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1 <html>
2 <head>
3 <title>pcrejit specification</title>
4 </head>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <h1>pcrejit man page</h1>
7 <p>
8 Return to the <a href="index.html">PCRE index page</a>.
9 </p>
10 <p>
11 This page is part of the PCRE HTML documentation. It was generated automatically
12 from the original man page. If there is any nonsense in it, please consult the
13 man page, in case the conversion went wrong.
14 <br>
15 <ul>
16 <li><a name="TOC1" href="#SEC1">PCRE JUST-IN-TIME COMPILER SUPPORT</a>
17 <li><a name="TOC2" href="#SEC2">8-BIT, 16-BIT AND 32-BIT SUPPORT</a>
18 <li><a name="TOC3" href="#SEC3">AVAILABILITY OF JIT SUPPORT</a>
19 <li><a name="TOC4" href="#SEC4">SIMPLE USE OF JIT</a>
21 <li><a name="TOC6" href="#SEC6">RETURN VALUES FROM JIT EXECUTION</a>
23 <li><a name="TOC8" href="#SEC8">CONTROLLING THE JIT STACK</a>
24 <li><a name="TOC9" href="#SEC9">JIT STACK FAQ</a>
25 <li><a name="TOC10" href="#SEC10">EXAMPLE CODE</a>
26 <li><a name="TOC11" href="#SEC11">JIT FAST PATH API</a>
27 <li><a name="TOC12" href="#SEC12">SEE ALSO</a>
28 <li><a name="TOC13" href="#SEC13">AUTHOR</a>
29 <li><a name="TOC14" href="#SEC14">REVISION</a>
30 </ul>
31 <br><a name="SEC1" href="#TOC1">PCRE JUST-IN-TIME COMPILER SUPPORT</a><br>
32 <P>
33 Just-in-time compiling is a heavyweight optimization that can greatly speed up
34 pattern matching. However, it comes at the cost of extra processing before the
35 match is performed. Therefore, it is of most benefit when the same pattern is
36 going to be matched many times. This does not necessarily mean many calls of a
37 matching function; if the pattern is not anchored, matching attempts may take
38 place many times at various positions in the subject, even for a single call.
39 Therefore, if the subject string is very long, it may still pay to use JIT for
40 one-off matches.
41 </P>
42 <P>
43 JIT support applies only to the traditional Perl-compatible matching function.
44 It does not apply when the DFA matching function is being used. The code for
45 this support was written by Zoltan Herczeg.
46 </P>
47 <br><a name="SEC2" href="#TOC1">8-BIT, 16-BIT AND 32-BIT SUPPORT</a><br>
48 <P>
49 JIT support is available for all of the 8-bit, 16-bit and 32-bit PCRE
50 libraries. To keep this documentation simple, only the 8-bit interface is
51 described in what follows. If you are using the 16-bit library, substitute the
52 16-bit functions and 16-bit structures (for example, <i>pcre16_jit_stack</i>
53 instead of <i>pcre_jit_stack</i>). If you are using the 32-bit library,
54 substitute the 32-bit functions and 32-bit structures (for example,
55 <i>pcre32_jit_stack</i> instead of <i>pcre_jit_stack</i>).
56 </P>
57 <br><a name="SEC3" href="#TOC1">AVAILABILITY OF JIT SUPPORT</a><br>
58 <P>
59 JIT support is an optional feature of PCRE. The "configure" option --enable-jit
60 (or equivalent CMake option) must be set when PCRE is built if you want to use
61 JIT. The support is limited to the following hardware platforms:
62 <pre>
63 ARM v5, v7, and Thumb2
64 Intel x86 32-bit and 64-bit
65 MIPS 32-bit
66 Power PC 32-bit and 64-bit
67 SPARC 32-bit (experimental)
68 </pre>
69 If --enable-jit is set on an unsupported platform, compilation fails.
70 </P>
71 <P>
72 A program that is linked with PCRE 8.20 or later can tell if JIT support is
73 available by calling <b>pcre_config()</b> with the PCRE_CONFIG_JIT option. The
74 result is 1 when JIT is available, and 0 otherwise. However, a simple program
75 does not need to check this in order to use JIT. The normal API is implemented
76 in a way that falls back to the interpretive code if JIT is not available. For
77 programs that need the best possible performance, there is also a "fast path"
78 API that is JIT-specific.
79 </P>
80 <P>
81 If your program may sometimes be linked with versions of PCRE that are older
82 than 8.20, but you want to use JIT when it is available, you can test
83 the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT macro such
84 as PCRE_CONFIG_JIT, for compile-time control of your code.
