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1 .TH PCREJIT 3 "04 May 2012" "PCRE 8.31"
3 PCRE - Perl-compatible regular expressions
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 .
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 .
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
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
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
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 .
142 .rs
143 .sp
144 The only \fBpcre_exec()\fP options that are supported for JIT execution are
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 (*PRUNE) )
153 (*SKIP) ) backtracking control verbs
154 (*THEN) )
155 .sp
156 Support for some of these may be added in future.
157 .
158 .
160 .rs
161 .sp
162 When a pattern is matched using JIT execution, the return values are the same
163 as those given by the interpretive \fBpcre_exec()\fP code, with the addition of
164 one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means that the memory used
165 for the JIT stack was insufficient. See
166 .\" HTML <a href="#stackcontrol">
167 .\" </a>
168 "Controlling the JIT stack"
169 .\"
170 below for a discussion of JIT stack usage. For compatibility with the
171 interpretive \fBpcre_exec()\fP code, no more than two-thirds of the
172 \fIovector\fP argument is used for passing back captured substrings.
173 .P
174 The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if searching a
175 very large pattern tree goes on for too long, as it is in the same circumstance
176 when JIT is not used, but the details of exactly what is counted are not the
177 same. The PCRE_ERROR_RECURSIONLIMIT error code is never returned by JIT
178 execution.
179 .
180 .
182 .rs
183 .sp
184 The code that is generated by the JIT compiler is architecture-specific, and is
185 also position dependent. For those reasons it cannot be saved (in a file or
186 database) and restored later like the bytecode and other data of a compiled
187 pattern. Saving and restoring compiled patterns is not something many people
188 do. More detail about this facility is given in the
189 .\" HREF
190 \fBpcreprecompile\fP
191 .\"
192 documentation. It should be possible to run \fBpcre_study()\fP on a saved and
193 restored pattern, and thereby recreate the JIT data, but because JIT
194 compilation uses significant resources, it is probably not worth doing this;
195 you might as well recompile the original pattern.
196 .
197 .
198 .\" HTML <a name="stackcontrol"></a>
200 .rs
201 .sp
202 When the compiled JIT code runs, it needs a block of memory to use as a stack.
203 By default, it uses 32K on the machine stack. However, some large or
204 complicated patterns need more than this. The error PCRE_ERROR_JIT_STACKLIMIT
205 is given when there is not enough stack. Three functions are provided for
206 managing blocks of memory for use as JIT stacks. There is further discussion
207 about the use of JIT stacks in the section entitled
208 .\" HTML <a href="#stackcontrol">
209 .\" </a>
210 "JIT stack FAQ"
211 .\"
212 below.
213 .P
214 The \fBpcre_jit_stack_alloc()\fP function creates a JIT stack. Its arguments
215 are a starting size and a maximum size, and it returns a pointer to an opaque
216 structure of type \fBpcre_jit_stack\fP, or NULL if there is an error. The
217 \fBpcre_jit_stack_free()\fP function can be used to free a stack that is no
218 longer needed. (For the technically minded: the address space is allocated by
219 mmap or VirtualAlloc.)
220 .P
221 JIT uses far less memory for recursion than the interpretive code,
222 and a maximum stack size of 512K to 1M should be more than enough for any
223 pattern.
224 .P
225 The \fBpcre_assign_jit_stack()\fP function specifies which stack JIT code
226 should use. Its arguments are as follows:
227 .sp
228 pcre_extra *extra
229 pcre_jit_callback callback
230 void *data
231 .sp
232 The \fIextra\fP argument must be the result of studying a pattern with
233 PCRE_STUDY_JIT_COMPILE etc. There are three cases for the values of the other
234 two options:
235 .sp
236 (1) If \fIcallback\fP is NULL and \fIdata\fP is NULL, an internal 32K block
237 on the machine stack is used.
238 .sp
239 (2) If \fIcallback\fP is NULL and \fIdata\fP is not NULL, \fIdata\fP must be
240 a valid JIT stack, the result of calling \fBpcre_jit_stack_alloc()\fP.
241 .sp
242 (3) If \fIcallback\fP is not NULL, it must point to a function that is
243 called with \fIdata\fP as an argument at the start of matching, in
244 order to set up a JIT stack. If the return from the callback
245 function is NULL, the internal 32K stack is used; otherwise the
246 return value must be a valid JIT stack, the result of calling
247 \fBpcre_jit_stack_alloc()\fP.
248 .sp
249 A callback function is obeyed whenever JIT code is about to be run; it is not
250 obeyed when \fBpcre_exec()\fP is called with options that are incompatible for
251 JIT execution. A callback function can therefore be used to determine whether a
252 match operation was executed by JIT or by the interpreter.
253 .P
254 You may safely use the same JIT stack for more than one pattern (either by
255 assigning directly or by callback), as long as the patterns are all matched
256 sequentially in the same thread. In a multithread application, if you do not
257 specify a JIT stack, or if you assign or pass back NULL from a callback, that
258 is thread-safe, because each thread has its own machine stack. However, if you
259 assign or pass back a non-NULL JIT stack, this must be a different stack for
260 each thread so that the application is thread-safe.
