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Wed Oct 5 15:58:51 2011 UTC (9 years, 10 months ago) by ph10
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Document Power PC JIT support as experimental.
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 (experimental)
32 .sp
33 The Power PC support is designated as experimental because it has not been
34 fully tested. If --enable-jit is set on an unsupported platform, compilation
35 fails.
36 .P
37 A program can tell if JIT support is available by calling \fBpcre_config()\fP
38 with the PCRE_CONFIG_JIT option. The result is 1 when JIT is available, and 0
39 otherwise. However, a simple program does not need to check this in order to
40 use JIT. The API is implemented in a way that falls back to the ordinary PCRE
41 code if JIT is not available.
42 .
43 .
45 .rs
46 .sp
47 You have to do two things to make use of the JIT support in the simplest way:
48 .sp
49 (1) Call \fBpcre_study()\fP with the PCRE_STUDY_JIT_COMPILE option for
50 each compiled pattern, and pass the resulting \fBpcre_extra\fP block to
51 \fBpcre_exec()\fP.
52 .sp
53 (2) Use \fBpcre_free_study()\fP to free the \fBpcre_extra\fP block when it is
54 no longer needed instead of just freeing it yourself. This
55 ensures that any JIT data is also freed.
56 .sp
57 In some circumstances you may need to call additional functions. These are
58 described in the section entitled
59 .\" HTML <a href="#stackcontrol">
60 .\" </a>
61 "Controlling the JIT stack"
62 .\"
63 below.
64 .P
65 If JIT support is not available, PCRE_STUDY_JIT_COMPILE is ignored, and no JIT
66 data is set up. Otherwise, the compiled pattern is passed to the JIT compiler,
67 which turns it into machine code that executes much faster than the normal
68 interpretive code. When \fBpcre_exec()\fP is passed a \fBpcre_extra\fP block
69 containing a pointer to JIT code, it obeys that instead of the normal code. The
70 result is identical, but the code runs much faster.
71 .P
72 There are some \fBpcre_exec()\fP options that are not supported for JIT
73 execution. There are also some pattern items that JIT cannot handle. Details
74 are given below. In both cases, execution automatically falls back to the
75 interpretive code.
76 .P
77 If the JIT compiler finds an unsupported item, no JIT data is generated. You
78 can find out if JIT execution is available after studying a pattern by calling
79 \fBpcre_fullinfo()\fP with the PCRE_INFO_JIT option. A result of 1 means that
80 JIT compilation was successful. A result of 0 means that JIT support is not
81 available, or the pattern was not studied with PCRE_STUDY_JIT_COMPILE, or the
82 JIT compiler was not able to handle the pattern.
83 .P
84 Once a pattern has been studied, with or without JIT, it can be used as many
85 times as you like for matching different subject strings.
86 .
87 .
89 .rs
90 .sp
91 The only \fBpcre_exec()\fP options that are supported for JIT execution are
93 PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not
94 supported.
95 .P
96 The unsupported pattern items are:
97 .sp
98 \eC match a single byte, even in UTF-8 mode
99 (?Cn) callouts
100 (?(<name>)... conditional test on setting of a named subpattern
101 (?(R)... conditional test on whole pattern recursion
102 (?(Rn)... conditional test on recursion, by number
103 (?(R&name)... conditional test on recursion, by name
104 (*COMMIT) )
105 (*MARK) )
106 (*PRUNE) ) the backtracking control verbs
107 (*SKIP) )
108 (*THEN) )
109 .sp
110 Support for some of these may be added in future.
111 .
112 .
114 .rs
115 .sp
116 When a pattern is matched using JIT execution, the return values are the same
117 as those given by the interpretive \fBpcre_exec()\fP code, with the addition of
118 one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means that the memory used
119 for the JIT stack was insufficient. See
120 .\" HTML <a href="#stackcontrol">
121 .\" </a>
122 "Controlling the JIT stack"
123 .\"
124 below for a discussion of JIT stack usage. For compatibility with the
125 interpretive \fBpcre_exec()\fP code, no more than two-thirds of the
126 \fIovector\fP argument is used for passing back captured substrings.
127 .P
128 The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if searching a
129 very large pattern tree goes on for too long, as it is in the same circumstance
130 when JIT is not used, but the details of exactly what is counted are not the
131 same. The PCRE_ERROR_RECURSIONLIMIT error code is never returned by JIT
132 execution.
133 .
134 .
136 .rs
137 .sp
138 The code that is generated by the JIT compiler is architecture-specific, and is
139 also position dependent. For those reasons it cannot be saved (in a file or
140 database) and restored later like the bytecode and other data of a compiled
141 pattern. Saving and restoring compiled patterns is not something many people
142 do. More detail about this facility is given in the
143 .\" HREF
144 \fBpcreprecompile\fP
145 .\"
146 documentation. It should be possible to run \fBpcre_study()\fP on a saved and
147 restored pattern, and thereby recreate the JIT data, but because JIT
148 compilation uses significant resources, it is probably not worth doing this;
149 you might as well recompile the original pattern.
