trunk/doc/pcrejit.3

.TH PCREJIT 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH "PCRE JUST-IN-TIME COMPILER SUPPORT"
.rs
.sp
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.
.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.
.
.
.SH "8-BIT and 16-BIT SUPPORT"
.rs
.sp
JIT support is available for both the 8-bit and 16-bit PCRE libraries. To keep
this documentation simple, only the 8-bit interface is described in what
follows. If you are using the 16-bit library, substitute the 16-bit functions
and 16-bit structures (for example, \fIpcre16_jit_stack\fP instead of
\fIpcre_jit_stack\fP).
.
.
.SH "AVAILABILITY OF JIT SUPPORT"
.rs
.sp
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:
.sp
  ARM v5, v7, and Thumb2
  Intel x86 32-bit and 64-bit
  MIPS 32-bit
  Power PC 32-bit and 64-bit
.sp
If --enable-jit is set on an unsupported platform, compilation fails.
.P
A program that is linked with PCRE 8.20 or later can tell if JIT support is
available by calling \fBpcre_config()\fP 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 interpretive code if JIT is not available.
.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.
.
.
.SH "SIMPLE USE OF JIT"
.rs
.sp
You have to do two things to make use of the JIT support in the simplest way:
.sp
  (1) Call \fBpcre_study()\fP with the PCRE_STUDY_JIT_COMPILE option for
      each compiled pattern, and pass the resulting \fBpcre_extra\fP block to
      \fBpcre_exec()\fP.
.sp
  (2) Use \fBpcre_free_study()\fP to free the \fBpcre_extra\fP block when it is
      no longer needed, instead of just freeing it yourself. This
      ensures that any JIT data is also freed.
.sp
For a program that may be linked with pre-8.20 versions of PCRE, you can insert
.sp
  #ifndef PCRE_STUDY_JIT_COMPILE
  #define PCRE_STUDY_JIT_COMPILE 0
  #endif
.sp
so that no option is passed to \fBpcre_study()\fP, and then use something like
this to free the study data:
.sp
  #ifdef PCRE_CONFIG_JIT
      pcre_free_study(study_ptr);
  #else
      pcre_free(study_ptr);
  #endif
.sp
PCRE_STUDY_JIT_COMPILE requests the JIT compiler to generate code for complete 
matches. If you want to run partial matches using the PCRE_PARTIAL_HARD or
PCRE_PARTIAL_SOFT options of \fBpcre_exec()\fP, you should set one or both of
the following options in addition to, or instead of, PCRE_STUDY_JIT_COMPILE
when you call \fBpcre_study()\fP:
.sp
  PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
  PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
.sp
The JIT compiler generates different optimized code for each of the three
modes (normal, soft partial, hard partial). When \fBpcre_exec()\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, PCRE_STUDY_JIT_COMPILE etc. are ignored, and
no JIT data is created. 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 \fBpcre_exec()\fP is passed a \fBpcre_extra\fP
block containing a pointer to JIT code of the appropriate mode (normal or 
hard/soft partial), it obeys that code instead of running the interpreter. The
result is identical, but the compiled JIT code runs much faster.
.P
There are some \fBpcre_exec()\fP 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. 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
execution 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 execution is available after studying a pattern by calling
\fBpcre_fullinfo()\fP 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 etc., or
the JIT compiler was not able to handle the pattern.
.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.
.
.
.SH "UNSUPPORTED OPTIONS AND PATTERN ITEMS"
.rs
.sp
The only \fBpcre_exec()\fP options that are supported for JIT execution are
PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,
PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT.
.P
The unsupported pattern items are:
.sp
  \eC             match a single byte; not supported in UTF-8 mode
  (?Cn)          callouts
  (*COMMIT)      )
  (*MARK)        )
  (*PRUNE)       ) the backtracking control verbs
  (*SKIP)        )
  (*THEN)        )
.sp
Support for some of these may be added in future.
.
.
.SH "RETURN VALUES FROM JIT EXECUTION"
.rs
.sp
When a pattern is matched using JIT execution, the return values are the same
as those given by the interpretive \fBpcre_exec()\fP 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
.\" HTML <a href="#stackcontrol">
.\" </a>
"Controlling the JIT stack"
.\"
below for a discussion of JIT stack usage. For compatibility with the
interpretive \fBpcre_exec()\fP code, no more than two-thirds of the
\fIovector\fP argument is used for passing back captured substrings.
.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.
