1PCREJIT(3)                 Library Functions Manual                 PCREJIT(3)
2
3
4

NAME

6       PCRE - Perl-compatible regular expressions
7

PCRE JUST-IN-TIME COMPILER SUPPORT

9
10       Just-in-time  compiling  is a heavyweight optimization that can greatly
11       speed up pattern matching. However, it comes at the cost of extra  pro‐
12       cessing before the match is performed. Therefore, it is of most benefit
13       when the same pattern is going to be matched many times. This does  not
14       necessarily  mean  many calls of a matching function; if the pattern is
15       not anchored, matching attempts may take place many  times  at  various
16       positions  in  the  subject, even for a single call.  Therefore, if the
17       subject string is very long, it may still pay to use  JIT  for  one-off
18       matches.
19
20       JIT  support  applies  only to the traditional Perl-compatible matching
21       function.  It does not apply when the DFA matching  function  is  being
22       used. The code for this support was written by Zoltan Herczeg.
23

8-BIT, 16-BIT AND 32-BIT SUPPORT

25
26       JIT  support  is available for all of the 8-bit, 16-bit and 32-bit PCRE
27       libraries. To keep this documentation simple, only the 8-bit  interface
28       is described in what follows. If you are using the 16-bit library, sub‐
29       stitute the  16-bit  functions  and  16-bit  structures  (for  example,
30       pcre16_jit_stack  instead  of  pcre_jit_stack).  If  you  are using the
31       32-bit library, substitute the 32-bit functions and  32-bit  structures
32       (for example, pcre32_jit_stack instead of pcre_jit_stack).
33

AVAILABILITY OF JIT SUPPORT

35
36       JIT  support  is  an  optional  feature of PCRE. The "configure" option
37       --enable-jit (or equivalent CMake option) must  be  set  when  PCRE  is
38       built  if  you want to use JIT. The support is limited to the following
39       hardware platforms:
40
41         ARM v5, v7, and Thumb2
42         Intel x86 32-bit and 64-bit
43         MIPS 32-bit
44         Power PC 32-bit and 64-bit
45         SPARC 32-bit (experimental)
46
47       If --enable-jit is set on an unsupported platform, compilation fails.
48
49       A program that is linked with PCRE 8.20 or later can tell if  JIT  sup‐
50       port is available by calling pcre_config() with the PCRE_CONFIG_JIT op‐
51       tion. The result is 1 when JIT is available, and 0 otherwise.  However,
52       a  simple  program does not need to check this in order to use JIT. The
53       normal API is implemented in a way that falls back to the  interpretive
54       code  if JIT is not available. For programs that need the best possible
55       performance, there is also a "fast path" API that is JIT-specific.
56
57       If your program may sometimes be linked with versions of PCRE that  are
58       older  than 8.20, but you want to use JIT when it is available, you can
59       test the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT
60       macro  such  as PCRE_CONFIG_JIT, for compile-time control of your code.
61       Also beware that the pcre_jit_exec() function was not available at  all
62       before  8.32,  and  may  not be available at all if PCRE isn't compiled
63       with --enable-jit. See the "JIT FAST PATH API" section  below  for  de‐
64       tails.
65