261 .P
262 Strictly speaking, even more is allowed. You can assign the same non-NULL stack
263 to any number of patterns as long as they are not used for matching by multiple
264 threads at the same time. For example, you can assign the same stack to all
265 compiled patterns, and use a global mutex in the callback to wait until the
266 stack is available for use. However, this is an inefficient solution, and not
267 recommended.
268 .P
269 This is a suggestion for how a multithreaded program that needs to set up
270 non-default JIT stacks might operate:
271 .sp
272 During thread initalization
273 thread_local_var = pcre_jit_stack_alloc(...)
274 .sp
275 During thread exit
276 pcre_jit_stack_free(thread_local_var)
277 .sp
278 Use a one-line callback function
279 return thread_local_var
280 .sp
281 All the functions described in this section do nothing if JIT is not available,
282 and \fBpcre_assign_jit_stack()\fP does nothing unless the \fBextra\fP argument
283 is non-NULL and points to a \fBpcre_extra\fP block that is the result of a
284 successful study with PCRE_STUDY_JIT_COMPILE etc.
285 .
286 .
287 .\" HTML <a name="stackfaq"></a>
289 .rs
290 .sp
291 (1) Why do we need JIT stacks?
292 .sp
293 PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack where
294 the local data of the current node is pushed before checking its child nodes.
295 Allocating real machine stack on some platforms is difficult. For example, the
296 stack chain needs to be updated every time if we extend the stack on PowerPC.
297 Although it is possible, its updating time overhead decreases performance. So
298 we do the recursion in memory.
299 .P
300 (2) Why don't we simply allocate blocks of memory with \fBmalloc()\fP?
301 .sp
302 Modern operating systems have a nice feature: they can reserve an address space
303 instead of allocating memory. We can safely allocate memory pages inside this
304 address space, so the stack could grow without moving memory data (this is
305 important because of pointers). Thus we can allocate 1M address space, and use
306 only a single memory page (usually 4K) if that is enough. However, we can still
307 grow up to 1M anytime if needed.
308 .P
309 (3) Who "owns" a JIT stack?
310 .sp
311 The owner of the stack is the user program, not the JIT studied pattern or
312 anything else. The user program must ensure that if a stack is used by
313 \fBpcre_exec()\fP, (that is, it is assigned to the pattern currently running),
314 that stack must not be used by any other threads (to avoid overwriting the same
315 memory area). The best practice for multithreaded programs is to allocate a
316 stack for each thread, and return this stack through the JIT callback function.
317 .P
318 (4) When should a JIT stack be freed?
319 .sp
320 You can free a JIT stack at any time, as long as it will not be used by
321 \fBpcre_exec()\fP again. When you assign the stack to a pattern, only a pointer
322 is set. There is no reference counting or any other magic. You can free the
323 patterns and stacks in any order, anytime. Just \fIdo not\fP call
324 \fBpcre_exec()\fP with a pattern pointing to an already freed stack, as that
325 will cause SEGFAULT. (Also, do not free a stack currently used by
326 \fBpcre_exec()\fP in another thread). You can also replace the stack for a
327 pattern at any time. You can even free the previous stack before assigning a
328 replacement.
329 .P
330 (5) Should I allocate/free a stack every time before/after calling
331 \fBpcre_exec()\fP?
332 .sp
333 No, because this is too costly in terms of resources. However, you could
334 implement some clever idea which release the stack if it is not used in let's
335 say two minutes. The JIT callback can help to achive this without keeping a
336 list of the currently JIT studied patterns.
337 .P
338 (6) OK, the stack is for long term memory allocation. But what happens if a
339 pattern causes stack overflow with a stack of 1M? Is that 1M kept until the
340 stack is freed?
341 .sp
342 Especially on embedded sytems, it might be a good idea to release memory
343 sometimes without freeing the stack. There is no API for this at the moment.
344 Probably a function call which returns with the currently allocated memory for
345 any stack and another which allows releasing memory (shrinking the stack) would
346 be a good idea if someone needs this.
347 .P
348 (7) This is too much of a headache. Isn't there any better solution for JIT
349 stack handling?
350 .sp
351 No, thanks to Windows. If POSIX threads were used everywhere, we could throw
352 out this complicated API.
353 .
354 .
356 .rs
357 .sp
358 This is a single-threaded example that specifies a JIT stack without using a
359 callback.
360 .sp
361 int rc;
362 int ovector[30];
363 pcre *re;
364 pcre_extra *extra;
365 pcre_jit_stack *jit_stack;
366 .sp
367 re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
368 /* Check for errors */
369 extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
370 jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024);
371 /* Check for error (NULL) */
372 pcre_assign_jit_stack(extra, NULL, jit_stack);
373 rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
374 /* Check results */
375 pcre_free(re);
376 pcre_free_study(extra);
377 pcre_jit_stack_free(jit_stack);
378 .sp
379 .
380 .
381 .SH "SEE ALSO"
382 .rs
383 .sp
384 \fBpcreapi\fP(3)
385 .
386 .
388 .rs
389 .sp
390 .nf
391 Philip Hazel (FAQ by Zoltan Herczeg)
392 University Computing Service
393 Cambridge CB2 3QH, England.
394 .fi
395 .
396 .
398 .rs
399 .sp
400 .nf
401 Last updated: 04 May 2012
402 Copyright (c) 1997-2012 University of Cambridge.
403 .fi

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