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
\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
\fBpcre_exec()\fP. 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 matching function,
\fBpcre_exec()\fP. It does not apply when \fBpcre_dfa_exec()\fP is being used.
The code for this support was written by Zoltan Herczeg.
.
.
.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 (experimental)
.sp
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
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 ordinary PCRE 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
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 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 \fBpcre_exec()\fP is passed a \fBpcre_extra\fP 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
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.
.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, 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, and
PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not
supported.
.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. 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 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 result 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
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.
.
.
.\" 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: 26 November 2011
Copyright (c) 1997-2011 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			going to be matched many times. This does not necessarily mean many calls of
11	ph10	678	\fPpcre_exec()\fP; 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 to
13	ph10	683	\fBpcre_exec()\fP. If the subject string is very long, it may still pay to use
14	ph10	678	JIT for one-off matches.
15			.P
16	ph10	683	JIT support applies only to the traditional matching function,
17	ph10	678	\fBpcre_exec()\fP. It does not apply when \fBpcre_dfa_exec()\fP is being used.
18			The code for this support was written by Zoltan Herczeg.
19			.
20			.
21			.SH "AVAILABILITY OF JIT SUPPORT"
22			.rs
23			.sp
24			JIT support is an optional feature of PCRE. The "configure" option --enable-jit
25			(or equivalent CMake option) must be set when PCRE is built if you want to use
26			JIT. The support is limited to the following hardware platforms:
27			.sp
28			ARM v5, v7, and Thumb2
29	ph10	683	Intel x86 32-bit and 64-bit
30	ph10	678	MIPS 32-bit
31	ph10	717	Power PC 32-bit and 64-bit (experimental)
32	ph10	683	.sp
33	ph10	733	The Power PC support is designated as experimental because it has not been
34	ph10	717	fully tested. If --enable-jit is set on an unsupported platform, compilation
35			fails.
36	ph10	678	.P
37	ph10	836	A program that is linked with PCRE 8.20 or later can tell if JIT support is
38			available by calling \fBpcre_config()\fP with the PCRE_CONFIG_JIT option. The
39			result is 1 when JIT is available, and 0 otherwise. However, a simple program
40			does not need to check this in order to use JIT. The API is implemented in a
41			way that falls back to the ordinary PCRE code if JIT is not available.
42			.P
43			If your program may sometimes be linked with versions of PCRE that are older
44			than 8.20, but you want to use JIT when it is available, you can test
45			the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT macro such
46			as PCRE_CONFIG_JIT, for compile-time control of your code.
47	ph10	678	.
48			.
49			.SH "SIMPLE USE OF JIT"
50			.rs
51			.sp
52			You have to do two things to make use of the JIT support in the simplest way:
53			.sp
54			(1) Call \fBpcre_study()\fP with the PCRE_STUDY_JIT_COMPILE option for
55			each compiled pattern, and pass the resulting \fBpcre_extra\fP block to
56			\fBpcre_exec()\fP.
57	ph10	691	.sp
58	ph10	678	(2) Use \fBpcre_free_study()\fP to free the \fBpcre_extra\fP block when it is
59	ph10	691	no longer needed instead of just freeing it yourself. This
60	ph10	683	ensures that any JIT data is also freed.
61	ph10	678	.sp
62	ph10	836	For a program that may be linked with pre-8.20 versions of PCRE, you can insert
63			.sp
64			#ifndef PCRE_STUDY_JIT_COMPILE
65			#define PCRE_STUDY_JIT_COMPILE 0
66			#endif
67			.sp
68			so that no option is passed to \fBpcre_study()\fP, and then use something like
69			this to free the study data:
70			.sp
71			#ifdef PCRE_CONFIG_JIT
72			pcre_free_study(study_ptr);
73			#else
74			pcre_free(study_ptr);
75			#endif
76			.sp
77	ph10	683	In some circumstances you may need to call additional functions. These are
78	ph10	678	described in the section entitled
79			.\" HTML <a href="#stackcontrol">
80			.\" </a>
81			"Controlling the JIT stack"
82			.\"
83			below.