85 </P>
86 <br><a name="SEC4" href="#TOC1">SIMPLE USE OF JIT</a><br>
87 <P>
88 You have to do two things to make use of the JIT support in the simplest way:
89 <pre>
90 (1) Call <b>pcre_study()</b> with the PCRE_STUDY_JIT_COMPILE option for
91 each compiled pattern, and pass the resulting <b>pcre_extra</b> block to
92 <b>pcre_exec()</b>.
94 (2) Use <b>pcre_free_study()</b> to free the <b>pcre_extra</b> block when it is
95 no longer needed, instead of just freeing it yourself. This ensures that
96 any JIT data is also freed.
97 </pre>
98 For a program that may be linked with pre-8.20 versions of PCRE, you can insert
99 <pre>
102 #endif
103 </pre>
104 so that no option is passed to <b>pcre_study()</b>, and then use something like
105 this to free the study data:
106 <pre>
107 #ifdef PCRE_CONFIG_JIT
108 pcre_free_study(study_ptr);
109 #else
110 pcre_free(study_ptr);
111 #endif
112 </pre>
113 PCRE_STUDY_JIT_COMPILE requests the JIT compiler to generate code for complete
114 matches. If you want to run partial matches using the PCRE_PARTIAL_HARD or
115 PCRE_PARTIAL_SOFT options of <b>pcre_exec()</b>, you should set one or both of
116 the following options in addition to, or instead of, PCRE_STUDY_JIT_COMPILE
117 when you call <b>pcre_study()</b>:
118 <pre>
121 </pre>
122 The JIT compiler generates different optimized code for each of the three
123 modes (normal, soft partial, hard partial). When <b>pcre_exec()</b> is called,
124 the appropriate code is run if it is available. Otherwise, the pattern is
125 matched using interpretive code.
126 </P>
127 <P>
128 In some circumstances you may need to call additional functions. These are
129 described in the section entitled
130 <a href="#stackcontrol">"Controlling the JIT stack"</a>
131 below.
132 </P>
133 <P>
134 If JIT support is not available, PCRE_STUDY_JIT_COMPILE etc. are ignored, and
135 no JIT data is created. Otherwise, the compiled pattern is passed to the JIT
136 compiler, which turns it into machine code that executes much faster than the
137 normal interpretive code. When <b>pcre_exec()</b> is passed a <b>pcre_extra</b>
138 block containing a pointer to JIT code of the appropriate mode (normal or
139 hard/soft partial), it obeys that code instead of running the interpreter. The
140 result is identical, but the compiled JIT code runs much faster.
141 </P>
142 <P>
143 There are some <b>pcre_exec()</b> options that are not supported for JIT
144 execution. There are also some pattern items that JIT cannot handle. Details
145 are given below. In both cases, execution automatically falls back to the
146 interpretive code. If you want to know whether JIT was actually used for a
147 particular match, you should arrange for a JIT callback function to be set up
148 as described in the section entitled
149 <a href="#stackcontrol">"Controlling the JIT stack"</a>
150 below, even if you do not need to supply a non-default JIT stack. Such a
151 callback function is called whenever JIT code is about to be obeyed. If the
152 execution options are not right for JIT execution, the callback function is not
153 obeyed.
154 </P>
155 <P>
156 If the JIT compiler finds an unsupported item, no JIT data is generated. You
157 can find out if JIT execution is available after studying a pattern by calling
158 <b>pcre_fullinfo()</b> with the PCRE_INFO_JIT option. A result of 1 means that
159 JIT compilation was successful. A result of 0 means that JIT support is not
160 available, or the pattern was not studied with PCRE_STUDY_JIT_COMPILE etc., or
161 the JIT compiler was not able to handle the pattern.
162 </P>
163 <P>
164 Once a pattern has been studied, with or without JIT, it can be used as many
165 times as you like for matching different subject strings.
