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Fri Sep 23 10:32:53 2011 UTC (9 years, 5 months ago) by ph10
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Documentation clarifications.
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
11 \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 \fBpcre_exec()\fP. If the subject string is very long, it may still pay to use
14 JIT for one-off matches.
15 .P
16 JIT support applies only to the traditional matching function,
17 \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 .
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 Intel x86 32-bit and 64-bit
30 MIPS 32-bit
31 Power PC 32-bit and 64-bit
32 .sp
33 If --enable-jit is set on an unsupported platform, compilation fails.
34 .P
35 A program can tell if JIT support is available by calling \fBpcre_config()\fP
36 with the PCRE_CONFIG_JIT option. The result is 1 when JIT is available, and 0
37 otherwise. However, a simple program does not need to check this in order to
38 use JIT. The API is implemented in a way that falls back to the ordinary PCRE
39 code if JIT is not available.
40 .
41 .
43 .rs
44 .sp
45 You have to do two things to make use of the JIT support in the simplest way:
46 .sp
47 (1) Call \fBpcre_study()\fP with the PCRE_STUDY_JIT_COMPILE option for
48 each compiled pattern, and pass the resulting \fBpcre_extra\fP block to
49 \fBpcre_exec()\fP.
50 .sp
51 (2) Use \fBpcre_free_study()\fP to free the \fBpcre_extra\fP block when it is
52 no longer needed instead of just freeing it yourself. This
53 ensures that any JIT data is also freed.
54 .sp
55 In some circumstances you may need to call additional functions. These are
56 described in the section entitled
57 .\" HTML <a href="#stackcontrol">
58 .\" </a>
59 "Controlling the JIT stack"
60 .\"
61 below.
62 .P
63 If JIT support is not available, PCRE_STUDY_JIT_COMPILE is ignored, and no JIT
64 data is set up. Otherwise, the compiled pattern is passed to the JIT compiler,
65 which turns it into machine code that executes much faster than the normal
66 interpretive code. When \fBpcre_exec()\fP is passed a \fBpcre_extra\fP block
67 containing a pointer to JIT code, it obeys that instead of the normal code. The
68 result is identical, but the code runs much faster.
69 .P
70 There are some \fBpcre_exec()\fP options that are not supported for JIT
71 execution. There are also some pattern items that JIT cannot handle. Details
72 are given below. In both cases, execution automatically falls back to the
73 interpretive code.
74 .P
75 If the JIT compiler finds an unsupported item, no JIT data is generated. You
76 can find out if JIT execution is available after studying a pattern by calling
77 \fBpcre_fullinfo()\fP with the PCRE_INFO_JIT option. A result of 1 means that
78 JIT compilation was successful. A result of 0 means that JIT support is not
79 available, or the pattern was not studied with PCRE_STUDY_JIT_COMPILE, or the
80 JIT compiler was not able to handle the pattern.
81 .P
82 Once a pattern has been studied, with or without JIT, it can be used as many
83 times as you like for matching different subject strings.
84 .
85 .
87 .rs
88 .sp
89 The only \fBpcre_exec()\fP options that are supported for JIT execution are
91 PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not
92 supported.
93 .P
94 The unsupported pattern items are:
95 .sp
96 \eC match a single byte, even in UTF-8 mode
97 (?Cn) callouts
98 (?(<name>)... conditional test on setting of a named subpattern
99 (?(R)... conditional test on whole pattern recursion
100 (?(Rn)... conditional test on recursion, by number
101 (?(R&name)... conditional test on recursion, by name
102 (*COMMIT) )
103 (*MARK) )
104 (*PRUNE) ) the backtracking control verbs
105 (*SKIP) )
106 (*THEN) )
107 .sp
108 Support for some of these may be added in future.
109 .
110 .
112 .rs
113 .sp
114 When a pattern is matched using JIT execution, the return values are the same
115 as those given by the interpretive \fBpcre_exec()\fP code, with the addition of
116 one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means that the memory used
117 for the JIT stack was insufficient. See
118 .\" HTML <a href="#stackcontrol">
119 .\" </a>
120 "Controlling the JIT stack"
121 .\"
122 below for a discussion of JIT stack usage. For compatibility with the
123 interpretive \fBpcre_exec()\fP code, no more than two-thirds of the
124 \fIovector\fP argument is used for passing back captured substrings.
125 .P
126 The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if searching a
127 very large pattern tree goes on for too long, as it is in the same circumstance
128 when JIT is not used, but the details of exactly what is counted are not the
129 same. The PCRE_ERROR_RECURSIONLIMIT error code is never returned by JIT
130 execution.
131 .
132 .