.
.
.SH "SAVING AND RESTORING COMPILED PATTERNS"
.rs
.sp
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
.\" HREF
\fBpcreprecompile\fP
.\"
documentation. It should be possible to run \fBpcre_study()\fP 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.
.
.
.\" HTML <a name="stackcontrol"></a>
.SH "CONTROLLING THE JIT STACK"
.rs
.sp
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
.\" HTML <a href="#stackcontrol">
.\" </a>
"JIT stack FAQ"
.\"
below.
.P
The \fBpcre_jit_stack_alloc()\fP 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 \fBpcre_jit_stack\fP, or NULL if there is an error. The
\fBpcre_jit_stack_free()\fP 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
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 \fBpcre_assign_jit_stack()\fP function specifies which stack JIT code
should use. Its arguments are as follows:
.sp
  pcre_extra         *extra
  pcre_jit_callback  callback
  void               *data
.sp
The \fIextra\fP argument must be the result of studying a pattern with
PCRE_STUDY_JIT_COMPILE etc. 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.
.sp
  (2) If \fIcallback\fP is NULL and \fIdata\fP is not NULL, \fIdata\fP must be
      a valid JIT stack, the result of calling \fBpcre_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 
      \fBpcre_jit_stack_alloc()\fP.
.sp
A callback function is obeyed whenever JIT code is about to be run; it is not 
obeyed when \fBpcre_exec()\fP is called with options that are incompatible for 
JIT execution. 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 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
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
This is a suggestion for how a typical multithreaded program might operate:
.sp
  During thread initalization
    thread_local_var = pcre_jit_stack_alloc(...)
.sp
  During thread exit
    pcre_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 \fBpcre_assign_jit_stack()\fP does nothing unless the \fBextra\fP argument
is non-NULL and points to a \fBpcre_extra\fP block that is the result of a
successful study with PCRE_STUDY_JIT_COMPILE etc.
.
.
.\" HTML <a name="stackfaq"></a>
.SH "JIT STACK FAQ"
.rs
.sp
(1) Why do we need JIT stacks?
.sp
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
(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
\fBpcre_exec()\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
\fBpcre_exec()\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
\fBpcre_exec()\fP with a pattern pointing to an already freed stack, as that
will cause SEGFAULT. (Also, do not free a stack currently used by
\fBpcre_exec()\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
\fBpcre_exec()\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 achive 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.
.
.
.SH "EXAMPLE CODE"
.rs
.sp
This is a single-threaded example that specifies a JIT stack without using a
callback.
.sp
  int rc;
  int ovector[30];
  pcre *re;
  pcre_extra *extra;
  pcre_jit_stack *jit_stack;
.sp
  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);
.sp
.
.
.SH "SEE ALSO"
.rs
.sp
\fBpcreapi\fP(3)
.
.
.SH AUTHOR
.rs
.sp
.nf
Philip Hazel (FAQ by Zoltan Herczeg)
University Computing Service
Cambridge CB2 3QH, England.
.fi
.
.
.SH REVISION
.rs
.sp
.nf
Last updated: 22 February 2012
Copyright (c) 1997-2012 University of Cambridge.
.fi
1	ph10	678	.TH PCREJIT 3
2			.SH NAME
3			PCRE - Perl-compatible regular expressions
4			.SH "PCRE JUST-IN-TIME COMPILER SUPPORT"
5			.rs
6			.sp
7			Just-in-time compiling is a heavyweight optimization that can greatly speed up
8	ph10	683	pattern matching. However, it comes at the cost of extra processing before the
9			match is performed. Therefore, it is of most benefit when the same pattern is
10	ph10	858	going to be matched many times. This does not necessarily mean many calls of a
11			matching function; if the pattern is not anchored, matching attempts may take
12			place many times at various positions in the subject, even for a single call.
13			Therefore, if the subject string is very long, it may still pay to use JIT for
14			one-off matches.
15	ph10	678	.P
16	ph10	858	JIT support applies only to the traditional Perl-compatible matching function.
17			It does not apply when the DFA matching function is being used. The code for
18			this support was written by Zoltan Herczeg.
19	ph10	678	.
20			.
21	ph10	858	.SH "8-BIT and 16-BIT SUPPORT"
22			.rs
23			.sp
24	ph10	903	JIT support is available for both the 8-bit and 16-bit PCRE libraries. To keep
25			this documentation simple, only the 8-bit interface is described in what
26			follows. If you are using the 16-bit library, substitute the 16-bit functions
27			and 16-bit structures (for example, \fIpcre16_jit_stack\fP instead of
28	ph10	858	\fIpcre_jit_stack\fP).
