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1 ph10 678 <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 ph10 691 <li><a name="TOC1" href="#SEC1">PCRE JUST-IN-TIME COMPILER SUPPORT</a>
17 ph10 869 <li><a name="TOC2" href="#SEC2">8-BIT and 16-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>
20     <li><a name="TOC5" href="#SEC5">UNSUPPORTED OPTIONS AND PATTERN ITEMS</a>
21     <li><a name="TOC6" href="#SEC6">RETURN VALUES FROM JIT EXECUTION</a>
22     <li><a name="TOC7" href="#SEC7">SAVING AND RESTORING COMPILED PATTERNS</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">SEE ALSO</a>
27     <li><a name="TOC12" href="#SEC12">AUTHOR</a>
28     <li><a name="TOC13" href="#SEC13">REVISION</a>
29 ph10 678 </ul>
30 ph10 691 <br><a name="SEC1" href="#TOC1">PCRE JUST-IN-TIME COMPILER SUPPORT</a><br>
31     <P>
32     Just-in-time compiling is a heavyweight optimization that can greatly speed up
33     pattern matching. However, it comes at the cost of extra processing before the
34     match is performed. Therefore, it is of most benefit when the same pattern is
35 ph10 869 going to be matched many times. This does not necessarily mean many calls of a
36     matching function; if the pattern is not anchored, matching attempts may take
37     place many times at various positions in the subject, even for a single call.
38     Therefore, if the subject string is very long, it may still pay to use JIT for
39     one-off matches.
40 ph10 691 </P>
41     <P>
42 ph10 869 JIT support applies only to the traditional Perl-compatible matching function.
43     It does not apply when the DFA matching function is being used. The code for
44     this support was written by Zoltan Herczeg.
45 ph10 691 </P>
46 ph10 869 <br><a name="SEC2" href="#TOC1">8-BIT and 16-BIT SUPPORT</a><br>
47 ph10 691 <P>
48 ph10 869 JIT support is available for both the 8-bit and 16-bit PCRE libraries. To keep
49     this documentation simple, only the 8-bit interface is described in what
50     follows. If you are using the 16-bit library, substitute the 16-bit functions
51     and 16-bit structures (for example, <i>pcre16_jit_stack</i> instead of
52     <i>pcre_jit_stack</i>).
53     </P>
54     <br><a name="SEC3" href="#TOC1">AVAILABILITY OF JIT SUPPORT</a><br>
55     <P>
56 ph10 691 JIT support is an optional feature of PCRE. The "configure" option --enable-jit
57     (or equivalent CMake option) must be set when PCRE is built if you want to use
58     JIT. The support is limited to the following hardware platforms:
59     <pre>
60     ARM v5, v7, and Thumb2
61     Intel x86 32-bit and 64-bit
62     MIPS 32-bit
63 ph10 869 Power PC 32-bit and 64-bit
64 ph10 691 </pre>
65 ph10 733 The Power PC support is designated as experimental because it has not been
66     fully tested. If --enable-jit is set on an unsupported platform, compilation
67     fails.
68 ph10 691 </P>
69     <P>
70 ph10 836 A program that is linked with PCRE 8.20 or later can tell if JIT support is
71     available by calling <b>pcre_config()</b> with the PCRE_CONFIG_JIT option. The
72     result is 1 when JIT is available, and 0 otherwise. However, a simple program
73     does not need to check this in order to use JIT. The API is implemented in a
74     way that falls back to the ordinary PCRE code if JIT is not available.
75 ph10 691 </P>
76 ph10 836 <P>
77     If your program may sometimes be linked with versions of PCRE that are older
78     than 8.20, but you want to use JIT when it is available, you can test
79     the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT macro such
80     as PCRE_CONFIG_JIT, for compile-time control of your code.
81     </P>
82 ph10 869 <br><a name="SEC4" href="#TOC1">SIMPLE USE OF JIT</a><br>
83 ph10 691 <P>
84     You have to do two things to make use of the JIT support in the simplest way:
85     <pre>
86     (1) Call <b>pcre_study()</b> with the PCRE_STUDY_JIT_COMPILE option for
87     each compiled pattern, and pass the resulting <b>pcre_extra</b> block to
88     <b>pcre_exec()</b>.
90     (2) Use <b>pcre_free_study()</b> to free the <b>pcre_extra</b> block when it is
91     no longer needed instead of just freeing it yourself. This
92     ensures that any JIT data is also freed.