134 .rs
135 .sp
136 The code that is generated by the JIT compiler is architecture-specific, and is
137 also position dependent. For those reasons it cannot be saved (in a file or
138 database) and restored later like the bytecode and other data of a compiled
139 pattern. Saving and restoring compiled patterns is not something many people
140 do. More detail about this facility is given in the
141 .\" HREF
142 \fBpcreprecompile\fP
143 .\"
144 documentation. It should be possible to run \fBpcre_study()\fP on a saved and
145 restored pattern, and thereby recreate the JIT data, but because JIT
146 compilation uses significant resources, it is probably not worth doing this;
147 you might as well recompile the original pattern.
148 .
149 .
150 .\" HTML <a name="stackcontrol"></a>
152 .rs
153 .sp
154 When the compiled JIT code runs, it needs a block of memory to use as a stack.
155 By default, it uses 32K on the machine stack. However, some large or
156 complicated patterns need more than this. The error PCRE_ERROR_JIT_STACKLIMIT
157 is given when there is not enough stack. Three functions are provided for
158 managing blocks of memory for use as JIT stacks.
159 .P
160 The \fBpcre_jit_stack_alloc()\fP function creates a JIT stack. Its arguments
161 are a starting size and a maximum size, and it returns a pointer to an opaque
162 structure of type \fBpcre_jit_stack\fP, or NULL if there is an error. The
163 \fBpcre_jit_stack_free()\fP function can be used to free a stack that is no
164 longer needed. (For the technically minded: the address space is allocated by
165 mmap or VirtualAlloc.)
166 .P
167 JIT uses far less memory for recursion than the interpretive code,
168 and a maximum stack size of 512K to 1M should be more than enough for any
169 pattern.
170 .P
171 The \fBpcre_assign_jit_stack()\fP function specifies which stack JIT code
172 should use. Its arguments are as follows:
173 .sp
174 pcre_extra *extra
175 pcre_jit_callback callback
176 void *data
177 .sp
178 The \fIextra\fP argument must be the result of studying a pattern with
179 PCRE_STUDY_JIT_COMPILE. There are three cases for the values of the other two
180 options:
181 .sp
182 (1) If \fIcallback\fP is NULL and \fIdata\fP is NULL, an internal 32K block
183 on the machine stack is used.
184 .sp
185 (2) If \fIcallback\fP is NULL and \fIdata\fP is not NULL, \fIdata\fP must be
186 a valid JIT stack, the result of calling \fBpcre_jit_stack_alloc()\fP.
187 .sp
188 (3) If \fIcallback\fP not NULL, it must point to a function that is called
189 with \fIdata\fP as an argument at the start of matching, in order to
190 set up a JIT stack. If the result is NULL, the internal 32K stack
191 is used; otherwise the return value must be a valid JIT stack,
192 the result of calling \fBpcre_jit_stack_alloc()\fP.
193 .sp
194 You may safely assign the same JIT stack to more than one pattern, as long as
195 they are all matched sequentially in the same thread. In a multithread
196 application, each thread must use its own JIT stack.
197 .P
198 Strictly speaking, even more is allowed. You can assign the same stack to any
199 number of patterns as long as they are not used for matching by multiple
200 threads at the same time. For example, you can assign the same stack to all
201 compiled patterns, and use a global mutex in the callback to wait until the
202 stack is available for use. However, this is an inefficient solution, and
203 not recommended.
204 .P
205 This is a suggestion for how a typical multithreaded program might operate:
206 .sp
207 During thread initalization
208 thread_local_var = pcre_jit_stack_alloc(...)
209 .sp
210 During thread exit
211 pcre_jit_stack_free(thread_local_var)
212 .sp
213 Use a one-line callback function
214 return thread_local_var
215 .sp
216 All the functions described in this section do nothing if JIT is not available,
217 and \fBpcre_assign_jit_stack()\fP does nothing unless the \fBextra\fP argument
218 is non-NULL and points to a \fBpcre_extra\fP block that is the result of a
219 successful study with PCRE_STUDY_JIT_COMPILE.
220 .
221 .
223 .rs
224 .sp
225 This is a single-threaded example that specifies a JIT stack without using a
226 callback.
227 .sp
228 int rc;
229 int ovector[30];
230 pcre *re;
231 pcre_extra *extra;
232 pcre_jit_stack *jit_stack;
233 .sp
234 re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
235 /* Check for errors */
236 extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
237 jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024);
238 /* Check for error (NULL) */
239 pcre_assign_jit_stack(extra, NULL, jit_stack);
240 rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
241 /* Check results */
242 pcre_free(re);
243 pcre_free_study(extra);
244 pcre_jit_stack_free(jit_stack);
245 .sp
246 .
247 .
248 .SH "SEE ALSO"
249 .rs
250 .sp
251 \fBpcreapi\fP(3)
252 .
253 .
255 .rs
256 .sp
257 .nf
258 Philip Hazel
259 University Computing Service
260 Cambridge CB2 3QH, England.
261 .fi
262 .
263 .
265 .rs
266 .sp
267 .nf
268 Last updated: 23 September 2011
269 Copyright (c) 1997-2011 University of Cambridge.
270 .fi

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