29			.
30			.
31	ph10	678	.SH "AVAILABILITY OF JIT SUPPORT"
32			.rs
33			.sp
34			JIT support is an optional feature of PCRE. The "configure" option --enable-jit
35			(or equivalent CMake option) must be set when PCRE is built if you want to use
36			JIT. The support is limited to the following hardware platforms:
37			.sp
38			ARM v5, v7, and Thumb2
39	ph10	683	Intel x86 32-bit and 64-bit
40	ph10	678	MIPS 32-bit
41	ph10	859	Power PC 32-bit and 64-bit
42	ph10	683	.sp
43	ph10	921	If --enable-jit is set on an unsupported platform, compilation fails.
44	ph10	678	.P
45	ph10	836	A program that is linked with PCRE 8.20 or later can tell if JIT support is
46			available by calling \fBpcre_config()\fP with the PCRE_CONFIG_JIT option. The
47			result is 1 when JIT is available, and 0 otherwise. However, a simple program
48			does not need to check this in order to use JIT. The API is implemented in a
49	ph10	921	way that falls back to the interpretive code if JIT is not available.
50	ph10	836	.P
51			If your program may sometimes be linked with versions of PCRE that are older
52			than 8.20, but you want to use JIT when it is available, you can test
53			the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT macro such
54			as PCRE_CONFIG_JIT, for compile-time control of your code.
55	ph10	678	.
56			.
57			.SH "SIMPLE USE OF JIT"
58			.rs
59			.sp
60			You have to do two things to make use of the JIT support in the simplest way:
61			.sp
62			(1) Call \fBpcre_study()\fP with the PCRE_STUDY_JIT_COMPILE option for
63			each compiled pattern, and pass the resulting \fBpcre_extra\fP block to
64			\fBpcre_exec()\fP.
65	ph10	691	.sp
66	ph10	678	(2) Use \fBpcre_free_study()\fP to free the \fBpcre_extra\fP block when it is
67	ph10	921	no longer needed, instead of just freeing it yourself. This
68	ph10	683	ensures that any JIT data is also freed.
69	ph10	678	.sp
70	ph10	836	For a program that may be linked with pre-8.20 versions of PCRE, you can insert
71			.sp
72			#ifndef PCRE_STUDY_JIT_COMPILE
73			#define PCRE_STUDY_JIT_COMPILE 0
74			#endif
75			.sp
76			so that no option is passed to \fBpcre_study()\fP, and then use something like
77			this to free the study data:
78			.sp
79			#ifdef PCRE_CONFIG_JIT
80			pcre_free_study(study_ptr);
81			#else
82			pcre_free(study_ptr);
83			#endif
84			.sp
85	ph10	921	PCRE_STUDY_JIT_COMPILE requests the JIT compiler to generate code for complete
86			matches. If you want to run partial matches using the PCRE_PARTIAL_HARD or
87			PCRE_PARTIAL_SOFT options of \fBpcre_exec()\fP, you should set one or both of
88			the following options in addition to, or instead of, PCRE_STUDY_JIT_COMPILE
89			when you call \fBpcre_study()\fP:
90			.sp
91			PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
92			PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
93			.sp
94			The JIT compiler generates different optimized code for each of the three
95			modes (normal, soft partial, hard partial). When \fBpcre_exec()\fP is called,
96			the appropriate code is run if it is available. Otherwise, the pattern is
97			matched using interpretive code.
98			.P
99	ph10	683	In some circumstances you may need to call additional functions. These are
100	ph10	678	described in the section entitled
101			.\" HTML <a href="#stackcontrol">
102			.\" </a>
103			"Controlling the JIT stack"
104			.\"
105			below.
106			.P
107	ph10	921	If JIT support is not available, PCRE_STUDY_JIT_COMPILE etc. are ignored, and
108			no JIT data is created. Otherwise, the compiled pattern is passed to the JIT
109			compiler, which turns it into machine code that executes much faster than the
110			normal interpretive code. When \fBpcre_exec()\fP is passed a \fBpcre_extra\fP
111			block containing a pointer to JIT code of the appropriate mode (normal or
112			hard/soft partial), it obeys that code instead of running the interpreter. The
113			result is identical, but the compiled JIT code runs much faster.
