.
192	ph10	691	</P>
193			<P>
194			The <b>pcre_jit_stack_alloc()</b> function creates a JIT stack. Its arguments
195			are a starting size and a maximum size, and it returns a pointer to an opaque
196			structure of type <b>pcre_jit_stack</b>, or NULL if there is an error. The
197			<b>pcre_jit_stack_free()</b> function can be used to free a stack that is no
198			longer needed. (For the technically minded: the address space is allocated by
199			mmap or VirtualAlloc.)
200			</P>
201			<P>
202			JIT uses far less memory for recursion than the interpretive code,
203			and a maximum stack size of 512K to 1M should be more than enough for any
204			pattern.
205			</P>
206			<P>
207			The <b>pcre_assign_jit_stack()</b> function specifies which stack JIT code
208			should use. Its arguments are as follows:
209			<pre>
210			pcre_extra *extra
211			pcre_jit_callback callback
212			void *data
213			</pre>
214			The <i>extra</i> argument must be the result of studying a pattern with
215			PCRE_STUDY_JIT_COMPILE. There are three cases for the values of the other two
216			options:
217			<pre>
218			(1) If <i>callback</i> is NULL and <i>data</i> is NULL, an internal 32K block
219			on the machine stack is used.
220
221			(2) If <i>callback</i> is NULL and <i>data</i> is not NULL, <i>data</i> must be
222			a valid JIT stack, the result of calling <b>pcre_jit_stack_alloc()</b>.
223
224			(3) If <i>callback</i> not NULL, it must point to a function that is called
225			with <i>data</i> as an argument at the start of matching, in order to
226			set up a JIT stack. If the result is NULL, the internal 32K stack
227			is used; otherwise the return value must be a valid JIT stack,
228			the result of calling <b>pcre_jit_stack_alloc()</b>.
229			</pre>
230			You may safely assign the same JIT stack to more than one pattern, as long as
231			they are all matched sequentially in the same thread. In a multithread
232			application, each thread must use its own JIT stack.
233			</P>
234			<P>
235			Strictly speaking, even more is allowed. You can assign the same stack to any
236			number of patterns as long as they are not used for matching by multiple
237			threads at the same time. For example, you can assign the same stack to all
238			compiled patterns, and use a global mutex in the callback to wait until the
239			stack is available for use. However, this is an inefficient solution, and
240			not recommended.
241			</P>
242			<P>
243			This is a suggestion for how a typical multithreaded program might operate:
244			<pre>
245			During thread initalization
246			thread_local_var = pcre_jit_stack_alloc(...)
247
248			During thread exit
249			pcre_jit_stack_free(thread_local_var)
250
251			Use a one-line callback function
252			return thread_local_var
253			</pre>
254			All the functions described in this section do nothing if JIT is not available,
255			and <b>pcre_assign_jit_stack()</b> does nothing unless the <b>extra</b> argument
256			is non-NULL and points to a <b>pcre_extra</b> block that is the result of a
257			successful study with PCRE_STUDY_JIT_COMPILE.
258	ph10	836	<a name="stackfaq"></a></P>
259			<br><a name="SEC8" href="#TOC1">JIT STACK FAQ</a><br>
260			<P>
261			(1) Why do we need JIT stacks?
262			<br>
263			<br>
264			PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack where
265			the local data of the current node is pushed before checking its child nodes.
266			Allocating real machine stack on some platforms is difficult. For example, the
267			stack chain needs to be updated every time if we extend the stack on PowerPC.
268			Although it is possible, its updating time overhead decreases performance. So
269			we do the recursion in memory.
270	ph10	691	</P>
271			<P>
272	ph10	836	(2) Why don't we simply allocate blocks of memory with <b>malloc()</b>?
273			<br>
274			<br>
275			Modern operating systems have a nice feature: they can reserve an address space
276			instead of allocating memory. We can safely allocate memory pages inside this
277			address space, so the stack could grow without moving memory data (this is
278			important because of pointers). Thus we can allocate 1M address space, and use
279			only a single memory page (usually 4K) if that is enough. However, we can still
280			grow up to 1M anytime if needed.