166 </P>
167 <br><a name="SEC5" href="#TOC1">UNSUPPORTED OPTIONS AND PATTERN ITEMS</a><br>
168 <P>
169 The only <b>pcre_exec()</b> options that are supported for JIT execution are
173 </P>
174 <P>
175 The unsupported pattern items are:
176 <pre>
177 \C match a single byte; not supported in UTF-8 mode
178 (?Cn) callouts
179 (*PRUNE) )
180 (*SKIP) ) backtracking control verbs
181 (*THEN) )
182 </pre>
183 Support for some of these may be added in future.
184 </P>
185 <br><a name="SEC6" href="#TOC1">RETURN VALUES FROM JIT EXECUTION</a><br>
186 <P>
187 When a pattern is matched using JIT execution, the return values are the same
188 as those given by the interpretive <b>pcre_exec()</b> code, with the addition of
189 one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means that the memory used
190 for the JIT stack was insufficient. See
191 <a href="#stackcontrol">"Controlling the JIT stack"</a>
192 below for a discussion of JIT stack usage. For compatibility with the
193 interpretive <b>pcre_exec()</b> code, no more than two-thirds of the
194 <i>ovector</i> argument is used for passing back captured substrings.
195 </P>
196 <P>
197 The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if searching a
198 very large pattern tree goes on for too long, as it is in the same circumstance
199 when JIT is not used, but the details of exactly what is counted are not the
200 same. The PCRE_ERROR_RECURSIONLIMIT error code is never returned by JIT
201 execution.
202 </P>
203 <br><a name="SEC7" href="#TOC1">SAVING AND RESTORING COMPILED PATTERNS</a><br>
204 <P>
205 The code that is generated by the JIT compiler is architecture-specific, and is
206 also position dependent. For those reasons it cannot be saved (in a file or
207 database) and restored later like the bytecode and other data of a compiled
208 pattern. Saving and restoring compiled patterns is not something many people
209 do. More detail about this facility is given in the
210 <a href="pcreprecompile.html"><b>pcreprecompile</b></a>
211 documentation. It should be possible to run <b>pcre_study()</b> on a saved and
212 restored pattern, and thereby recreate the JIT data, but because JIT
213 compilation uses significant resources, it is probably not worth doing this;
214 you might as well recompile the original pattern.
215 <a name="stackcontrol"></a></P>
216 <br><a name="SEC8" href="#TOC1">CONTROLLING THE JIT STACK</a><br>
217 <P>
218 When the compiled JIT code runs, it needs a block of memory to use as a stack.
219 By default, it uses 32K on the machine stack. However, some large or
220 complicated patterns need more than this. The error PCRE_ERROR_JIT_STACKLIMIT
221 is given when there is not enough stack. Three functions are provided for
222 managing blocks of memory for use as JIT stacks. There is further discussion
223 about the use of JIT stacks in the section entitled
224 <a href="#stackcontrol">"JIT stack FAQ"</a>
225 below.
226 </P>
227 <P>
228 The <b>pcre_jit_stack_alloc()</b> function creates a JIT stack. Its arguments
229 are a starting size and a maximum size, and it returns a pointer to an opaque
230 structure of type <b>pcre_jit_stack</b>, or NULL if there is an error. The
231 <b>pcre_jit_stack_free()</b> function can be used to free a stack that is no
232 longer needed. (For the technically minded: the address space is allocated by
233 mmap or VirtualAlloc.)
234 </P>
235 <P>
236 JIT uses far less memory for recursion than the interpretive code,
237 and a maximum stack size of 512K to 1M should be more than enough for any
238 pattern.
239 </P>
240 <P>
241 The <b>pcre_assign_jit_stack()</b> function specifies which stack JIT code
242 should use. Its arguments are as follows:
243 <pre>
244 pcre_extra *extra
245 pcre_jit_callback callback
246 void *data
247 </pre>
248 The <i>extra</i> argument must be the result of studying a pattern with
249 PCRE_STUDY_JIT_COMPILE etc. There are three cases for the values of the other
250 two options:
251 <pre>
252 (1) If <i>callback</i> is NULL and <i>data</i> is NULL, an internal 32K block
253 on the machine stack is used.
255 (2) If <i>callback</i> is NULL and <i>data</i> is not NULL, <i>data</i> must be
256 a valid JIT stack, the result of calling <b>pcre_jit_stack_alloc()</b>.