114	ph10	678	.P
115			There are some \fBpcre_exec()\fP options that are not supported for JIT
116	ph10	683	execution. There are also some pattern items that JIT cannot handle. Details
117			are given below. In both cases, execution automatically falls back to the
118	ph10	926	interpretive code. If you want to know whether JIT was actually used for a
119			particular match, you should arrange for a JIT callback function to be set up
120			as described in the section entitled
121			.\" HTML <a href="#stackcontrol">
122			.\" </a>
123			"Controlling the JIT stack"
124			.\"
125			below, even if you do not need to supply a non-default JIT stack. Such a
126			callback function is called whenever JIT code is about to be obeyed. If the
127			execution options are not right for JIT execution, the callback function is not
128			obeyed.
129	ph10	678	.P
130			If the JIT compiler finds an unsupported item, no JIT data is generated. You
131			can find out if JIT execution is available after studying a pattern by calling
132			\fBpcre_fullinfo()\fP with the PCRE_INFO_JIT option. A result of 1 means that
133	ph10	693	JIT compilation was successful. A result of 0 means that JIT support is not
134	ph10	921	available, or the pattern was not studied with PCRE_STUDY_JIT_COMPILE etc., or
135			the JIT compiler was not able to handle the pattern.
136	ph10	707	.P
137	ph10	708	Once a pattern has been studied, with or without JIT, it can be used as many
138	ph10	707	times as you like for matching different subject strings.
139	ph10	678	.
140			.
141			.SH "UNSUPPORTED OPTIONS AND PATTERN ITEMS"
142			.rs
143			.sp
144			The only \fBpcre_exec()\fP options that are supported for JIT execution are
145	ph10	921	PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,
146			PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT.
147	ph10	678	.P
148			The unsupported pattern items are:
149			.sp
150	ph10	836	\eC match a single byte; not supported in UTF-8 mode
151	ph10	678	(?Cn) callouts
152			(*COMMIT) )
153			(*MARK) )
154			(*PRUNE) ) the backtracking control verbs
155			(*SKIP) )
156			(*THEN) )
157	ph10	683	.sp
158	ph10	678	Support for some of these may be added in future.
159			.
160			.
161			.SH "RETURN VALUES FROM JIT EXECUTION"
162			.rs
163			.sp
164	ph10	683	When a pattern is matched using JIT execution, the return values are the same
165			as those given by the interpretive \fBpcre_exec()\fP code, with the addition of
166			one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means that the memory used
167	ph10	678	for the JIT stack was insufficient. See
168			.\" HTML <a href="#stackcontrol">
169			.\" </a>
170			"Controlling the JIT stack"
171			.\"
172	ph10	683	below for a discussion of JIT stack usage. For compatibility with the
173			interpretive \fBpcre_exec()\fP code, no more than two-thirds of the
174			\fIovector\fP argument is used for passing back captured substrings.
175	ph10	678	.P
176			The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if searching a
177			very large pattern tree goes on for too long, as it is in the same circumstance
178			when JIT is not used, but the details of exactly what is counted are not the
179			same. The PCRE_ERROR_RECURSIONLIMIT error code is never returned by JIT
180			execution.
181			.
182			.
183			.SH "SAVING AND RESTORING COMPILED PATTERNS"
184			.rs
185			.sp
186	ph10	683	The code that is generated by the JIT compiler is architecture-specific, and is
187	ph10	708	also position dependent. For those reasons it cannot be saved (in a file or
188	ph10	707	database) and restored later like the bytecode and other data of a compiled
189	ph10	708	pattern. Saving and restoring compiled patterns is not something many people
190	ph10	707	do. More detail about this facility is given in the
191			.\" HREF
192			\fBpcreprecompile\fP
193			.\"
194			documentation. It should be possible to run \fBpcre_study()\fP on a saved and
195			restored pattern, and thereby recreate the JIT data, but because JIT
196	ph10	708	compilation uses significant resources, it is probably not worth doing this;
197	ph10	707	you might as well recompile the original pattern.
198	ph10	678	.
199			.
200			.\" HTML <a name="stackcontrol"></a>
201			.SH "CONTROLLING THE JIT STACK"
202			.rs
203			.sp
204	ph10	683	When the compiled JIT code runs, it needs a block of memory to use as a stack.