SIMPLE USE OF JIT

67
68       You  have  to  do two things to make use of the JIT support in the sim‐
69       plest way:
70
71         (1) Call pcre_study() with the PCRE_STUDY_JIT_COMPILE option for
72             each compiled pattern, and pass the resulting pcre_extra block to
73             pcre_exec().
74
75         (2) Use pcre_free_study() to free the pcre_extra block when it is
76             no longer needed, instead of just freeing it yourself.  This  en‐
77       sures that
78             any JIT data is also freed.
79
80       For  a  program  that may be linked with pre-8.20 versions of PCRE, you
81       can insert
82
83         #ifndef PCRE_STUDY_JIT_COMPILE
84         #define PCRE_STUDY_JIT_COMPILE 0
85         #endif
86
87       so that no option is passed to pcre_study(),  and  then  use  something
88       like this to free the study data:
89
90         #ifdef PCRE_CONFIG_JIT
91             pcre_free_study(study_ptr);
92         #else
93             pcre_free(study_ptr);
94         #endif
95
96       PCRE_STUDY_JIT_COMPILE  requests  the JIT compiler to generate code for
97       complete matches.  If  you  want  to  run  partial  matches  using  the
98       PCRE_PARTIAL_HARD  or  PCRE_PARTIAL_SOFT  options  of  pcre_exec(), you
99       should set one or both of the following options in addition to, or  in‐
100       stead of, PCRE_STUDY_JIT_COMPILE when you call pcre_study():
101
102         PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
103         PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
104
105       If using pcre_jit_exec() and supporting a pre-8.32 version of PCRE, you
106       can insert:
107
108          #if PCRE_MAJOR >= 8 && PCRE_MINOR >= 32
109          pcre_jit_exec(...);
110          #else
111          pcre_exec(...)
112          #endif
113
114       but as described in the "JIT FAST PATH API" section below this  assumes
115       version 8.32 and later are compiled with --enable-jit, which may break.
116
117       The  JIT  compiler  generates  different optimized code for each of the
118       three modes (normal, soft partial, hard partial). When  pcre_exec()  is
119       called,  the appropriate code is run if it is available. Otherwise, the
120       pattern is matched using interpretive code.
121
122       In some circumstances you may need to call additional functions.  These
123       are  described  in the section entitled "Controlling the JIT stack" be‐
124       low.
125
126       If JIT support is not available, PCRE_STUDY_JIT_COMPILE  etc.  are  ig‐
127       nored,  and  no JIT data is created. Otherwise, the compiled pattern is
128       passed to the JIT compiler, which turns it into machine code that  exe‐
129       cutes  much  faster than the normal interpretive code. When pcre_exec()
130       is passed a pcre_extra block containing a pointer to JIT  code  of  the
131       appropriate  mode (normal or hard/soft partial), it obeys that code in‐
132       stead of running the interpreter. The result is identical, but the com‐
133       piled JIT code runs much faster.
134
135       There  are some pcre_exec() options that are not supported for JIT exe‐
136       cution. There are also some pattern items that JIT cannot  handle.  De‐
137       tails  are  given  below.  In both cases, execution automatically falls
138       back to the interpretive code. If you want to know whether JIT was  ac‐
139       tually  used for a particular match, you should arrange for a JIT call‐
140       back function to be set up as described in the section  entitled  "Con‐
141       trolling the JIT stack" below, even if you do not need to supply a non-
142       default JIT stack. Such a callback function is called whenever JIT code
143       is  about  to be obeyed. If the execution options are not right for JIT
144       execution, the callback function is not obeyed.
145
146       If the JIT compiler finds an unsupported item, no JIT  data  is  gener‐
147       ated.  You  can find out if JIT execution is available after studying a
148       pattern by calling pcre_fullinfo() with the PCRE_INFO_JIT option. A re‐
149       sult  of  1  means  that  JIT compilation was successful. A result of 0
150       means that JIT support is not available, or the pattern was not studied
151       with  PCRE_STUDY_JIT_COMPILE  etc., or the JIT compiler was not able to
152       handle the pattern.
153
154       Once a pattern has been studied, with or without JIT, it can be used as
155       many times as you like for matching different subject strings.
156

UNSUPPORTED OPTIONS AND PATTERN ITEMS

158
159       The  only  pcre_exec() options that are supported for JIT execution are
160       PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK, PCRE_NO_UTF32_CHECK, PCRE_NOT‐
161       BOL,   PCRE_NOTEOL,   PCRE_NOTEMPTY,  PCRE_NOTEMPTY_ATSTART,  PCRE_PAR‐
162       TIAL_HARD, and PCRE_PARTIAL_SOFT.
163
164       The only unsupported pattern items are \C (match a  single  data  unit)
165       when  running in a UTF mode, and a callout immediately before an asser‐
166       tion condition in a conditional group.
167

