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> 8Return to the <a href="index.html">PCRE index page</a>. 9</p> 10<p> 11This page is part of the PCRE HTML documentation. It was generated automatically 12from the original man page. If there is any nonsense in it, please consult the 13man 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 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</ul> 30<br><a name="SEC1" href="#TOC1">PCRE JUST-IN-TIME COMPILER SUPPORT</a><br> 31<P> 32Just-in-time compiling is a heavyweight optimization that can greatly speed up 33pattern matching. However, it comes at the cost of extra processing before the 34match is performed. Therefore, it is of most benefit when the same pattern is 35going to be matched many times. This does not necessarily mean many calls of a 36matching function; if the pattern is not anchored, matching attempts may take 37place many times at various positions in the subject, even for a single call. 38Therefore, if the subject string is very long, it may still pay to use JIT for 39one-off matches. 40</P> 41<P> 42JIT support applies only to the traditional Perl-compatible matching function. 43It does not apply when the DFA matching function is being used. The code for 44this support was written by Zoltan Herczeg. 45</P> 46<br><a name="SEC2" href="#TOC1">8-BIT and 16-BIT SUPPORT</a><br> 47<P> 48JIT support is available for both the 8-bit and 16-bit PCRE libraries. To keep 49this documentation simple, only the 8-bit interface is described in what 50follows. If you are using the 16-bit library, substitute the 16-bit functions 51and 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> 56JIT 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 58JIT. 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 Power PC 32-bit and 64-bit 64</pre> 65If --enable-jit is set on an unsupported platform, compilation fails. 66</P> 67<P> 68A program that is linked with PCRE 8.20 or later can tell if JIT support is 69available by calling <b>pcre_config()</b> with the PCRE_CONFIG_JIT option. The 70result is 1 when JIT is available, and 0 otherwise. However, a simple program 71does not need to check this in order to use JIT. The API is implemented in a 72way that falls back to the interpretive code if JIT is not available. 73</P> 74<P> 75If your program may sometimes be linked with versions of PCRE that are older 76than 8.20, but you want to use JIT when it is available, you can test 77the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT macro such 78as PCRE_CONFIG_JIT, for compile-time control of your code. 79</P> 80<br><a name="SEC4" href="#TOC1">SIMPLE USE OF JIT</a><br> 81<P> 82You have to do two things to make use of the JIT support in the simplest way: 83<pre> 84 (1) Call <b>pcre_study()</b> with the PCRE_STUDY_JIT_COMPILE option for 85 each compiled pattern, and pass the resulting <b>pcre_extra</b> block to 86 <b>pcre_exec()</b>. 87 88 (2) Use <b>pcre_free_study()</b> to free the <b>pcre_extra</b> block when it is 89 no longer needed, instead of just freeing it yourself. This 90 ensures that any JIT data is also freed. 91</pre> 92For a program that may be linked with pre-8.20 versions of PCRE, you can insert 93<pre> 94 #ifndef PCRE_STUDY_JIT_COMPILE 95 #define PCRE_STUDY_JIT_COMPILE 0 96 #endif 97</pre> 98so that no option is passed to <b>pcre_study()</b>, and then use something like 99this to free the study data: 100<pre> 101 #ifdef PCRE_CONFIG_JIT 102 pcre_free_study(study_ptr); 103 #else 104 pcre_free(study_ptr); 105 #endif 106</pre> 107PCRE_STUDY_JIT_COMPILE requests the JIT compiler to generate code for complete 108matches. If you want to run partial matches using the PCRE_PARTIAL_HARD or 109PCRE_PARTIAL_SOFT options of <b>pcre_exec()</b>, you should set one or both of 110the following options in addition to, or instead of, PCRE_STUDY_JIT_COMPILE 111when you call <b>pcre_study()</b>: 112<pre> 113 PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE 114 PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE 115</pre> 116The JIT compiler generates different optimized code for each of the three 117modes (normal, soft partial, hard partial). When <b>pcre_exec()</b> is called, 118the appropriate code is run if it is available. Otherwise, the pattern is 119matched using interpretive code. 120</P> 121<P> 122In some circumstances you may need to call additional functions. These are 123described in the section entitled 124<a href="#stackcontrol">"Controlling the JIT stack"</a> 125below. 