205			By default, it uses 32K on the machine stack. However, some large or
206			complicated patterns need more than this. The error PCRE_ERROR_JIT_STACKLIMIT
207			is given when there is not enough stack. Three functions are provided for
208	ph10	836	managing blocks of memory for use as JIT stacks. There is further discussion
209			about the use of JIT stacks in the section entitled
210			.\" HTML <a href="#stackcontrol">
211			.\" </a>
212			"JIT stack FAQ"
213			.\"
214			below.
215	ph10	678	.P
216	ph10	683	The \fBpcre_jit_stack_alloc()\fP function creates a JIT stack. Its arguments
217	ph10	691	are a starting size and a maximum size, and it returns a pointer to an opaque
218	ph10	686	structure of type \fBpcre_jit_stack\fP, or NULL if there is an error. The
219			\fBpcre_jit_stack_free()\fP function can be used to free a stack that is no
220			longer needed. (For the technically minded: the address space is allocated by
221			mmap or VirtualAlloc.)
222	ph10	678	.P
223	ph10	691	JIT uses far less memory for recursion than the interpretive code,
224	ph10	683	and a maximum stack size of 512K to 1M should be more than enough for any
225			pattern.
226			.P
227			The \fBpcre_assign_jit_stack()\fP function specifies which stack JIT code
228	ph10	678	should use. Its arguments are as follows:
229			.sp
230			pcre_extra *extra
231			pcre_jit_callback callback
232			void *data
233	ph10	683	.sp
234			The \fIextra\fP argument must be the result of studying a pattern with
235	ph10	921	PCRE_STUDY_JIT_COMPILE etc. There are three cases for the values of the other
236			two options:
237	ph10	678	.sp
238			(1) If \fIcallback\fP is NULL and \fIdata\fP is NULL, an internal 32K block
239			on the machine stack is used.
240			.sp
241			(2) If \fIcallback\fP is NULL and \fIdata\fP is not NULL, \fIdata\fP must be
242			a valid JIT stack, the result of calling \fBpcre_jit_stack_alloc()\fP.
243			.sp
244	ph10	926	(3) If \fIcallback\fP is not NULL, it must point to a function that is
245			called with \fIdata\fP as an argument at the start of matching, in
246			order to set up a JIT stack. If the return from the callback
247			function is NULL, the internal 32K stack is used; otherwise the
248			return value must be a valid JIT stack, the result of calling
249			\fBpcre_jit_stack_alloc()\fP.
250	ph10	678	.sp
251	ph10	926	A callback function is obeyed whenever JIT code is about to be run; it is not
252			obeyed when \fBpcre_exec()\fP is called with options that are incompatible for
253			JIT execution. A callback function can therefore be used to determine whether a
254			match operation was executed by JIT or by the interpreter.
255			.P
256	ph10	678	You may safely assign the same JIT stack to more than one pattern, as long as
257			they are all matched sequentially in the same thread. In a multithread
258			application, each thread must use its own JIT stack.
259			.P
260	ph10	683	Strictly speaking, even more is allowed. You can assign the same stack to any
261			number of patterns as long as they are not used for matching by multiple
262			threads at the same time. For example, you can assign the same stack to all
263			compiled patterns, and use a global mutex in the callback to wait until the
264			stack is available for use. However, this is an inefficient solution, and
265			not recommended.
266			.P
267			This is a suggestion for how a typical multithreaded program might operate:
268			.sp
269			During thread initalization
270			thread_local_var = pcre_jit_stack_alloc(...)
271	ph10	691	.sp
272	ph10	683	During thread exit
273			pcre_jit_stack_free(thread_local_var)
274	ph10	691	.sp
275	ph10	683	Use a one-line callback function
276			return thread_local_var
277			.sp
278	ph10	678	All the functions described in this section do nothing if JIT is not available,
279	ph10	683	and \fBpcre_assign_jit_stack()\fP does nothing unless the \fBextra\fP argument
280			is non-NULL and points to a \fBpcre_extra\fP block that is the result of a
281	ph10	921	successful study with PCRE_STUDY_JIT_COMPILE etc.
282	ph10	678	.
283			.
284	ph10	836	.\" HTML <a name="stackfaq"></a>
285			.SH "JIT STACK FAQ"
286			.rs
287			.sp
288			(1) Why do we need JIT stacks?
289			.sp
290			PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack where
291			the local data of the current node is pushed before checking its child nodes.
292			Allocating real machine stack on some platforms is difficult. For example, the
293			stack chain needs to be updated every time if we extend the stack on PowerPC.