93     </pre>
94 ph10 836 For a program that may be linked with pre-8.20 versions of PCRE, you can insert
95     <pre>
97     #define PCRE_STUDY_JIT_COMPILE 0
98     #endif
99     </pre>
100     so that no option is passed to <b>pcre_study()</b>, and then use something like
101     this to free the study data:
102     <pre>
103     #ifdef PCRE_CONFIG_JIT
104     pcre_free_study(study_ptr);
105     #else
106     pcre_free(study_ptr);
107     #endif
108     </pre>
109 ph10 691 In some circumstances you may need to call additional functions. These are
110     described in the section entitled
111     <a href="#stackcontrol">"Controlling the JIT stack"</a>
112     below.
113     </P>
114     <P>
115     If JIT support is not available, PCRE_STUDY_JIT_COMPILE is ignored, and no JIT
116     data is set up. Otherwise, the compiled pattern is passed to the JIT compiler,
117     which turns it into machine code that executes much faster than the normal
118     interpretive code. When <b>pcre_exec()</b> is passed a <b>pcre_extra</b> block
119     containing a pointer to JIT code, it obeys that instead of the normal code. The
120     result is identical, but the code runs much faster.
121     </P>
122     <P>
123     There are some <b>pcre_exec()</b> options that are not supported for JIT
124     execution. There are also some pattern items that JIT cannot handle. Details
125     are given below. In both cases, execution automatically falls back to the
126     interpretive code.
127     </P>
128     <P>
129     If the JIT compiler finds an unsupported item, no JIT data is generated. You
130     can find out if JIT execution is available after studying a pattern by calling
131     <b>pcre_fullinfo()</b> with the PCRE_INFO_JIT option. A result of 1 means that
132 ph10 708 JIT compilation was successful. A result of 0 means that JIT support is not
133 ph10 691 available, or the pattern was not studied with PCRE_STUDY_JIT_COMPILE, or the
134     JIT compiler was not able to handle the pattern.
135     </P>
136 ph10 708 <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     </P>
140 ph10 869 <br><a name="SEC5" href="#TOC1">UNSUPPORTED OPTIONS AND PATTERN ITEMS</a><br>
141 ph10 691 <P>
142     The only <b>pcre_exec()</b> options that are supported for JIT execution are
144     PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not
145     supported.
146     </P>
147     <P>
148     The unsupported pattern items are:
149     <pre>
150 ph10 836 \C match a single byte; not supported in UTF-8 mode
151 ph10 691 (?Cn) callouts
152     (*COMMIT) )
153     (*MARK) )
154     (*PRUNE) ) the backtracking control verbs
155     (*SKIP) )
156     (*THEN) )
157     </pre>
158     Support for some of these may be added in future.
159     </P>
160 ph10 869 <br><a name="SEC6" href="#TOC1">RETURN VALUES FROM JIT EXECUTION</a><br>
161 ph10 691 <P>
162     When a pattern is matched using JIT execution, the return values are the same
163     as those given by the interpretive <b>pcre_exec()</b> 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     <a href="#stackcontrol">"Controlling the JIT stack"</a>
167     below for a discussion of JIT stack usage. For compatibility with the
168     interpretive <b>pcre_exec()</b> code, no more than two-thirds of the
169     <i>ovector</i> argument is used for passing back captured substrings.
170     </P>
171     <P>
172     The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if searching a
173     very large pattern tree goes on for too long, as it is in the same circumstance
174     when JIT is not used, but the details of exactly what is counted are not the
175     same. The PCRE_ERROR_RECURSIONLIMIT error code is never returned by JIT
176     execution.
177     </P>
178 ph10 869 <br><a name="SEC7" href="#TOC1">SAVING AND RESTORING COMPILED PATTERNS</a><br>
179 ph10 691 <P>
180     The code that is generated by the JIT compiler is architecture-specific, and is
181 ph10 708 also position dependent. For those reasons it cannot be saved (in a file or
182     database) and restored later like the bytecode and other data of a compiled
183     pattern. Saving and restoring compiled patterns is not something many people
184     do. More detail about this facility is given in the
185     <a href="pcreprecompile.html"><b>pcreprecompile</b></a>
186     documentation. It should be possible to run <b>pcre_study()</b> on a saved and
187     restored pattern, and thereby recreate the JIT data, but because JIT
188     compilation uses significant resources, it is probably not worth doing this;
189     you might as well recompile the original pattern.