126</P> 127<P> 128If JIT support is not available, PCRE_STUDY_JIT_COMPILE etc. are ignored, and 129no JIT data is created. Otherwise, the compiled pattern is passed to the JIT 130compiler, which turns it into machine code that executes much faster than the 131normal interpretive code. When <b>pcre_exec()</b> is passed a <b>pcre_extra</b> 132block containing a pointer to JIT code of the appropriate mode (normal or 133hard/soft partial), it obeys that code instead of running the interpreter. The 134result is identical, but the compiled JIT code runs much faster. 135</P> 136<P> 137There are some <b>pcre_exec()</b> options that are not supported for JIT 138execution. There are also some pattern items that JIT cannot handle. Details 139are given below. In both cases, execution automatically falls back to the 140interpretive code. If you want to know whether JIT was actually used for a 141particular match, you should arrange for a JIT callback function to be set up 142as described in the section entitled 143<a href="#stackcontrol">"Controlling the JIT stack"</a> 144below, even if you do not need to supply a non-default JIT stack. Such a 145callback function is called whenever JIT code is about to be obeyed. If the 146execution options are not right for JIT execution, the callback function is not 147obeyed. 148</P> 149<P> 150If the JIT compiler finds an unsupported item, no JIT data is generated. You 151can find out if JIT execution is available after studying a pattern by calling 152<b>pcre_fullinfo()</b> with the PCRE_INFO_JIT option. A result of 1 means that 153JIT compilation was successful. A result of 0 means that JIT support is not 154available, or the pattern was not studied with PCRE_STUDY_JIT_COMPILE etc., or 155the JIT compiler was not able to handle the pattern. 156</P> 157<P> 158Once a pattern has been studied, with or without JIT, it can be used as many 159times as you like for matching different subject strings. 160</P> 161<br><a name="SEC5" href="#TOC1">UNSUPPORTED OPTIONS AND PATTERN ITEMS</a><br> 162<P> 163The only <b>pcre_exec()</b> options that are supported for JIT execution are 164PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, 165PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT. 166</P> 167<P> 168The unsupported pattern items are: 169<pre> 170 \C match a single byte; not supported in UTF-8 mode 171 (?Cn) callouts 172 (*PRUNE) ) 173 (*SKIP) ) backtracking control verbs 174 (*THEN) ) 175</pre> 176Support for some of these may be added in future. 177</P> 178<br><a name="SEC6" href="#TOC1">RETURN VALUES FROM JIT EXECUTION</a><br> 179<P> 180When a pattern is matched using JIT execution, the return values are the same 181as those given by the interpretive <b>pcre_exec()</b> code, with the addition of 182one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means that the memory used 183for the JIT stack was insufficient. See 184<a href="#stackcontrol">"Controlling the JIT stack"</a> 185below for a discussion of JIT stack usage. For compatibility with the 186interpretive <b>pcre_exec()</b> code, no more than two-thirds of the 187<i>ovector</i> argument is used for passing back captured substrings. 188</P> 189<P> 190The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if searching a 191very large pattern tree goes on for too long, as it is in the same circumstance 192when JIT is not used, but the details of exactly what is counted are not the 193same. The PCRE_ERROR_RECURSIONLIMIT error code is never returned by JIT 194execution. 195</P> 196<br><a name="SEC7" href="#TOC1">SAVING AND RESTORING COMPILED PATTERNS</a><br> 197<P> 198The code that is generated by the JIT compiler is architecture-specific, and is 199also position dependent. For those reasons it cannot be saved (in a file or 200database) and restored later like the bytecode and other data of a compiled 201pattern. Saving and restoring compiled patterns is not something many people 202do. More detail about this facility is given in the 203<a href="pcreprecompile.html"><b>pcreprecompile</b></a> 204documentation. It should be possible to run <b>pcre_study()</b> on a saved and 205restored pattern, and thereby recreate the JIT data, but because JIT 206compilation uses significant resources, it is probably not worth doing this; 207you might as well recompile the original pattern. 208<a name="stackcontrol"></a></P> 209<br><a name="SEC8" href="#TOC1">CONTROLLING THE JIT STACK</a><br> 210<P> 211When the compiled JIT code runs, it needs a block of memory to use as a stack. 