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

Diff of /code/trunk/doc/pcrejit.3

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-revision 691 by ph10,
Sun Sep 11 14:31:21 2011 UTC
+revision 761 by ph10,
Tue Nov 22 12:24:26 2011 UTC
 Line 28 
 JIT. The support is limited to the follo
    ARM v5, v7, and Thumb2
    Intel x86 32-bit and 64-bit
    MIPS 32-bit
-   Power PC 32-bit and 64-bit
+   Power PC 32-bit and 64-bit (experimental)
  .sp
- If --enable-jit is set on an unsupported platform, compilation fails.
+ The Power PC support is designated as experimental because it has not been
+ 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
  with the PCRE_CONFIG_JIT option. The result is 1 when JIT is available, and 0
-Line 75 
 interpretive code.
+Line 77 
 interpretive code.
  If the JIT compiler finds an unsupported item, no JIT data is generated. You
  can find out if JIT execution is available after studying a pattern by calling
  \fBpcre_fullinfo()\fP with the PCRE_INFO_JIT option. A result of 1 means that
- JIT compilationw was successful. A result of 0 means that JIT support is not
+ JIT compilation was successful. A result of 0 means that JIT support is not
  available, or the pattern was not studied with PCRE_STUDY_JIT_COMPILE, or the
  JIT compiler was not able to handle the pattern.
+ .P
+ Once a pattern has been studied, with or without JIT, it can be used as many
+ times as you like for matching different subject strings.
  .
  .
  .SH "UNSUPPORTED OPTIONS AND PATTERN ITEMS"
-Line 90 
 supported.
+Line 95 
 supported.
  .P
  The unsupported pattern items are:
  .sp
-   \eC            match a single byte, even in UTF-8 mode
+   \eC            match a single byte; not supported in UTF-8 mode
    (?Cn)          callouts
-   (?(<name>)...  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
    (*COMMIT)      )
    (*MARK)        )
    (*PRUNE)       ) the backtracking control verbs
-Line 131 
 execution.
+Line 132 
 execution.
  .rs
  .sp
  The code that is generated by the JIT compiler is architecture-specific, and is
- also position dependent. For those reasons it cannot be saved and restored like
+ also position dependent. For those reasons it cannot be saved (in a file or
- the bytecode and other data of a compiled pattern. You should be able run
+ database) and restored later like the bytecode and other data of a compiled
- \fBpcre_study()\fP on a saved and restored pattern, and thereby recreate the
+ pattern. Saving and restoring compiled patterns is not something many people
- JIT data, but because JIT compilation uses significant resources, it is
+ do. More detail about this facility is given in the
- probably not worth doing this.
+ .\" HREF
+ \fBpcreprecompile\fP
+ .\"
+ documentation. It should be possible to run \fBpcre_study()\fP on a saved and
+ restored pattern, and thereby recreate the JIT data, but because JIT
+ compilation uses significant resources, it is probably not worth doing this;
+ you might as well recompile the original pattern.
  .
  .
  .\" HTML <a name="stackcontrol"></a>
-Line 146 
 When the compiled JIT code runs, it need
+Line 153 
 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 210 
 is non-NULL and points to a \fBpcre_extr
+Line 223 
 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 246 
 callback.
+Line 327 
 callback.
  .rs
  .sp
  .nf
- Philip Hazel
+ Philip Hazel (FAQ by Zoltan Herczeg)
  University Computing Service
  Cambridge CB2 3QH, England.
  .fi
-Line 256 
 Cambridge CB2 3QH, England.
+Line 337 
 Cambridge CB2 3QH, England.
  .rs
  .sp
  .nf
- Last updated: 06 September 2011
+ Last updated: 22 November 2011
  Copyright (c) 1997-2011 University of Cambridge.
  .fi

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