190 ph10 691 <a name="stackcontrol"></a></P>
191 ph10 869 <br><a name="SEC8" href="#TOC1">CONTROLLING THE JIT STACK</a><br>
192 ph10 691 <P>
193     When the compiled JIT code runs, it needs a block of memory to use as a stack.
194     By default, it uses 32K on the machine stack. However, some large or
195     complicated patterns need more than this. The error PCRE_ERROR_JIT_STACKLIMIT
196     is given when there is not enough stack. Three functions are provided for
197 ph10 836 managing blocks of memory for use as JIT stacks. There is further discussion
198     about the use of JIT stacks in the section entitled
199     <a href="#stackcontrol">"JIT stack FAQ"</a>
200     below.
201 ph10 691 </P>
202     <P>
203     The <b>pcre_jit_stack_alloc()</b> function creates a JIT stack. Its arguments
204     are a starting size and a maximum size, and it returns a pointer to an opaque
205     structure of type <b>pcre_jit_stack</b>, or NULL if there is an error. The
206     <b>pcre_jit_stack_free()</b> function can be used to free a stack that is no
207     longer needed. (For the technically minded: the address space is allocated by
208     mmap or VirtualAlloc.)
209     </P>
210     <P>
211     JIT uses far less memory for recursion than the interpretive code,
212     and a maximum stack size of 512K to 1M should be more than enough for any
213     pattern.
214     </P>
215     <P>
216     The <b>pcre_assign_jit_stack()</b> function specifies which stack JIT code
217     should use. Its arguments are as follows:
218     <pre>
219     pcre_extra *extra
220     pcre_jit_callback callback
221     void *data
222     </pre>
223     The <i>extra</i> argument must be the result of studying a pattern with
224     PCRE_STUDY_JIT_COMPILE. There are three cases for the values of the other two
225     options:
226     <pre>
227     (1) If <i>callback</i> is NULL and <i>data</i> is NULL, an internal 32K block
228     on the machine stack is used.
230     (2) If <i>callback</i> is NULL and <i>data</i> is not NULL, <i>data</i> must be
231     a valid JIT stack, the result of calling <b>pcre_jit_stack_alloc()</b>.
233     (3) If <i>callback</i> not NULL, it must point to a function that is called
234     with <i>data</i> as an argument at the start of matching, in order to
235     set up a JIT stack. If the result is NULL, the internal 32K stack
236     is used; otherwise the return value must be a valid JIT stack,
237     the result of calling <b>pcre_jit_stack_alloc()</b>.
238     </pre>
239     You may safely assign the same JIT stack to more than one pattern, as long as
240     they are all matched sequentially in the same thread. In a multithread
241     application, each thread must use its own JIT stack.
242     </P>
243     <P>
244     Strictly speaking, even more is allowed. You can assign the same stack to any
245     number of patterns as long as they are not used for matching by multiple
246     threads at the same time. For example, you can assign the same stack to all
247     compiled patterns, and use a global mutex in the callback to wait until the
248     stack is available for use. However, this is an inefficient solution, and
249     not recommended.
250     </P>
251     <P>
252     This is a suggestion for how a typical multithreaded program might operate:
253     <pre>
254     During thread initalization
255     thread_local_var = pcre_jit_stack_alloc(...)
257     During thread exit
258     pcre_jit_stack_free(thread_local_var)
260     Use a one-line callback function
261     return thread_local_var
262     </pre>
263     All the functions described in this section do nothing if JIT is not available,
264     and <b>pcre_assign_jit_stack()</b> does nothing unless the <b>extra</b> argument
265     is non-NULL and points to a <b>pcre_extra</b> block that is the result of a
266     successful study with PCRE_STUDY_JIT_COMPILE.
267 ph10 836 <a name="stackfaq"></a></P>
268 ph10 869 <br><a name="SEC9" href="#TOC1">JIT STACK FAQ</a><br>
269 ph10 836 <P>
270     (1) Why do we need JIT stacks?
271     <br>
272     <br>
273     PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack where
274     the local data of the current node is pushed before checking its child nodes.
275     Allocating real machine stack on some platforms is difficult. For example, the
276     stack chain needs to be updated every time if we extend the stack on PowerPC.
277     Although it is possible, its updating time overhead decreases performance. So
278     we do the recursion in memory.
279 ph10 691 </P>
280     <P>
281 ph10 836 (2) Why don't we simply allocate blocks of memory with <b>malloc()</b>?