RETURN VALUES FROM JIT EXECUTION

169
170       When a pattern is matched using JIT execution, the  return  values  are
171       the  same as those given by the interpretive pcre_exec() code, with the
172       addition of one new error code: PCRE_ERROR_JIT_STACKLIMIT.  This  means
173       that  the memory used for the JIT stack was insufficient. See "Control‐
174       ling the JIT stack" below for a discussion of JIT stack usage. For com‐
175       patibility  with  the  interpretive pcre_exec() code, no more than two-
176       thirds of the ovector argument is used for passing back  captured  sub‐
177       strings.
178
179       The  error  code  PCRE_ERROR_MATCHLIMIT  is returned by the JIT code if
180       searching a very large pattern tree goes on for too long, as it  is  in
181       the  same circumstance when JIT is not used, but the details of exactly
182       what is counted are not the same. The  PCRE_ERROR_RECURSIONLIMIT  error
183       code is never returned by JIT execution.
184

SAVING AND RESTORING COMPILED PATTERNS

186
187       The  code  that  is  generated by the JIT compiler is architecture-spe‐
188       cific, and is also position dependent. For those reasons it  cannot  be
189       saved  (in a file or database) and restored later like the bytecode and
190       other data of a compiled pattern. Saving and  restoring  compiled  pat‐
191       terns  is not something many people do. More detail about this facility
192       is given in the pcreprecompile documentation. It should be possible  to
193       run  pcre_study() on a saved and restored pattern, and thereby recreate
194       the JIT data, but because JIT compilation uses  significant  resources,
195       it  is  probably  not worth doing this; you might as well recompile the
196       original pattern.
197

CONTROLLING THE JIT STACK

199
200       When the compiled JIT code runs, it needs a block of memory to use as a
201       stack.   By  default,  it  uses 32K on the machine stack. However, some
202       large or complicated patterns need more than this. The  error  PCRE_ER‐
203       ROR_JIT_STACKLIMIT is given when there is not enough stack. Three func‐
204       tions are provided for managing blocks of memory for use as JIT stacks.
205       There  is further discussion about the use of JIT stacks in the section
206       entitled "JIT stack FAQ" below.
207
208       The pcre_jit_stack_alloc() function creates a JIT stack. Its  arguments
209       are  a starting size and a maximum size, and it returns a pointer to an
210       opaque structure of type pcre_jit_stack, or NULL if there is an  error.
211       The  pcre_jit_stack_free() function can be used to free a stack that is
212       no longer needed. (For the technically minded: the address space is al‐
213       located by mmap or VirtualAlloc.)
214
215       JIT  uses far less memory for recursion than the interpretive code, and
216       a maximum stack size of 512K to 1M should be more than enough  for  any
217       pattern.
218
219       The  pcre_assign_jit_stack()  function  specifies  which stack JIT code
220       should use. Its arguments are as follows:
221
222         pcre_extra         *extra
223         pcre_jit_callback  callback
224         void               *data
225
226       The extra argument must be  the  result  of  studying  a  pattern  with
227       PCRE_STUDY_JIT_COMPILE etc. There are three cases for the values of the
228       other two options:
229
230         (1) If callback is NULL and data is NULL, an internal 32K block
231             on the machine stack is used.
232
233         (2) If callback is NULL and data is not NULL, data must be
234             a valid JIT stack, the result of calling pcre_jit_stack_alloc().
235
236         (3) If callback is not NULL, it must point to a function that is
237             called with data as an argument at the start of matching, in
238             order to set up a JIT stack. If the return from the callback
239             function is NULL, the internal 32K stack is used; otherwise the
240             return value must be a valid JIT stack, the result of calling
241             pcre_jit_stack_alloc().
242
243       A callback function is obeyed whenever JIT code is about to be run;  it
244       is  not  obeyed when pcre_exec() is called with options that are incom‐
245       patible for JIT execution. A callback function can therefore be used to
246       determine  whether  a match operation was executed by JIT or by the in‐
247       terpreter.
248
249       You may safely use the same JIT stack for more than one pattern (either
250       by  assigning directly or by callback), as long as the patterns are all
251       matched sequentially in the same thread. In a multithread  application,
252       if  you  do not specify a JIT stack, or if you assign or pass back NULL
253       from a callback, that is thread-safe, because each thread has  its  own
254       machine  stack.  However,  if  you  assign  or pass back a non-NULL JIT
255       stack, this must be a different stack for each thread so that  the  ap‐
256       plication is thread-safe.
257
258       Strictly  speaking,  even more is allowed. You can assign the same non-
259       NULL stack to any number of patterns as long as they are not  used  for
260       matching by multiple threads at the same time. For example, you can as‐
261       sign the same stack to all compiled patterns, and use a global mutex in
262       the  callback  to  wait  until the stack is available for use. However,
263       this is an inefficient solution, and not recommended.
264
265       This is a suggestion for how a multithreaded program that needs to  set
266       up non-default JIT stacks might operate:
267
268         During thread initialization
269           thread_local_var = pcre_jit_stack_alloc(...)
270
271         During thread exit
272           pcre_jit_stack_free(thread_local_var)
273
274         Use a one-line callback function
275           return thread_local_var
276
277       All  the  functions  described in this section do nothing if JIT is not
278       available, and pcre_assign_jit_stack() does nothing  unless  the  extra
279       argument  is  non-NULL and points to a pcre_extra block that is the re‐
280       sult of a successful study with PCRE_STUDY_JIT_COMPILE etc.
281