212By default, it uses 32K on the machine stack. However, some large or 213complicated patterns need more than this. The error PCRE_ERROR_JIT_STACKLIMIT 214is given when there is not enough stack. Three functions are provided for 215managing blocks of memory for use as JIT stacks. There is further discussion 216about the use of JIT stacks in the section entitled 217<a href="#stackcontrol">"JIT stack FAQ"</a> 218below. 219</P> 220<P> 221The <b>pcre_jit_stack_alloc()</b> function creates a JIT stack. Its arguments 222are a starting size and a maximum size, and it returns a pointer to an opaque 223structure of type <b>pcre_jit_stack</b>, or NULL if there is an error. The 224<b>pcre_jit_stack_free()</b> function can be used to free a stack that is no 225longer needed. (For the technically minded: the address space is allocated by 226mmap or VirtualAlloc.) 227</P> 228<P> 229JIT uses far less memory for recursion than the interpretive code, 230and a maximum stack size of 512K to 1M should be more than enough for any 231pattern. 232</P> 233<P> 234The <b>pcre_assign_jit_stack()</b> function specifies which stack JIT code 235should use. Its arguments are as follows: 236<pre> 237 pcre_extra *extra 238 pcre_jit_callback callback 239 void *data 240</pre> 241The <i>extra</i> argument must be the result of studying a pattern with 242PCRE_STUDY_JIT_COMPILE etc. There are three cases for the values of the other 243two options: 244<pre> 245 (1) If <i>callback</i> is NULL and <i>data</i> is NULL, an internal 32K block 246 on the machine stack is used. 247 248 (2) If <i>callback</i> is NULL and <i>data</i> is not NULL, <i>data</i> must be 249 a valid JIT stack, the result of calling <b>pcre_jit_stack_alloc()</b>. 250 251 (3) If <i>callback</i> is not NULL, it must point to a function that is 252 called with <i>data</i> as an argument at the start of matching, in 253 order to set up a JIT stack. If the return from the callback 254 function is NULL, the internal 32K stack is used; otherwise the 255 return value must be a valid JIT stack, the result of calling 256 <b>pcre_jit_stack_alloc()</b>. 257</pre> 258A callback function is obeyed whenever JIT code is about to be run; it is not 259obeyed when <b>pcre_exec()</b> is called with options that are incompatible for 260JIT execution. A callback function can therefore be used to determine whether a 261match operation was executed by JIT or by the interpreter. 262</P> 263<P> 264You may safely use the same JIT stack for more than one pattern (either by 265assigning directly or by callback), as long as the patterns are all matched 266sequentially in the same thread. In a multithread application, if you do not 267specify a JIT stack, or if you assign or pass back NULL from a callback, that 268is thread-safe, because each thread has its own machine stack. However, if you 269assign or pass back a non-NULL JIT stack, this must be a different stack for 270each thread so that the application is thread-safe. 271</P> 272<P> 273Strictly speaking, even more is allowed. You can assign the same non-NULL stack 274to any number of patterns as long as they are not used for matching by multiple 275threads at the same time. For example, you can assign the same stack to all 276compiled patterns, and use a global mutex in the callback to wait until the 277stack is available for use. However, this is an inefficient solution, and not 278recommended. 279</P> 280<P> 281This is a suggestion for how a multithreaded program that needs to set up 282non-default JIT stacks might operate: 283<pre> 284 During thread initalization 285 thread_local_var = pcre_jit_stack_alloc(...) 286 287 During thread exit 288 pcre_jit_stack_free(thread_local_var) 289 290 Use a one-line callback function 291 return thread_local_var 292</pre> 293All the functions described in this section do nothing if JIT is not available, 294and <b>pcre_assign_jit_stack()</b> does nothing unless the <b>extra</b> argument 295is non-NULL and points to a <b>pcre_extra</b> block that is the result of a 296successful study with PCRE_STUDY_JIT_COMPILE etc. 297<a name="stackfaq"></a></P> 298<br><a name="SEC9" href="#TOC1">JIT STACK FAQ</a><br> 299<P> 300(1) Why do we need JIT stacks? 301<br> 302<br> 303PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack where 304the local data of the current node is pushed before checking its child nodes. 