258 (3) If <i>callback</i> is not NULL, it must point to a function that is
259 called with <i>data</i> as an argument at the start of matching, in
260 order to set up a JIT stack. If the return from the callback
261 function is NULL, the internal 32K stack is used; otherwise the
262 return value must be a valid JIT stack, the result of calling
263 <b>pcre_jit_stack_alloc()</b>.
264 </pre>
265 A callback function is obeyed whenever JIT code is about to be run; it is not
266 obeyed when <b>pcre_exec()</b> is called with options that are incompatible for
267 JIT execution. A callback function can therefore be used to determine whether a
268 match operation was executed by JIT or by the interpreter.
269 </P>
270 <P>
271 You may safely use the same JIT stack for more than one pattern (either by
272 assigning directly or by callback), as long as the patterns are all matched
273 sequentially in the same thread. In a multithread application, if you do not
274 specify a JIT stack, or if you assign or pass back NULL from a callback, that
275 is thread-safe, because each thread has its own machine stack. However, if you
276 assign or pass back a non-NULL JIT stack, this must be a different stack for
277 each thread so that the application is thread-safe.
278 </P>
279 <P>
280 Strictly speaking, even more is allowed. You can assign the same non-NULL stack
281 to any number of patterns as long as they are not used for matching by multiple
282 threads at the same time. For example, you can assign the same stack to all
283 compiled patterns, and use a global mutex in the callback to wait until the
284 stack is available for use. However, this is an inefficient solution, and not
285 recommended.
286 </P>
287 <P>
288 This is a suggestion for how a multithreaded program that needs to set up
289 non-default JIT stacks might operate:
290 <pre>
291 During thread initalization
292 thread_local_var = pcre_jit_stack_alloc(...)
294 During thread exit
295 pcre_jit_stack_free(thread_local_var)
297 Use a one-line callback function
298 return thread_local_var
299 </pre>
300 All the functions described in this section do nothing if JIT is not available,
301 and <b>pcre_assign_jit_stack()</b> does nothing unless the <b>extra</b> argument
302 is non-NULL and points to a <b>pcre_extra</b> block that is the result of a
303 successful study with PCRE_STUDY_JIT_COMPILE etc.
304 <a name="stackfaq"></a></P>
305 <br><a name="SEC9" href="#TOC1">JIT STACK FAQ</a><br>
306 <P>
307 (1) Why do we need JIT stacks?
308 <br>
309 <br>
310 PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack where
311 the local data of the current node is pushed before checking its child nodes.
312 Allocating real machine stack on some platforms is difficult. For example, the
313 stack chain needs to be updated every time if we extend the stack on PowerPC.
314 Although it is possible, its updating time overhead decreases performance. So
315 we do the recursion in memory.
316 </P>
317 <P>
318 (2) Why don't we simply allocate blocks of memory with <b>malloc()</b>?
319 <br>
320 <br>
321 Modern operating systems have a nice feature: they can reserve an address space
322 instead of allocating memory. We can safely allocate memory pages inside this
323 address space, so the stack could grow without moving memory data (this is
324 important because of pointers). Thus we can allocate 1M address space, and use
325 only a single memory page (usually 4K) if that is enough. However, we can still
326 grow up to 1M anytime if needed.
327 </P>
328 <P>
329 (3) Who "owns" a JIT stack?
330 <br>
331 <br>
332 The owner of the stack is the user program, not the JIT studied pattern or
333 anything else. The user program must ensure that if a stack is used by
334 <b>pcre_exec()</b>, (that is, it is assigned to the pattern currently running),
335 that stack must not be used by any other threads (to avoid overwriting the same
336 memory area). The best practice for multithreaded programs is to allocate a
337 stack for each thread, and return this stack through the JIT callback function.
338 </P>
339 <P>
340 (4) When should a JIT stack be freed?
341 <br>
342 <br>
343 You can free a JIT stack at any time, as long as it will not be used by
344 <b>pcre_exec()</b> again. When you assign the stack to a pattern, only a pointer
345 is set. There is no reference counting or any other magic. You can free the
346 patterns and stacks in any order, anytime. Just <i>do not</i> call
347 <b>pcre_exec()</b> with a pattern pointing to an already freed stack, as that
348 will cause SEGFAULT. (Also, do not free a stack currently used by
349 <b>pcre_exec()</b> in another thread). You can also replace the stack for a
350 pattern at any time. You can even free the previous stack before assigning a
351 replacement.