84			.P
85	ph10	683	If JIT support is not available, PCRE_STUDY_JIT_COMPILE is ignored, and no JIT
86			data is set up. Otherwise, the compiled pattern is passed to the JIT compiler,
87			which turns it into machine code that executes much faster than the normal
88			interpretive code. When \fBpcre_exec()\fP is passed a \fBpcre_extra\fP block
89			containing a pointer to JIT code, it obeys that instead of the normal code. The
90			result is identical, but the code runs much faster.
91	ph10	678	.P
92			There are some \fBpcre_exec()\fP options that are not supported for JIT
93	ph10	683	execution. There are also some pattern items that JIT cannot handle. Details
94			are given below. In both cases, execution automatically falls back to the
95	ph10	678	interpretive code.
96			.P
97			If the JIT compiler finds an unsupported item, no JIT data is generated. You
98			can find out if JIT execution is available after studying a pattern by calling
99			\fBpcre_fullinfo()\fP with the PCRE_INFO_JIT option. A result of 1 means that
100	ph10	693	JIT compilation was successful. A result of 0 means that JIT support is not
101	ph10	678	available, or the pattern was not studied with PCRE_STUDY_JIT_COMPILE, or the
102			JIT compiler was not able to handle the pattern.
103	ph10	707	.P
104	ph10	708	Once a pattern has been studied, with or without JIT, it can be used as many
105	ph10	707	times as you like for matching different subject strings.
106	ph10	678	.
107			.
108			.SH "UNSUPPORTED OPTIONS AND PATTERN ITEMS"
109			.rs
110			.sp
111			The only \fBpcre_exec()\fP options that are supported for JIT execution are
112	ph10	683	PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and
113			PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not
114	ph10	678	supported.
115			.P
116			The unsupported pattern items are:
117			.sp
118	ph10	836	\eC match a single byte; not supported in UTF-8 mode
119	ph10	678	(?Cn) callouts
120			(*COMMIT) )
121			(*MARK) )
122			(*PRUNE) ) the backtracking control verbs
123			(*SKIP) )
124			(*THEN) )
125	ph10	683	.sp
126	ph10	678	Support for some of these may be added in future.
127			.
128			.
129			.SH "RETURN VALUES FROM JIT EXECUTION"
130			.rs
131			.sp
132	ph10	683	When a pattern is matched using JIT execution, the return values are the same
133			as those given by the interpretive \fBpcre_exec()\fP code, with the addition of
134			one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means that the memory used
135	ph10	678	for the JIT stack was insufficient. See
136			.\" HTML <a href="#stackcontrol">
137			.\" </a>
138			"Controlling the JIT stack"
139			.\"
140	ph10	683	below for a discussion of JIT stack usage. For compatibility with the
141			interpretive \fBpcre_exec()\fP code, no more than two-thirds of the
142			\fIovector\fP argument is used for passing back captured substrings.
143	ph10	678	.P
144			The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if searching a
145			very large pattern tree goes on for too long, as it is in the same circumstance
146			when JIT is not used, but the details of exactly what is counted are not the
147			same. The PCRE_ERROR_RECURSIONLIMIT error code is never returned by JIT
148			execution.
149			.
150			.
151			.SH "SAVING AND RESTORING COMPILED PATTERNS"
152			.rs
153			.sp
154	ph10	683	The code that is generated by the JIT compiler is architecture-specific, and is
155	ph10	708	also position dependent. For those reasons it cannot be saved (in a file or
156	ph10	707	database) and restored later like the bytecode and other data of a compiled
157	ph10	708	pattern. Saving and restoring compiled patterns is not something many people
158	ph10	707	do. More detail about this facility is given in the
159			.\" HREF
160			\fBpcreprecompile\fP
161			.\"
162			documentation. It should be possible to run \fBpcre_study()\fP on a saved and
163			restored pattern, and thereby recreate the JIT data, but because JIT
164	ph10	708	compilation uses significant resources, it is probably not worth doing this;
165	ph10	707	you might as well recompile the original pattern.
166	ph10	678	.
167			.
168			.\" HTML <a name="stackcontrol"></a>
169			.SH "CONTROLLING THE JIT STACK"
170			.rs
171			.sp
172	ph10	683	When the compiled JIT code runs, it needs a block of memory to use as a stack.