305Allocating real machine stack on some platforms is difficult. For example, the 306stack chain needs to be updated every time if we extend the stack on PowerPC. 307Although it is possible, its updating time overhead decreases performance. So 308we do the recursion in memory. 309</P> 310<P> 311(2) Why don't we simply allocate blocks of memory with <b>malloc()</b>? 312<br> 313<br> 314Modern operating systems have a nice feature: they can reserve an address space 315instead of allocating memory. We can safely allocate memory pages inside this 316address space, so the stack could grow without moving memory data (this is 317important because of pointers). Thus we can allocate 1M address space, and use 318only a single memory page (usually 4K) if that is enough. However, we can still 319grow up to 1M anytime if needed. 320</P> 321<P> 322(3) Who "owns" a JIT stack? 323<br> 324<br> 325The owner of the stack is the user program, not the JIT studied pattern or 326anything else. The user program must ensure that if a stack is used by 327<b>pcre_exec()</b>, (that is, it is assigned to the pattern currently running), 328that stack must not be used by any other threads (to avoid overwriting the same 329memory area). The best practice for multithreaded programs is to allocate a 330stack for each thread, and return this stack through the JIT callback function. 331</P> 332<P> 333(4) When should a JIT stack be freed? 334<br> 335<br> 336You can free a JIT stack at any time, as long as it will not be used by 337<b>pcre_exec()</b> again. When you assign the stack to a pattern, only a pointer 338is set. There is no reference counting or any other magic. You can free the 339patterns and stacks in any order, anytime. Just <i>do not</i> call 340<b>pcre_exec()</b> with a pattern pointing to an already freed stack, as that 341will cause SEGFAULT. (Also, do not free a stack currently used by 342<b>pcre_exec()</b> in another thread). You can also replace the stack for a 343pattern at any time. You can even free the previous stack before assigning a 344replacement. 345</P> 346<P> 347(5) Should I allocate/free a stack every time before/after calling 348<b>pcre_exec()</b>? 349<br> 350<br> 351No, because this is too costly in terms of resources. However, you could 352implement some clever idea which release the stack if it is not used in let's 353say two minutes. The JIT callback can help to achive this without keeping a 354list of the currently JIT studied patterns. 355</P> 356<P> 357(6) OK, the stack is for long term memory allocation. But what happens if a 358pattern causes stack overflow with a stack of 1M? Is that 1M kept until the 359stack is freed? 360<br> 361<br> 362Especially on embedded sytems, it might be a good idea to release memory 363sometimes without freeing the stack. There is no API for this at the moment. 364Probably a function call which returns with the currently allocated memory for 365any stack and another which allows releasing memory (shrinking the stack) would 366be a good idea if someone needs this. 367</P> 368<P> 369(7) This is too much of a headache. Isn't there any better solution for JIT 370stack handling? 371<br> 372<br> 373No, thanks to Windows. If POSIX threads were used everywhere, we could throw 374out this complicated API. 375</P> 376<br><a name="SEC10" href="#TOC1">EXAMPLE CODE</a><br> 377<P> 378This is a single-threaded example that specifies a JIT stack without using a 379callback. 380<pre> 381 int rc; 382 int ovector[30]; 383 pcre *re; 384 pcre_extra *extra; 385 pcre_jit_stack *jit_stack; 386 387 re = pcre_compile(pattern, 0, &error, &erroffset, NULL); 388 /* Check for errors */ 389 extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error); 390 jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024); 391 /* Check for error (NULL) */ 392 pcre_assign_jit_stack(extra, NULL, jit_stack); 393 rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30); 394 /* Check results */ 395 pcre_free(re); 396 pcre_free_study(extra); 397 pcre_jit_stack_free(jit_stack); 398 399</PRE> 400</P> 401<br><a name="SEC11" href="#TOC1">SEE ALSO</a><br> 402<P> 403<b>pcreapi</b>(3) 404</P> 405<br><a name="SEC12" href="#TOC1">AUTHOR</a><br> 406<P> 407Philip Hazel (FAQ by Zoltan Herczeg) 408<br> 409University Computing Service 410<br> 411Cambridge CB2 3QH, England. 412<br> 413</P> 414<br><a name="SEC13" href="#TOC1">REVISION</a><br> 415<P> 416Last updated: 04 May 2012 417<br> 418Copyright © 1997-2012 University of Cambridge. 419<br> 420<p> 421Return to the <a href="index.html">PCRE index page</a>. 422</p> 423