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# Line 20  man page, in case the conversion went wr Line 20  man page, in case the conversion went wr
20  <li><a name="TOC5" href="#SEC5">RETURN VALUES FROM JIT EXECUTION</a>  <li><a name="TOC5" href="#SEC5">RETURN VALUES FROM JIT EXECUTION</a>
21  <li><a name="TOC6" href="#SEC6">SAVING AND RESTORING COMPILED PATTERNS</a>  <li><a name="TOC6" href="#SEC6">SAVING AND RESTORING COMPILED PATTERNS</a>
22  <li><a name="TOC7" href="#SEC7">CONTROLLING THE JIT STACK</a>  <li><a name="TOC7" href="#SEC7">CONTROLLING THE JIT STACK</a>
23  <li><a name="TOC8" href="#SEC8">EXAMPLE CODE</a>  <li><a name="TOC8" href="#SEC8">JIT STACK FAQ</a>
24  <li><a name="TOC9" href="#SEC9">SEE ALSO</a>  <li><a name="TOC9" href="#SEC9">EXAMPLE CODE</a>
25  <li><a name="TOC10" href="#SEC10">AUTHOR</a>  <li><a name="TOC10" href="#SEC10">SEE ALSO</a>
26  <li><a name="TOC11" href="#SEC11">REVISION</a>  <li><a name="TOC11" href="#SEC11">AUTHOR</a>
27    <li><a name="TOC12" href="#SEC12">REVISION</a>
28  </ul>  </ul>
29  <br><a name="SEC1" href="#TOC1">PCRE JUST-IN-TIME COMPILER SUPPORT</a><br>  <br><a name="SEC1" href="#TOC1">PCRE JUST-IN-TIME COMPILER SUPPORT</a><br>
30  <P>  <P>
# Line 57  fully tested. If --enable-jit is set on Line 58  fully tested. If --enable-jit is set on
58  fails.  fails.
59  </P>  </P>
60  <P>  <P>
61  A program can tell if JIT support is available by calling <b>pcre_config()</b>  A program that is linked with PCRE 8.20 or later can tell if JIT support is
62  with the PCRE_CONFIG_JIT option. The result is 1 when JIT is available, and 0  available by calling <b>pcre_config()</b> with the PCRE_CONFIG_JIT option. The
63  otherwise. However, a simple program does not need to check this in order to  result is 1 when JIT is available, and 0 otherwise. However, a simple program
64  use JIT. The API is implemented in a way that falls back to the ordinary PCRE  does not need to check this in order to use JIT. The API is implemented in a
65  code if JIT is not available.  way that falls back to the ordinary PCRE code if JIT is not available.
66    </P>
67    <P>
68    If your program may sometimes be linked with versions of PCRE that are older
69    than 8.20, but you want to use JIT when it is available, you can test
70    the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT macro such
71    as PCRE_CONFIG_JIT, for compile-time control of your code.
72  </P>  </P>
73  <br><a name="SEC3" href="#TOC1">SIMPLE USE OF JIT</a><br>  <br><a name="SEC3" href="#TOC1">SIMPLE USE OF JIT</a><br>
74  <P>  <P>
# Line 75  You have to do two things to make use of Line 82  You have to do two things to make use of
82        no longer needed instead of just freeing it yourself. This        no longer needed instead of just freeing it yourself. This
83        ensures that any JIT data is also freed.        ensures that any JIT data is also freed.