JIT STACK FAQ

283
284       (1) Why do we need JIT stacks?
285
286       PCRE (and JIT) is a recursive, depth-first engine, so it needs a  stack
287       where  the local data of the current node is pushed before checking its
288       child nodes.  Allocating real machine stack on some platforms is diffi‐
289       cult. For example, the stack chain needs to be updated every time if we
290       extend the stack on PowerPC.  Although it  is  possible,  its  updating
291       time overhead decreases performance. So we do the recursion in memory.
292
293       (2) Why don't we simply allocate blocks of memory with malloc()?
294
295       Modern  operating  systems have a nice feature: they can reserve an ad‐
296       dress space instead of allocating memory. We can safely allocate memory
297       pages inside this address space, so the stack could grow without moving
298       memory data (this is important because of pointers). Thus we can  allo‐
299       cate  1M  address space, and use only a single memory page (usually 4K)
300       if that is enough. However, we can still  grow  up  to  1M  anytime  if
301       needed.
302
303       (3) Who "owns" a JIT stack?
304
305       The owner of the stack is the user program, not the JIT studied pattern
306       or anything else. The user program must ensure that if a stack is  used
307       by  pcre_exec(), (that is, it is assigned to the pattern currently run‐
308       ning), that stack must not be used by any other threads (to avoid over‐
309       writing the same memory area). The best practice for multithreaded pro‐
310       grams is to allocate a stack for each thread,  and  return  this  stack
311       through the JIT callback function.
312
313       (4) When should a JIT stack be freed?
314
315       You can free a JIT stack at any time, as long as it will not be used by
316       pcre_exec() again. When you assign the  stack  to  a  pattern,  only  a
317       pointer  is set. There is no reference counting or any other magic. You
318       can free the patterns and stacks in any order,  anytime.  Just  do  not
319       call  pcre_exec() with a pattern pointing to an already freed stack, as
320       that will cause SEGFAULT. (Also, do not free a stack currently used  by
321       pcre_exec()  in  another  thread). You can also replace the stack for a
322       pattern at any time. You can even free the previous  stack  before  as‐
323       signing a replacement.
324
325       (5)  Should  I  allocate/free  a  stack every time before/after calling
326       pcre_exec()?
327
328       No, because this is too costly in  terms  of  resources.  However,  you
329       could  implement  some clever idea which release the stack if it is not
330       used in let's say two minutes. The JIT callback  can  help  to  achieve
331       this without keeping a list of the currently JIT studied patterns.
332
333       (6)  OK, the stack is for long term memory allocation. But what happens
334       if a pattern causes stack overflow with a stack of 1M? Is that 1M  kept
335       until the stack is freed?
336
337       Especially  on embedded sytems, it might be a good idea to release mem‐
338       ory sometimes without freeing the stack. There is no API  for  this  at
339       the  moment.  Probably a function call which returns with the currently
340       allocated memory for any stack and another which allows releasing  mem‐
341       ory (shrinking the stack) would be a good idea if someone needs this.
342
343       (7) This is too much of a headache. Isn't there any better solution for
344       JIT stack handling?
345
346       No, thanks to Windows. If POSIX threads were used everywhere, we  could
347       throw out this complicated API.
348

