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Revision 678 - (show annotations)
Sun Aug 28 15:23:03 2011 UTC (9 years, 3 months ago) by ph10
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Documentation for JIT support.
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
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 .
21 .SH "AVAILABILITY OF JIT SUPPORT"
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 MIPS 32-bit
30 Power PC 32-bit and 64-bit
31 Intel x86 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 .
42 .SH "SIMPLE USE OF JIT"
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
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 ensures that
53 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 compilationw 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 .
82 .
83 .SH "UNSUPPORTED OPTIONS AND PATTERN ITEMS"
84 .rs
85 .sp
86 The only \fBpcre_exec()\fP options that are supported for JIT execution are
87 PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and
88 PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not
89 supported.
90 .P
91 The unsupported pattern items are:
92 .sp
93 \eC match a single byte, even in UTF-8 mode
94 (?Cn) callouts
95 (?(<name>)... conditional test on setting of a named subpattern
96 (?(R)... conditional test on whole pattern recursion
97 (?(Rn)... conditional test on recursion, by number
98 (?(R&name)... conditional test on recursion, by name
99 (*COMMIT) )
100 (*MARK) )
101 (*PRUNE) ) the backtracking control verbs
102 (*SKIP) )
103 (*THEN) )
104 .sp
105 Support for some of these may be added in future.
106 .
107 .
108 .SH "RETURN VALUES FROM JIT EXECUTION"
109 .rs
110 .sp
111 When a pattern is matched using JIT execution, the return values are the same
112 as those given by the interpretive \fBpcre_exec()\fP code, with the addition of
113 one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means that the memory used
114 for the JIT stack was insufficient. See
115 .\" HTML <a href="#stackcontrol">
116 .\" </a>
117 "Controlling the JIT stack"
118 .\"
119 below for a discussion of JIT stack usage.
120 .P
121 The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if searching a
122 very large pattern tree goes on for too long, as it is in the same circumstance
123 when JIT is not used, but the details of exactly what is counted are not the
124 same. The PCRE_ERROR_RECURSIONLIMIT error code is never returned by JIT
125 execution.
126 .
127 .
128 .SH "SAVING AND RESTORING COMPILED PATTERNS"
129 .rs
130 .sp
131 The code that is generated by the JIT compiler is architecture-specific, and is
132 also position dependent. For those reasons it cannot be saved and restored like
133 the bytecode and other data of a compiled pattern. You should be able run
134 \fBpcre_study()\fP on a saved and restored pattern, and thereby recreate the
135 JIT data, but because JIT compilation uses significant resources, it is
136 probably not worth doing.
137 .
138 .
139 .\" HTML <a name="stackcontrol"></a>
140 .SH "CONTROLLING THE JIT STACK"
141 .rs
142 .sp
143 When the compiled JIT code runs, it needs a block of memory to use as a stack.
144 By default, it uses 32K on the machine stack. However, some large or
145 complicated patterns need more than this. The error PCRE_ERROR_JIT_STACKLIMIT
146 is given when there is not enough stack. Three functions are provided for
147 setting up alternative blocks of memory for use as JIT stacks.
148 .P
149 The \fBpcre_jit_stack_alloc()\fP function creates a JIT stack. Its arguments
150 are a starting size and a maximum size, and it returns an opaque value
151 of type \fBpcre_jit_stack\fP that represents a JIT stack, or NULL if there is
152 an error. The \fBpcre_jit_stack_free()\fP function can be used to free a stack
153 that is no longer needed.
154 .P
155 The \fBpcre_assign_jit_stack()\fP function specifies which stack JIT code
156 should use. Its arguments are as follows:
157 .sp
158 pcre_extra *extra
159 pcre_jit_callback callback
160 void *data
161 .sp
162 The \fIextra\fP argument must be the result of studying a pattern with
163 PCRE_STUDY_JIT_COMPILE. There are three cases for the values of the other two
164 options:
165 .sp
166 (1) If \fIcallback\fP is NULL and \fIdata\fP is NULL, an internal 32K block
167 on the machine stack is used.
168 .sp
169 (2) If \fIcallback\fP is NULL and \fIdata\fP is not NULL, \fIdata\fP must be
170 a valid JIT stack, the result of calling \fBpcre_jit_stack_alloc()\fP.
171 .sp
172 (3) If \fIcallback\fP not NULL, it must point to a function that is called
173 with \fIdata\fP as an argument at the start of matching, in order to
174 set up a JIT stack. If the result is NULL, the internal 32K stack
175 is used; otherwise the return value must be a valid JIT stack,
176 the result of calling \fBpcre_jit_stack_alloc()\fP.
177 .sp
178 You may safely assign the same JIT stack to more than one pattern, as long as
179 they are all matched sequentially in the same thread. In a multithread
180 application, each thread must use its own JIT stack.
181 .P
182 All the functions described in this section do nothing if JIT is not available,
183 and \fBpcre_assign_jit_stack()\fP does nothing unless the \fBextra\fP argument
184 is non-NULL and points to a \fBpcre_extra\fP block that is the result of a
185 successful study with PCRE_STUDY_JIT_COMPILE.
186 .
187 .
188 .SH "EXAMPLE CODE"
189 .rs
190 .sp
191 This is a single-threaded example that specifies a JIT stack without using a
192 callback.
193 .sp
194 int rc;
195 pcre *re;
196 pcre_extra *extra;
197 pcre_jit_stack *jit_stack;
198 .sp
199 re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
200 /* Check for errors */
201 extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
202 jit_stack = pcre_jit_stack_alloc(1, 512 * 1024);
203 /* Check for error (NULL) */
204 pcre_assign_jit_stack(extra, NULL, jit_stack);
205 rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, ovecsize);
206 /* Check results */
207 pcre_free(re);
208 pcre_free_study(extra);
209 .sp
210 .
211 .
212 .SH "SEE ALSO"
213 .rs
214 .sp
215 \fBpcreapi\fP(3)
216 .
217 .
218 .SH AUTHOR
219 .rs
220 .sp
221 .nf
222 Philip Hazel
223 University Computing Service
224 Cambridge CB2 3QH, England.
225 .fi
226 .
227 .
228 .SH REVISION
229 .rs
230 .sp
231 .nf
232 Last updated: 28 August 2011
233 Copyright (c) 1997-2011 University of Cambridge.
234 .fi

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