84  </pre>  </pre>
85    For a program that may be linked with pre-8.20 versions of PCRE, you can insert
86    <pre>
87      #ifndef PCRE_STUDY_JIT_COMPILE
88      #define PCRE_STUDY_JIT_COMPILE 0
89      #endif
90    </pre>
91    so that no option is passed to <b>pcre_study()</b>, and then use something like
92    this to free the study data:
93    <pre>
94      #ifdef PCRE_CONFIG_JIT
95          pcre_free_study(study_ptr);
96      #else
97          pcre_free(study_ptr);
98      #endif
99    </pre>
100  In some circumstances you may need to call additional functions. These are  In some circumstances you may need to call additional functions. These are
101  described in the section entitled  described in the section entitled
102  <a href="#stackcontrol">"Controlling the JIT stack"</a>  <a href="#stackcontrol">"Controlling the JIT stack"</a>
# Line 116  supported. Line 138  supported.
138  <P>  <P>
139  The unsupported pattern items are:  The unsupported pattern items are:
140  <pre>  <pre>
141    \C            match a single byte; not supported in UTF-8 mode    \C             match a single byte; not supported in UTF-8 mode
142    (?Cn)          callouts    (?Cn)          callouts
   (?(&#60;name&#62;)...  conditional test on setting of a named subpattern  
   (?(R)...       conditional test on whole pattern recursion  
   (?(Rn)...      conditional test on recursion, by number  
   (?(R&name)...  conditional test on recursion, by name  
143    (*COMMIT)      )    (*COMMIT)      )
144    (*MARK)        )    (*MARK)        )
145    (*PRUNE)       ) the backtracking control verbs    (*PRUNE)       ) the backtracking control verbs
# Line 167  When the compiled JIT code runs, it need Line 185  When the compiled JIT code runs, it need
185  By default, it uses 32K on the machine stack. However, some large or  By default, it uses 32K on the machine stack. However, some large or
186  complicated patterns need more than this. The error PCRE_ERROR_JIT_STACKLIMIT  complicated patterns need more than this. The error PCRE_ERROR_JIT_STACKLIMIT
187  is given when there is not enough stack. Three functions are provided for  is given when there is not enough stack. Three functions are provided for
188  managing blocks of memory for use as JIT stacks.  managing blocks of memory for use as JIT stacks. There is further discussion
189    about the use of JIT stacks in the section entitled
190    <a href="#stackcontrol">"JIT stack FAQ"</a>
191    below.
192  </P>  </P>
193  <P>  <P>
194  The <b>pcre_jit_stack_alloc()</b> function creates a JIT stack. Its arguments  The <b>pcre_jit_stack_alloc()</b> function creates a JIT stack. Its arguments
# Line 234  All the functions described in this sect Line 255  All the functions described in this sect
255  and <b>pcre_assign_jit_stack()</b> does nothing unless the <b>extra</b> argument  and <b>pcre_assign_jit_stack()</b> does nothing unless the <b>extra</b> argument
256  is non-NULL and points to a <b>pcre_extra</b> block that is the result of a  is non-NULL and points to a <b>pcre_extra</b> block that is the result of a
257  successful study with PCRE_STUDY_JIT_COMPILE.  successful study with PCRE_STUDY_JIT_COMPILE.
258    <a name="stackfaq"></a></P>
259    <br><a name="SEC8" href="#TOC1">JIT STACK FAQ</a><br>
260    <P>
261    (1) Why do we need JIT stacks?
262    <br>
263    <br>
264    PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack where
265    the local data of the current node is pushed before checking its child nodes.
266    Allocating real machine stack on some platforms is difficult. For example, the
267    stack chain needs to be updated every time if we extend the stack on PowerPC.
268    Although it is possible, its updating time overhead decreases performance. So
269    we do the recursion in memory.
270    </P>
271    <P>
272    (2) Why don't we simply allocate blocks of memory with <b>malloc()</b>?
273    <br>
274    <br>
275    Modern operating systems have a nice feature: they can reserve an address space
276    instead of allocating memory. We can safely allocate memory pages inside this
277    address space, so the stack could grow without moving memory data (this is
278    important because of pointers). Thus we can allocate 1M address space, and use
279    only a single memory page (usually 4K) if that is enough. However, we can still
280    grow up to 1M anytime if needed.
281    </P>
282    <P>
283    (3) Who "owns" a JIT stack?