352 </P>
353 <P>
354 (5) Should I allocate/free a stack every time before/after calling
355 <b>pcre_exec()</b>?
356 <br>
357 <br>
358 No, because this is too costly in terms of resources. However, you could
359 implement some clever idea which release the stack if it is not used in let's
360 say two minutes. The JIT callback can help to achieve this without keeping a
361 list of the currently JIT studied patterns.
362 </P>
363 <P>
364 (6) OK, the stack is for long term memory allocation. But what happens if a
365 pattern causes stack overflow with a stack of 1M? Is that 1M kept until the
366 stack is freed?
367 <br>
368 <br>
369 Especially on embedded sytems, it might be a good idea to release memory
370 sometimes without freeing the stack. There is no API for this at the moment.
371 Probably a function call which returns with the currently allocated memory for
372 any stack and another which allows releasing memory (shrinking the stack) would
373 be a good idea if someone needs this.
374 </P>
375 <P>
376 (7) This is too much of a headache. Isn't there any better solution for JIT
377 stack handling?
378 <br>
379 <br>
380 No, thanks to Windows. If POSIX threads were used everywhere, we could throw
381 out this complicated API.
382 </P>
383 <br><a name="SEC10" href="#TOC1">EXAMPLE CODE</a><br>
384 <P>
385 This is a single-threaded example that specifies a JIT stack without using a
386 callback.
387 <pre>
388 int rc;
389 int ovector[30];
390 pcre *re;
391 pcre_extra *extra;
392 pcre_jit_stack *jit_stack;
394 re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
395 /* Check for errors */
396 extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
397 jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024);
398 /* Check for error (NULL) */
399 pcre_assign_jit_stack(extra, NULL, jit_stack);
400 rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
401 /* Check results */
402 pcre_free(re);
403 pcre_free_study(extra);
404 pcre_jit_stack_free(jit_stack);
406 </PRE>
407 </P>
408 <br><a name="SEC11" href="#TOC1">JIT FAST PATH API</a><br>
409 <P>
410 Because the API described above falls back to interpreted execution when JIT is
411 not available, it is convenient for programs that are written for general use
412 in many environments. However, calling JIT via <b>pcre_exec()</b> does have a
413 performance impact. Programs that are written for use where JIT is known to be
414 available, and which need the best possible performance, can instead use a
415 "fast path" API to call JIT execution directly instead of calling
416 <b>pcre_exec()</b> (obviously only for patterns that have been successfully
417 studied by JIT).
418 </P>
419 <P>
420 The fast path function is called <b>pcre_jit_exec()</b>, and it takes exactly
421 the same arguments as <b>pcre_exec()</b>, plus one additional argument that
422 must point to a JIT stack. The JIT stack arrangements described above do not
423 apply. The return values are the same as for <b>pcre_exec()</b>.
424 </P>
425 <P>
426 When you call <b>pcre_exec()</b>, as well as testing for invalid options, a
427 number of other sanity checks are performed on the arguments. For example, if
428 the subject pointer is NULL, or its length is negative, an immediate error is
429 given. Also, unless PCRE_NO_UTF[8|16|32] is set, a UTF subject string is tested
430 for validity. In the interests of speed, these checks do not happen on the JIT
431 fast path, and if invalid data is passed, the result is undefined.
432 </P>
433 <P>
434 Bypassing the sanity checks and the <b>pcre_exec()</b> wrapping can give
435 speedups of more than 10%.
436 </P>
437 <br><a name="SEC12" href="#TOC1">SEE ALSO</a><br>
438 <P>
439 <b>pcreapi</b>(3)
440 </P>
441 <br><a name="SEC13" href="#TOC1">AUTHOR</a><br>
442 <P>
443 Philip Hazel (FAQ by Zoltan Herczeg)
444 <br>
445 University Computing Service
446 <br>
447 Cambridge CB2 3QH, England.
448 <br>
449 </P>
450 <br><a name="SEC14" href="#TOC1">REVISION</a><br>
451 <P>
452 Last updated: 31 October 2012
453 <br>
454 Copyright &copy; 1997-2012 University of Cambridge.
455 <br>
456 <p>
457 Return to the <a href="index.html">PCRE index page</a>.
458 </p>

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