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	.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	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 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	.P
16	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	.
20	.
21	.SH "8-BIT and 16-BIT SUPPORT"
22	.rs
23	.sp
24	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	\fIpcre_jit_stack\fP).
29	.
30	.
31	.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	Intel x86 32-bit and 64-bit
40	MIPS 32-bit
41	Power PC 32-bit and 64-bit
42	.sp
43	If --enable-jit is set on an unsupported platform, compilation fails.
44	.P
45	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	way that falls back to the interpretive code if JIT is not available.
50	.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	.
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	.sp
66	(2) Use \fBpcre_free_study()\fP to free the \fBpcre_extra\fP block when it is
67	no longer needed, instead of just freeing it yourself. This
68	ensures that any JIT data is also freed.
69	.sp
70	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	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	In some circumstances you may need to call additional functions. These are
100	described in the section entitled
101	.\" HTML <a href="#stackcontrol">
102	.\" </a>
103	"Controlling the JIT stack"
104	.\"
105	below.
106	.P
107	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	.P
115	There are some \fBpcre_exec()\fP options that are not supported for JIT
116	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	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	.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	JIT compilation was successful. A result of 0 means that JIT support is not
134	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	.P
137	Once a pattern has been studied, with or without JIT, it can be used as many
138	times as you like for matching different subject strings.
139	.
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	PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,
146	PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT.
147	.P
148	The unsupported pattern items are:
149	.sp
150	\eC match a single byte; not supported in UTF-8 mode
151	(?Cn) callouts
152	(*COMMIT) )
153	(*MARK) )
154	(*PRUNE) ) the backtracking control verbs
155	(*SKIP) )
156	(*THEN) )
157	.sp
158	Support for some of these may be added in future.
159	.
160	.
161	.SH "RETURN VALUES FROM JIT EXECUTION"
162	.rs
163	.sp
164	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	for the JIT stack was insufficient. See
168	.\" HTML <a href="#stackcontrol">
169	.\" </a>
170	"Controlling the JIT stack"
171	.\"
172	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	.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	The code that is generated by the JIT compiler is architecture-specific, and is
187	also position dependent. For those reasons it cannot be saved (in a file or
188	database) and restored later like the bytecode and other data of a compiled
189	pattern. Saving and restoring compiled patterns is not something many people
190	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	compilation uses significant resources, it is probably not worth doing this;
197	you might as well recompile the original pattern.
198	.
199	.
200	.\" HTML <a name="stackcontrol"></a>
201	.SH "CONTROLLING THE JIT STACK"
202	.rs
203	.sp
204	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	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	.P
216	The \fBpcre_jit_stack_alloc()\fP function creates a JIT stack. Its arguments
217	are a starting size and a maximum size, and it returns a pointer to an opaque
218	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	.P
223	JIT uses far less memory for recursion than the interpretive code,
224	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	should use. Its arguments are as follows:
229	.sp
230	pcre_extra *extra
231	pcre_jit_callback callback
232	void *data
233	.sp
234	The \fIextra\fP argument must be the result of studying a pattern with
235	PCRE_STUDY_JIT_COMPILE etc. There are three cases for the values of the other
236	two options:
237	.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	(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	.sp
251	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	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	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	.sp
272	During thread exit
273	pcre_jit_stack_free(thread_local_var)
274	.sp
275	Use a one-line callback function
276	return thread_local_var
277	.sp
278	All the functions described in this section do nothing if JIT is not available,
279	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	successful study with PCRE_STUDY_JIT_COMPILE etc.
282	.
283	.
284	.\" 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	.SH "EXAMPLE CODE"
353	.rs
354	.sp
355	This is a single-threaded example that specifies a JIT stack without using a
356	callback.
357	.sp
358	int rc;
359	int ovector[30];
360	pcre *re;
361	pcre_extra *extra;
362	pcre_jit_stack *jit_stack;
363	.sp
364	re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
365	/* Check for errors */
366	extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
367	jit_stack = pcre_jit_stack_alloc(321024, 5121024);
368	/* Check for error (NULL) */
369	pcre_assign_jit_stack(extra, NULL, jit_stack);
370	rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
371	/* Check results */
372	pcre_free(re);
373	pcre_free_study(extra);
374	pcre_jit_stack_free(jit_stack);
375	.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	Philip Hazel (FAQ by Zoltan Herczeg)
389	University Computing Service
390	Cambridge CB2 3QH, England.
391	.fi
392	.
393	.
394	.SH REVISION
395	.rs
396	.sp
397	.nf
398	Last updated: 22 February 2012
399	Copyright (c) 1997-2012 University of Cambridge.
400	.fi