173			By default, it uses 32K on the machine stack. However, some large or
174			complicated patterns need more than this. The error PCRE_ERROR_JIT_STACKLIMIT
175			is given when there is not enough stack. Three functions are provided for
176	ph10	836	managing blocks of memory for use as JIT stacks. There is further discussion
177			about the use of JIT stacks in the section entitled
178			.\" HTML <a href="#stackcontrol">
179			.\" </a>
180			"JIT stack FAQ"
181			.\"
182			below.
183	ph10	678	.P
184	ph10	683	The \fBpcre_jit_stack_alloc()\fP function creates a JIT stack. Its arguments
185	ph10	691	are a starting size and a maximum size, and it returns a pointer to an opaque
186	ph10	686	structure of type \fBpcre_jit_stack\fP, or NULL if there is an error. The
187			\fBpcre_jit_stack_free()\fP function can be used to free a stack that is no
188			longer needed. (For the technically minded: the address space is allocated by
189			mmap or VirtualAlloc.)
190	ph10	678	.P
191	ph10	691	JIT uses far less memory for recursion than the interpretive code,
192	ph10	683	and a maximum stack size of 512K to 1M should be more than enough for any
193			pattern.
194			.P
195			The \fBpcre_assign_jit_stack()\fP function specifies which stack JIT code
196	ph10	678	should use. Its arguments are as follows:
197			.sp
198			pcre_extra *extra
199			pcre_jit_callback callback
200			void *data
201	ph10	683	.sp
202			The \fIextra\fP argument must be the result of studying a pattern with
203			PCRE_STUDY_JIT_COMPILE. There are three cases for the values of the other two
204	ph10	678	options:
205			.sp
206			(1) If \fIcallback\fP is NULL and \fIdata\fP is NULL, an internal 32K block
207			on the machine stack is used.
208			.sp
209			(2) If \fIcallback\fP is NULL and \fIdata\fP is not NULL, \fIdata\fP must be
210			a valid JIT stack, the result of calling \fBpcre_jit_stack_alloc()\fP.
211			.sp
212			(3) If \fIcallback\fP not NULL, it must point to a function that is called
213	ph10	683	with \fIdata\fP as an argument at the start of matching, in order to
214			set up a JIT stack. If the result is NULL, the internal 32K stack
215			is used; otherwise the return value must be a valid JIT stack,
216	ph10	678	the result of calling \fBpcre_jit_stack_alloc()\fP.
217			.sp
218			You may safely assign the same JIT stack to more than one pattern, as long as
219			they are all matched sequentially in the same thread. In a multithread
220			application, each thread must use its own JIT stack.
221			.P
222	ph10	683	Strictly speaking, even more is allowed. You can assign the same stack to any
223			number of patterns as long as they are not used for matching by multiple
224			threads at the same time. For example, you can assign the same stack to all
225			compiled patterns, and use a global mutex in the callback to wait until the
226			stack is available for use. However, this is an inefficient solution, and
227			not recommended.
228			.P
229			This is a suggestion for how a typical multithreaded program might operate:
230			.sp
231			During thread initalization
232			thread_local_var = pcre_jit_stack_alloc(...)
233	ph10	691	.sp
234	ph10	683	During thread exit
235			pcre_jit_stack_free(thread_local_var)
236	ph10	691	.sp
237	ph10	683	Use a one-line callback function
238			return thread_local_var
239			.sp
240	ph10	678	All the functions described in this section do nothing if JIT is not available,
241	ph10	683	and \fBpcre_assign_jit_stack()\fP does nothing unless the \fBextra\fP argument
242			is non-NULL and points to a \fBpcre_extra\fP block that is the result of a
243	ph10	678	successful study with PCRE_STUDY_JIT_COMPILE.
244			.
245			.
246	ph10	836	.\" HTML <a name="stackfaq"></a>
247			.SH "JIT STACK FAQ"
248			.rs
249			.sp
250			(1) Why do we need JIT stacks?
251			.sp
252			PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack where
253			the local data of the current node is pushed before checking its child nodes.
254			Allocating real machine stack on some platforms is difficult. For example, the
255			stack chain needs to be updated every time if we extend the stack on PowerPC.
256			Although it is possible, its updating time overhead decreases performance. So
257			we do the recursion in memory.
258			.P
259			(2) Why don't we simply allocate blocks of memory with \fBmalloc()\fP?
