/[pcre]/code/trunk/doc/pcrejit.3

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-revision 835 by ph10,
Wed Dec 28 16:10:09 2011 UTC
+revision 836 by ph10,
Wed Dec 28 17:16:11 2011 UTC
 Line 34 
 The Power PC support is designated as ex
  fully tested. If --enable-jit is set on an unsupported platform, compilation
  fails.
  .P
- A program can tell if JIT support is available by calling \fBpcre_config()\fP
+ A program that is linked with PCRE 8.20 or later can tell if JIT support is
- with the PCRE_CONFIG_JIT option. The result is 1 when JIT is available, and 0
+ available by calling \fBpcre_config()\fP with the PCRE_CONFIG_JIT option. The
- 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
- 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
- code if JIT is not available.
+ way that falls back to the ordinary PCRE code if JIT is not available.
+ .P
+ If your program may sometimes be linked with versions of PCRE that are older
+ than 8.20, but you want to use JIT when it is available, you can test
+ the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT macro such
+ as PCRE_CONFIG_JIT, for compile-time control of your code.
  .
  .
  .SH "SIMPLE USE OF JIT"
-Line 54 
 You have to do two things to make use of
+Line 59 
 You have to do two things to make use of
        no longer needed instead of just freeing it yourself. This
        ensures that any JIT data is also freed.
  .sp
+ For a program that may be linked with pre-8.20 versions of PCRE, you can insert
+ .sp
+   #ifndef PCRE_STUDY_JIT_COMPILE
+   #define PCRE_STUDY_JIT_COMPILE 0
+   #endif
+ .sp
+ so that no option is passed to \fBpcre_study()\fP, and then use something like
+ this to free the study data:
+ .sp
+   #ifdef PCRE_CONFIG_JIT
+       pcre_free_study(study_ptr);
+   #else
+       pcre_free(study_ptr);
+   #endif
+ .sp
  In some circumstances you may need to call additional functions. These are
  described in the section entitled
  .\" HTML <a href="#stackcontrol">
-Line 95 
 supported.
+Line 115 
 supported.
  .P
  The unsupported pattern items are:
  .sp
-   \eC            match a single byte; not supported in UTF-8 mode
+   \eC             match a single byte; not supported in UTF-8 mode
    (?Cn)          callouts
    (*COMMIT)      )
    (*MARK)        )
-Line 153 
 When the compiled JIT code runs, it need
+Line 173 
 When the compiled JIT code runs, it need
  By default, it uses 32K on the machine stack. However, some large or
  complicated patterns need more than this. The error PCRE_ERROR_JIT_STACKLIMIT
  is given when there is not enough stack. Three functions are provided for
- managing blocks of memory for use as JIT stacks.
+ managing blocks of memory for use as JIT stacks. There is further discussion
+ about the use of JIT stacks in the section entitled
+ .\" HTML <a href="#stackcontrol">
+ .\" </a>
+ "JIT stack FAQ"
+ .\"
+ below.
  .P
  The \fBpcre_jit_stack_alloc()\fP function creates a JIT stack. Its arguments
  are a starting size and a maximum size, and it returns a pointer to an opaque
-Line 217 
 is non-NULL and points to a \fBpcre_extr
+Line 243 
 is non-NULL and points to a \fBpcre_extr
  successful study with PCRE_STUDY_JIT_COMPILE.
  .
  .
+ .\" HTML <a name="stackfaq"></a>
+ .SH "JIT STACK FAQ"
+ .rs
+ .sp
+ (1) Why do we need JIT stacks?
+ .sp
+ PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack where
+ the local data of the current node is pushed before checking its child nodes.
+ Allocating real machine stack on some platforms is difficult. For example, the
+ stack chain needs to be updated every time if we extend the stack on PowerPC.
+ Although it is possible, its updating time overhead decreases performance. So
+ we do the recursion in memory.
+ .P
+ (2) Why don't we simply allocate blocks of memory with \fBmalloc()\fP?
+ .sp
+ Modern operating systems have a nice feature: they can reserve an address space
+ instead of allocating memory. We can safely allocate memory pages inside this
+ address space, so the stack could grow without moving memory data (this is
+ important because of pointers). Thus we can allocate 1M address space, and use
+ only a single memory page (usually 4K) if that is enough. However, we can still
+ grow up to 1M anytime if needed.
+ .P
+ (3) Who "owns" a JIT stack?
+ .sp
+ The owner of the stack is the user program, not the JIT studied pattern or
+ anything else. The user program must ensure that if a stack is used by
+ \fBpcre_exec()\fP, (that is, it is assigned to the pattern currently running),
+ that stack must not be used by any other threads (to avoid overwriting the same
+ memory area). The best practice for multithreaded programs is to allocate a
+ stack for each thread, and return this stack through the JIT callback function.
+ .P
+ (4) When should a JIT stack be freed?
+ .sp
+ You can free a JIT stack at any time, as long as it will not be used by
+ \fBpcre_exec()\fP again. When you assign the stack to a pattern, only a pointer
+ is set. There is no reference counting or any other magic. You can free the
+ patterns and stacks in any order, anytime. Just \fIdo not\fP call
+ \fBpcre_exec()\fP with a pattern pointing to an already freed stack, as that
+ will cause SEGFAULT. (Also, do not free a stack currently used by
+ \fBpcre_exec()\fP in another thread). You can also replace the stack for a
+ pattern at any time. You can even free the previous stack before assigning a
+ replacement.
+ .P
+ (5) Should I allocate/free a stack every time before/after calling
+ \fBpcre_exec()\fP?
+ .sp
+ No, because this is too costly in terms of resources. However, you could
+ implement some clever idea which release the stack if it is not used in let's
+ say two minutes. The JIT callback can help to achive this without keeping a
+ list of the currently JIT studied patterns.
+ .P
+ (6) OK, the stack is for long term memory allocation. But what happens if a
+ pattern causes stack overflow with a stack of 1M? Is that 1M kept until the
+ stack is freed?
+ .sp
+ Especially on embedded sytems, it might be a good idea to release
+ memory sometimes without freeing the stack. There is no API for this at the
+ moment. Probably a function call which returns with the currently allocated
+ memory for any stack and another which allows releasing memory (shrinking the
+ stack) would be a good idea if someone needs this.
+ .P
+ (7) This is too much of a headache. Isn't there any better solution for JIT
+ stack handling?
+ .sp
+ No, thanks to Windows. If POSIX threads were used everywhere, we could throw
+ out this complicated API.
+ .
+ .
  .SH "EXAMPLE CODE"
  .rs
  .sp
-Line 253 
 callback.
+Line 347 
 callback.
  .rs
  .sp
  .nf
- Philip Hazel
+ Philip Hazel (FAQ by Zoltan Herczeg)
  University Computing Service
  Cambridge CB2 3QH, England.
  .fi
-Line 263 
 Cambridge CB2 3QH, England.
+Line 357 
 Cambridge CB2 3QH, England.
  .rs
  .sp
  .nf
- Last updated: 15 November 2011
+ Last updated: 26 November 2011
  Copyright (c) 1997-2011 University of Cambridge.
  .fi

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