284    <br>
285    <br>
286    The owner of the stack is the user program, not the JIT studied pattern or
287    anything else. The user program must ensure that if a stack is used by
288    <b>pcre_exec()</b>, (that is, it is assigned to the pattern currently running),
289    that stack must not be used by any other threads (to avoid overwriting the same
290    memory area). The best practice for multithreaded programs is to allocate a
291    stack for each thread, and return this stack through the JIT callback function.
292    </P>
293    <P>
294    (4) When should a JIT stack be freed?
295    <br>
296    <br>
297    You can free a JIT stack at any time, as long as it will not be used by
298    <b>pcre_exec()</b> again. When you assign the stack to a pattern, only a pointer
299    is set. There is no reference counting or any other magic. You can free the
300    patterns and stacks in any order, anytime. Just <i>do not</i> call
301    <b>pcre_exec()</b> with a pattern pointing to an already freed stack, as that
302    will cause SEGFAULT. (Also, do not free a stack currently used by
303    <b>pcre_exec()</b> in another thread). You can also replace the stack for a
304    pattern at any time. You can even free the previous stack before assigning a
305    replacement.
306    </P>
307    <P>
308    (5) Should I allocate/free a stack every time before/after calling
309    <b>pcre_exec()</b>?
310    <br>
311    <br>
312    No, because this is too costly in terms of resources. However, you could
313    implement some clever idea which release the stack if it is not used in let's
314    say two minutes. The JIT callback can help to achive this without keeping a
315    list of the currently JIT studied patterns.
316    </P>
317    <P>
318    (6) OK, the stack is for long term memory allocation. But what happens if a
319    pattern causes stack overflow with a stack of 1M? Is that 1M kept until the
320    stack is freed?
321    <br>
322    <br>
323    Especially on embedded sytems, it might be a good idea to release
324    memory sometimes without freeing the stack. There is no API for this at the
325    moment. Probably a function call which returns with the currently allocated
326    memory for any stack and another which allows releasing memory (shrinking the
327    stack) would be a good idea if someone needs this.
328    </P>
329    <P>
330    (7) This is too much of a headache. Isn't there any better solution for JIT
331    stack handling?
332    <br>
333    <br>
334    No, thanks to Windows. If POSIX threads were used everywhere, we could throw
335    out this complicated API.
336  </P>  </P>
337  <br><a name="SEC8" href="#TOC1">EXAMPLE CODE</a><br>  <br><a name="SEC9" href="#TOC1">EXAMPLE CODE</a><br>
338  <P>  <P>
339  This is a single-threaded example that specifies a JIT stack without using a  This is a single-threaded example that specifies a JIT stack without using a
340  callback.  callback.
# Line 260  callback. Line 359  callback.
359    
360  </PRE>  </PRE>
361  </P>  </P>
362  <br><a name="SEC9" href="#TOC1">SEE ALSO</a><br>  <br><a name="SEC10" href="#TOC1">SEE ALSO</a><br>
363  <P>  <P>
364  <b>pcreapi</b>(3)  <b>pcreapi</b>(3)
365  </P>  </P>
366  <br><a name="SEC10" href="#TOC1">AUTHOR</a><br>  <br><a name="SEC11" href="#TOC1">AUTHOR</a><br>
367  <P>  <P>
368  Philip Hazel  Philip Hazel (FAQ by Zoltan Herczeg)
369  <br>  <br>
370  University Computing Service  University Computing Service
371  <br>  <br>
372  Cambridge CB2 3QH, England.  Cambridge CB2 3QH, England.
373  <br>  <br>
374  </P>  </P>
375  <br><a name="SEC11" href="#TOC1">REVISION</a><br>  <br><a name="SEC12" href="#TOC1">REVISION</a><br>
376  <P>  <P>
377  Last updated: 19 October 2011  Last updated: 26 November 2011
378  <br>  <br>
379  Copyright &copy; 1997-2011 University of Cambridge.  Copyright &copy; 1997-2011 University of Cambridge.
380  <br>  <br>

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