260			.sp
261			Modern operating systems have a nice feature: they can reserve an address space
262			instead of allocating memory. We can safely allocate memory pages inside this
263			address space, so the stack could grow without moving memory data (this is
264			important because of pointers). Thus we can allocate 1M address space, and use
265			only a single memory page (usually 4K) if that is enough. However, we can still
266			grow up to 1M anytime if needed.
267			.P
268			(3) Who "owns" a JIT stack?
269			.sp
270			The owner of the stack is the user program, not the JIT studied pattern or
271			anything else. The user program must ensure that if a stack is used by
272			\fBpcre_exec()\fP, (that is, it is assigned to the pattern currently running),
273			that stack must not be used by any other threads (to avoid overwriting the same
274			memory area). The best practice for multithreaded programs is to allocate a
275			stack for each thread, and return this stack through the JIT callback function.
276			.P
277			(4) When should a JIT stack be freed?
278			.sp
279			You can free a JIT stack at any time, as long as it will not be used by
280			\fBpcre_exec()\fP again. When you assign the stack to a pattern, only a pointer
281			is set. There is no reference counting or any other magic. You can free the
282			patterns and stacks in any order, anytime. Just \fIdo not\fP call
283			\fBpcre_exec()\fP with a pattern pointing to an already freed stack, as that
284			will cause SEGFAULT. (Also, do not free a stack currently used by
285			\fBpcre_exec()\fP in another thread). You can also replace the stack for a
286			pattern at any time. You can even free the previous stack before assigning a
287			replacement.
288			.P
289			(5) Should I allocate/free a stack every time before/after calling
290			\fBpcre_exec()\fP?
291			.sp
292			No, because this is too costly in terms of resources. However, you could
293			implement some clever idea which release the stack if it is not used in let's
294			say two minutes. The JIT callback can help to achive this without keeping a
295			list of the currently JIT studied patterns.
296			.P
297			(6) OK, the stack is for long term memory allocation. But what happens if a
298			pattern causes stack overflow with a stack of 1M? Is that 1M kept until the
299			stack is freed?
300			.sp
301			Especially on embedded sytems, it might be a good idea to release
302			memory sometimes without freeing the stack. There is no API for this at the
303			moment. Probably a function call which returns with the currently allocated
304			memory for any stack and another which allows releasing memory (shrinking the
305			stack) would be a good idea if someone needs this.
306			.P
307			(7) This is too much of a headache. Isn't there any better solution for JIT
308			stack handling?
309			.sp
310			No, thanks to Windows. If POSIX threads were used everywhere, we could throw
311			out this complicated API.
312			.
313			.
314	ph10	678	.SH "EXAMPLE CODE"
315			.rs
316			.sp
317	ph10	683	This is a single-threaded example that specifies a JIT stack without using a
318	ph10	691	callback.
319	ph10	678	.sp
320			int rc;
321	ph10	683	int ovector[30];
322	ph10	678	pcre *re;
323	ph10	683	pcre_extra *extra;
324			pcre_jit_stack *jit_stack;
325			.sp
326	ph10	678	re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
327			/* Check for errors */
328			extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
329	ph10	683	jit_stack = pcre_jit_stack_alloc(321024, 5121024);
330	ph10	678	/* Check for error (NULL) */
331			pcre_assign_jit_stack(extra, NULL, jit_stack);
332	ph10	683	rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
333	ph10	678	/* Check results */
334			pcre_free(re);
335	ph10	683	pcre_free_study(extra);
336	ph10	691	pcre_jit_stack_free(jit_stack);
337	ph10	678	.sp
338			.
339			.
340			.SH "SEE ALSO"
341			.rs
342			.sp
343			\fBpcreapi\fP(3)
344			.
345			.
346			.SH AUTHOR
347			.rs
348			.sp
349			.nf
350	ph10	836	Philip Hazel (FAQ by Zoltan Herczeg)
351	ph10	678	University Computing Service
352			Cambridge CB2 3QH, England.
353			.fi
354			.
355			.
356			.SH REVISION
357			.rs
358			.sp
359			.nf
360	ph10	836	Last updated: 26 November 2011
361	ph10	678	Copyright (c) 1997-2011 University of Cambridge.
362			.fi