150 .
151 .
152 .\" HTML <a name="stackcontrol"></a>
154 .rs
155 .sp
156 When the compiled JIT code runs, it needs a block of memory to use as a stack.
157 By default, it uses 32K on the machine stack. However, some large or
158 complicated patterns need more than this. The error PCRE_ERROR_JIT_STACKLIMIT
159 is given when there is not enough stack. Three functions are provided for
160 managing blocks of memory for use as JIT stacks.
161 .P
162 The \fBpcre_jit_stack_alloc()\fP function creates a JIT stack. Its arguments
163 are a starting size and a maximum size, and it returns a pointer to an opaque
164 structure of type \fBpcre_jit_stack\fP, or NULL if there is an error. The
165 \fBpcre_jit_stack_free()\fP function can be used to free a stack that is no
166 longer needed. (For the technically minded: the address space is allocated by
167 mmap or VirtualAlloc.)
168 .P
169 JIT uses far less memory for recursion than the interpretive code,
170 and a maximum stack size of 512K to 1M should be more than enough for any
171 pattern.
172 .P
173 The \fBpcre_assign_jit_stack()\fP function specifies which stack JIT code
174 should use. Its arguments are as follows:
175 .sp
176 pcre_extra *extra
177 pcre_jit_callback callback
178 void *data
179 .sp
180 The \fIextra\fP argument must be the result of studying a pattern with
181 PCRE_STUDY_JIT_COMPILE. There are three cases for the values of the other two
182 options:
183 .sp
184 (1) If \fIcallback\fP is NULL and \fIdata\fP is NULL, an internal 32K block
185 on the machine stack is used.
186 .sp
187 (2) If \fIcallback\fP is NULL and \fIdata\fP is not NULL, \fIdata\fP must be
188 a valid JIT stack, the result of calling \fBpcre_jit_stack_alloc()\fP.
189 .sp
190 (3) If \fIcallback\fP not NULL, it must point to a function that is called
191 with \fIdata\fP as an argument at the start of matching, in order to
192 set up a JIT stack. If the result is NULL, the internal 32K stack
193 is used; otherwise the return value must be a valid JIT stack,
194 the result of calling \fBpcre_jit_stack_alloc()\fP.
195 .sp
196 You may safely assign the same JIT stack to more than one pattern, as long as
197 they are all matched sequentially in the same thread. In a multithread
198 application, each thread must use its own JIT stack.
199 .P
200 Strictly speaking, even more is allowed. You can assign the same stack to any
201 number of patterns as long as they are not used for matching by multiple
202 threads at the same time. For example, you can assign the same stack to all
203 compiled patterns, and use a global mutex in the callback to wait until the
204 stack is available for use. However, this is an inefficient solution, and
205 not recommended.
206 .P
207 This is a suggestion for how a typical multithreaded program might operate:
208 .sp
209 During thread initalization
210 thread_local_var = pcre_jit_stack_alloc(...)
211 .sp
212 During thread exit
213 pcre_jit_stack_free(thread_local_var)
214 .sp
215 Use a one-line callback function
216 return thread_local_var
217 .sp
218 All the functions described in this section do nothing if JIT is not available,
219 and \fBpcre_assign_jit_stack()\fP does nothing unless the \fBextra\fP argument
220 is non-NULL and points to a \fBpcre_extra\fP block that is the result of a
221 successful study with PCRE_STUDY_JIT_COMPILE.
222 .
223 .
225 .rs
226 .sp
227 This is a single-threaded example that specifies a JIT stack without using a
228 callback.
229 .sp
230 int rc;
231 int ovector[30];
232 pcre *re;
233 pcre_extra *extra;
234 pcre_jit_stack *jit_stack;
235 .sp
236 re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
237 /* Check for errors */
238 extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
239 jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024);
240 /* Check for error (NULL) */
241 pcre_assign_jit_stack(extra, NULL, jit_stack);
242 rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
243 /* Check results */
244 pcre_free(re);
245 pcre_free_study(extra);
246 pcre_jit_stack_free(jit_stack);
247 .sp
248 .
249 .
250 .SH "SEE ALSO"
251 .rs
252 .sp
253 \fBpcreapi\fP(3)
254 .
255 .
257 .rs
258 .sp
259 .nf
260 Philip Hazel
261 University Computing Service
262 Cambridge CB2 3QH, England.
263 .fi
264 .
265 .
267 .rs
268 .sp
269 .nf
270 Last updated: 05 October 2011
271 Copyright (c) 1997-2011 University of Cambridge.
272 .fi

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