281			</P>
282			<P>
283			(3) Who "owns" a JIT stack?
284			<br>
285			<br>
286			The owner of the stack is the user program, not the JIT studied pattern or
287			anything else. The user program must ensure that if a stack is used by
288			<b>pcre_exec()</b>, (that is, it is assigned to the pattern currently running),
289			that stack must not be used by any other threads (to avoid overwriting the same
290			memory area). The best practice for multithreaded programs is to allocate a
291			stack for each thread, and return this stack through the JIT callback function.
292			</P>
293			<P>
294			(4) When should a JIT stack be freed?
295			<br>
296			<br>
297			You can free a JIT stack at any time, as long as it will not be used by
298			<b>pcre_exec()</b> again. When you assign the stack to a pattern, only a pointer
299			is set. There is no reference counting or any other magic. You can free the
300			patterns and stacks in any order, anytime. Just <i>do not</i> call
301			<b>pcre_exec()</b> with a pattern pointing to an already freed stack, as that
302			will cause SEGFAULT. (Also, do not free a stack currently used by
303			<b>pcre_exec()</b> in another thread). You can also replace the stack for a
304			pattern at any time. You can even free the previous stack before assigning a
305			replacement.
306			</P>
307			<P>
308			(5) Should I allocate/free a stack every time before/after calling
309			<b>pcre_exec()</b>?
310			<br>
311			<br>
312			No, because this is too costly in terms of resources. However, you could
313			implement some clever idea which release the stack if it is not used in let's
314			say two minutes. The JIT callback can help to achive this without keeping a
315			list of the currently JIT studied patterns.
316			</P>
317			<P>
318			(6) OK, the stack is for long term memory allocation. But what happens if a
319			pattern causes stack overflow with a stack of 1M? Is that 1M kept until the
320			stack is freed?
321			<br>
322			<br>
323			Especially on embedded sytems, it might be a good idea to release
324			memory sometimes without freeing the stack. There is no API for this at the
325			moment. Probably a function call which returns with the currently allocated
326			memory for any stack and another which allows releasing memory (shrinking the
327			stack) would be a good idea if someone needs this.
328			</P>
329			<P>
330			(7) This is too much of a headache. Isn't there any better solution for JIT
331			stack handling?
332			<br>
333			<br>
334			No, thanks to Windows. If POSIX threads were used everywhere, we could throw
335			out this complicated API.
336			</P>
337			<br><a name="SEC9" href="#TOC1">EXAMPLE CODE</a><br>
338			<P>
339	ph10	691	This is a single-threaded example that specifies a JIT stack without using a
340			callback.
341			<pre>
342			int rc;
343			int ovector[30];
344			pcre *re;
345			pcre_extra *extra;
346			pcre_jit_stack *jit_stack;
347
348			re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
349			/* Check for errors */
350			extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
351			jit_stack = pcre_jit_stack_alloc(321024, 5121024);
352			/* Check for error (NULL) */
353			pcre_assign_jit_stack(extra, NULL, jit_stack);
354			rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
355			/* Check results */
356			pcre_free(re);
357			pcre_free_study(extra);
358			pcre_jit_stack_free(jit_stack);
359
360			</PRE>
361			</P>
362	ph10	836	<br><a name="SEC10" href="#TOC1">SEE ALSO</a><br>
363	ph10	691	<P>
364			<b>pcreapi</b>(3)
365			</P>
366	ph10	836	<br><a name="SEC11" href="#TOC1">AUTHOR</a><br>
367	ph10	691	<P>
368	ph10	836	Philip Hazel (FAQ by Zoltan Herczeg)
369	ph10	691	<br>
370			University Computing Service
371			<br>
372			Cambridge CB2 3QH, England.
373			<br>
374			</P>
375	ph10	836	<br><a name="SEC12" href="#TOC1">REVISION</a><br>
376	ph10	691	<P>
377	ph10	836	Last updated: 26 November 2011
378	ph10	691	<br>
379			Copyright © 1997-2011 University of Cambridge.
380			<br>
381	ph10	678	<p>
382			Return to the <a href="index.html">PCRE index page</a>.
383			</p>