282     <br>
283     <br>
284     Modern operating systems have a nice feature: they can reserve an address space
285     instead of allocating memory. We can safely allocate memory pages inside this
286     address space, so the stack could grow without moving memory data (this is
287     important because of pointers). Thus we can allocate 1M address space, and use
288     only a single memory page (usually 4K) if that is enough. However, we can still
289     grow up to 1M anytime if needed.
290     </P>
291     <P>
292     (3) Who "owns" a JIT stack?
293     <br>
294     <br>
295     The owner of the stack is the user program, not the JIT studied pattern or
296     anything else. The user program must ensure that if a stack is used by
297     <b>pcre_exec()</b>, (that is, it is assigned to the pattern currently running),
298     that stack must not be used by any other threads (to avoid overwriting the same
299     memory area). The best practice for multithreaded programs is to allocate a
300     stack for each thread, and return this stack through the JIT callback function.
301     </P>
302     <P>
303     (4) When should a JIT stack be freed?
304     <br>
305     <br>
306     You can free a JIT stack at any time, as long as it will not be used by
307     <b>pcre_exec()</b> again. When you assign the stack to a pattern, only a pointer
308     is set. There is no reference counting or any other magic. You can free the
309     patterns and stacks in any order, anytime. Just <i>do not</i> call
310     <b>pcre_exec()</b> with a pattern pointing to an already freed stack, as that
311     will cause SEGFAULT. (Also, do not free a stack currently used by
312     <b>pcre_exec()</b> in another thread). You can also replace the stack for a
313     pattern at any time. You can even free the previous stack before assigning a
314     replacement.
315     </P>
316     <P>
317     (5) Should I allocate/free a stack every time before/after calling
318     <b>pcre_exec()</b>?
319     <br>
320     <br>
321     No, because this is too costly in terms of resources. However, you could
322     implement some clever idea which release the stack if it is not used in let's
323     say two minutes. The JIT callback can help to achive this without keeping a
324     list of the currently JIT studied patterns.
325     </P>
326     <P>
327     (6) OK, the stack is for long term memory allocation. But what happens if a
328     pattern causes stack overflow with a stack of 1M? Is that 1M kept until the
329     stack is freed?
330     <br>
331     <br>
332     Especially on embedded sytems, it might be a good idea to release
333     memory sometimes without freeing the stack. There is no API for this at the
334     moment. Probably a function call which returns with the currently allocated
335     memory for any stack and another which allows releasing memory (shrinking the
336     stack) would be a good idea if someone needs this.
337     </P>
338     <P>
339     (7) This is too much of a headache. Isn't there any better solution for JIT
340     stack handling?
341     <br>
342     <br>
343     No, thanks to Windows. If POSIX threads were used everywhere, we could throw
344     out this complicated API.
345     </P>
346 ph10 869 <br><a name="SEC10" href="#TOC1">EXAMPLE CODE</a><br>
347 ph10 836 <P>
348 ph10 691 This is a single-threaded example that specifies a JIT stack without using a
349     callback.
350     <pre>
351     int rc;
352     int ovector[30];
353     pcre *re;
354     pcre_extra *extra;
355     pcre_jit_stack *jit_stack;
357     re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
358     /* Check for errors */
359     extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
360     jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024);
361     /* Check for error (NULL) */
362     pcre_assign_jit_stack(extra, NULL, jit_stack);
363     rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
364     /* Check results */
365     pcre_free(re);
366     pcre_free_study(extra);
367     pcre_jit_stack_free(jit_stack);
369     </PRE>
370     </P>
371 ph10 869 <br><a name="SEC11" href="#TOC1">SEE ALSO</a><br>
372 ph10 691 <P>
373     <b>pcreapi</b>(3)
374     </P>
375 ph10 869 <br><a name="SEC12" href="#TOC1">AUTHOR</a><br>
376 ph10 691 <P>
377 ph10 836 Philip Hazel (FAQ by Zoltan Herczeg)
378 ph10 691 <br>
379     University Computing Service
380     <br>
381     Cambridge CB2 3QH, England.
382     <br>
383     </P>
384 ph10 869 <br><a name="SEC13" href="#TOC1">REVISION</a><br>
385 ph10 691 <P>
386 ph10 869 Last updated: 08 January 2012
387 ph10 691 <br>
388 ph10 869 Copyright &copy; 1997-2012 University of Cambridge.
389 ph10 691 <br>
390 ph10 678 <p>
391     Return to the <a href="index.html">PCRE index page</a>.
392     </p>

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