294			Although it is possible, its updating time overhead decreases performance. So
295			we do the recursion in memory.
296			.P
297			(2) Why don't we simply allocate blocks of memory with \fBmalloc()\fP?
298			.sp
299			Modern operating systems have a nice feature: they can reserve an address space
300			instead of allocating memory. We can safely allocate memory pages inside this
301			address space, so the stack could grow without moving memory data (this is
302			important because of pointers). Thus we can allocate 1M address space, and use
303			only a single memory page (usually 4K) if that is enough. However, we can still
304			grow up to 1M anytime if needed.
305			.P
306			(3) Who "owns" a JIT stack?
307			.sp
308			The owner of the stack is the user program, not the JIT studied pattern or
309			anything else. The user program must ensure that if a stack is used by
310			\fBpcre_exec()\fP, (that is, it is assigned to the pattern currently running),
311			that stack must not be used by any other threads (to avoid overwriting the same
312			memory area). The best practice for multithreaded programs is to allocate a
313			stack for each thread, and return this stack through the JIT callback function.
314			.P
315			(4) When should a JIT stack be freed?
316			.sp
317			You can free a JIT stack at any time, as long as it will not be used by
318			\fBpcre_exec()\fP again. When you assign the stack to a pattern, only a pointer
319			is set. There is no reference counting or any other magic. You can free the
320			patterns and stacks in any order, anytime. Just \fIdo not\fP call
321			\fBpcre_exec()\fP with a pattern pointing to an already freed stack, as that
322			will cause SEGFAULT. (Also, do not free a stack currently used by
323			\fBpcre_exec()\fP in another thread). You can also replace the stack for a
324			pattern at any time. You can even free the previous stack before assigning a
325			replacement.
326			.P
327			(5) Should I allocate/free a stack every time before/after calling
328			\fBpcre_exec()\fP?
329			.sp
330			No, because this is too costly in terms of resources. However, you could
331			implement some clever idea which release the stack if it is not used in let's
332			say two minutes. The JIT callback can help to achive this without keeping a
333			list of the currently JIT studied patterns.
334			.P
335			(6) OK, the stack is for long term memory allocation. But what happens if a
336			pattern causes stack overflow with a stack of 1M? Is that 1M kept until the
337			stack is freed?
338			.sp
339			Especially on embedded sytems, it might be a good idea to release
340			memory sometimes without freeing the stack. There is no API for this at the
341			moment. Probably a function call which returns with the currently allocated
342			memory for any stack and another which allows releasing memory (shrinking the
343			stack) would be a good idea if someone needs this.
344			.P
345			(7) This is too much of a headache. Isn't there any better solution for JIT
346			stack handling?
347			.sp
348			No, thanks to Windows. If POSIX threads were used everywhere, we could throw
349			out this complicated API.
350			.
351			.
352	ph10	678	.SH "EXAMPLE CODE"
353			.rs
354			.sp
355	ph10	683	This is a single-threaded example that specifies a JIT stack without using a
356	ph10	691	callback.
357	ph10	678	.sp
358			int rc;
359	ph10	683	int ovector[30];
360	ph10	678	pcre *re;
361	ph10	683	pcre_extra *extra;
362			pcre_jit_stack *jit_stack;
363			.sp
364	ph10	678	re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
365			/* Check for errors */
366			extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
367	ph10	683	jit_stack = pcre_jit_stack_alloc(321024, 5121024);
368	ph10	678	/* Check for error (NULL) */
369			pcre_assign_jit_stack(extra, NULL, jit_stack);
370	ph10	683	rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
371	ph10	678	/* Check results */
372			pcre_free(re);
373	ph10	683	pcre_free_study(extra);
374	ph10	691	pcre_jit_stack_free(jit_stack);
375	ph10	678	.sp
376			.
377			.
378			.SH "SEE ALSO"
379			.rs
380			.sp
381			\fBpcreapi\fP(3)
382			.
383			.
384			.SH AUTHOR
385			.rs
386			.sp
387			.nf
388	ph10	836	Philip Hazel (FAQ by Zoltan Herczeg)
389	ph10	678	University Computing Service
390			Cambridge CB2 3QH, England.
391			.fi
392			.
393			.
394			.SH REVISION
395			.rs
396			.sp
397			.nf
398	ph10	926	Last updated: 22 February 2012
399	ph10	858	Copyright (c) 1997-2012 University of Cambridge.
400	ph10	678	.fi