EXAMPLE CODE

350
351       This  is  a  single-threaded example that specifies a JIT stack without
352       using a callback.
353
354         int rc;
355         int ovector[30];
356         pcre *re;
357         pcre_extra *extra;
358         pcre_jit_stack *jit_stack;
359
360         re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
361         /* Check for errors */
362         extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
363         jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024);
364         /* Check for error (NULL) */
365         pcre_assign_jit_stack(extra, NULL, jit_stack);
366         rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
367         /* Check results */
368         pcre_free(re);
369         pcre_free_study(extra);
370         pcre_jit_stack_free(jit_stack);
371
372

JIT FAST PATH API

374
375       Because the API described above falls  back  to  interpreted  execution
376       when JIT is not available, it is convenient for programs that are writ‐
377       ten for general use in many  environments.  However,  calling  JIT  via
378       pcre_exec()  does  have a performance impact. Programs that are written
379       for use where JIT is known to be available, and  which  need  the  best
380       possible performance, can instead use a "fast path" API to call JIT ex‐
381       ecution directly instead of calling  pcre_exec()  (obviously  only  for
382       patterns that have been successfully studied by JIT).
383
384       The  fast path function is called pcre_jit_exec(), and it takes exactly
385       the same arguments as pcre_exec(), plus one  additional  argument  that
386       must  point  to a JIT stack. The JIT stack arrangements described above
387       do not apply. The return values are the same as for pcre_exec().
388
389       When you call pcre_exec(), as well as testing for  invalid  options,  a
390       number of other sanity checks are performed on the arguments. For exam‐
391       ple, if the subject pointer is NULL, or its length is negative, an  im‐
392       mediate error is given. Also, unless PCRE_NO_UTF[8|16|32] is set, a UTF
393       subject string is tested for validity. In the interests of speed, these
394       checks  do  not  happen  on  the  JIT fast path, and if invalid data is
395       passed, the result is undefined.
396
397       Bypassing the sanity checks  and  the  pcre_exec()  wrapping  can  give
398       speedups of more than 10%.
399
400       Note  that the pcre_jit_exec() function is not available in versions of
401       PCRE before 8.32 (released in November 2012). If you  need  to  support
402       versions that old you must either use the slower pcre_exec(), or switch
403       between the two codepaths by checking  the  values  of  PCRE_MAJOR  and
404       PCRE_MINOR.
405
406       Due  to  an unfortunate implementation oversight, even in versions 8.32
407       and later there will be no pcre_jit_exec() stub function  defined  when
408       PCRE  is compiled with --disable-jit, which is the default, and there's
409       no way to detect whether PCRE was  compiled  with  --enable-jit  via  a
410       macro.
411
412       If  you  need to support versions older than 8.32, or versions that may
413       not  build  with  --enable-jit,  you  must  either   use   the   slower
414       pcre_exec(), or switch between the two codepaths by checking the values
415       of PCRE_MAJOR and PCRE_MINOR.
416
417       Switching between the two by checking the version assumes that all  the
418       versions  being  targeted  are built with --enable-jit. To also support
419       builds that may use --disable-jit either pcre_exec() must be used, or a
420       compile-time check for JIT via pcre_config() (which assumes the runtime
421       environment will be the same), or as the Git  project  decided  to  do,
422       simply assume that pcre_jit_exec() is present in 8.32 or later unless a
423       compile-time flag is provided, see the "grep:  un-break  building  with
424       PCRE  >= 8.32 without --enable-jit" commit in git.git for an example of
425       that.
426

SEE ALSO

428
429       pcreapi(3)
430

AUTHOR

432
433       Philip Hazel (FAQ by Zoltan Herczeg)
434       University Computing Service
435       Cambridge CB2 3QH, England.
436

REVISION

438
439       Last updated: 05 July 2017
440       Copyright (c) 1997-2017 University of Cambridge.
441
442
443
444PCRE 8.41                        05 July 2017                       PCREJIT(3)
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