Just-in-time compiling is a heavyweight optimization that can greatly speed up pattern matching. However, it comes at the cost of extra processing before the match is performed. Therefore, it is of most benefit when the same pattern is -going to be matched many times. This does not necessarily mean many calls of -\fPpcre_exec()\fP; if the pattern is not anchored, matching attempts may take -place many times at various positions in the subject, even for a single call to -pcre_exec(). If the subject string is very long, it may still pay to use -JIT for one-off matches. +going to be matched many times. This does not necessarily mean many calls of a +matching function; if the pattern is not anchored, matching attempts may take +place many times at various positions in the subject, even for a single call. +Therefore, if the subject string is very long, it may still pay to use JIT for +one-off matches.
-JIT support applies only to the traditional matching function, -pcre_exec(). It does not apply when pcre_dfa_exec() is being used. -The code for this support was written by Zoltan Herczeg. +JIT support applies only to the traditional Perl-compatible matching function. +It does not apply when the DFA matching function is being used. The code for +this support was written by Zoltan Herczeg.
-+JIT support is available for both the 8-bit and 16-bit PCRE libraries. To keep +this documentation simple, only the 8-bit interface is described in what +follows. If you are using the 16-bit library, substitute the 16-bit functions +and 16-bit structures (for example, pcre16_jit_stack instead of +pcre_jit_stack). +
+JIT support is an optional feature of PCRE. The "configure" option --enable-jit (or equivalent CMake option) must be set when PCRE is built if you want to use JIT. The support is limited to the following hardware platforms: @@ -51,18 +60,16 @@ JIT. The support is limited to the following hardware ARM v5, v7, and Thumb2 Intel x86 32-bit and 64-bit MIPS 32-bit - Power PC 32-bit and 64-bit (experimental) + Power PC 32-bit and 64-bit -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. +If --enable-jit is set on an unsupported platform, compilation fails.
A program that is linked with PCRE 8.20 or later can tell if JIT support is available by calling pcre_config() with the PCRE_CONFIG_JIT option. The result is 1 when JIT is available, and 0 otherwise. However, a simple program does not need to check this in order to use JIT. The API is implemented in a -way that falls back to the ordinary PCRE code if JIT is not available. +way that falls back to the interpretive code if JIT is not available.
If your program may sometimes be linked with versions of PCRE that are older @@ -70,7 +77,7 @@ than 8.20, but you want to use JIT when it is availabl 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.
-You have to do two things to make use of the JIT support in the simplest way:
@@ -79,7 +86,7 @@ You have to do two things to make use of the JIT suppo
pcre_exec().
(2) Use pcre_free_study() to free the pcre_extra block when it is
- no longer needed instead of just freeing it yourself. This
+ no longer needed, instead of just freeing it yourself. This
ensures that any JIT data is also freed.
For a program that may be linked with pre-8.20 versions of PCRE, you can insert
@@ -97,58 +104,78 @@ this to free the study data:
pcre_free(study_ptr);
#endif
+PCRE_STUDY_JIT_COMPILE requests the JIT compiler to generate code for complete
+matches. If you want to run partial matches using the PCRE_PARTIAL_HARD or
+PCRE_PARTIAL_SOFT options of pcre_exec(), you should set one or both of
+the following options in addition to, or instead of, PCRE_STUDY_JIT_COMPILE
+when you call pcre_study():
++ PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE + PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE ++The JIT compiler generates different optimized code for each of the three +modes (normal, soft partial, hard partial). When pcre_exec() is called, +the appropriate code is run if it is available. Otherwise, the pattern is +matched using interpretive code. + +
In some circumstances you may need to call additional functions. These are described in the section entitled "Controlling the JIT stack" below.
-If JIT support is not available, PCRE_STUDY_JIT_COMPILE is ignored, and no JIT -data is set up. Otherwise, the compiled pattern is passed to the JIT compiler, -which turns it into machine code that executes much faster than the normal -interpretive code. When pcre_exec() is passed a pcre_extra block -containing a pointer to JIT code, it obeys that instead of the normal code. The -result is identical, but the code runs much faster. +If JIT support is not available, PCRE_STUDY_JIT_COMPILE etc. are ignored, and +no JIT data is created. Otherwise, the compiled pattern is passed to the JIT +compiler, which turns it into machine code that executes much faster than the +normal interpretive code. When pcre_exec() is passed a pcre_extra +block containing a pointer to JIT code of the appropriate mode (normal or +hard/soft partial), it obeys that code instead of running the interpreter. The +result is identical, but the compiled JIT code runs much faster.
There are some pcre_exec() options that are not supported for JIT execution. There are also some pattern items that JIT cannot handle. Details are given below. In both cases, execution automatically falls back to the -interpretive code. +interpretive code. If you want to know whether JIT was actually used for a +particular match, you should arrange for a JIT callback function to be set up +as described in the section entitled +"Controlling the JIT stack" +below, even if you do not need to supply a non-default JIT stack. Such a +callback function is called whenever JIT code is about to be obeyed. If the +execution options are not right for JIT execution, the callback function is not +obeyed.
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 pcre_fullinfo() with the PCRE_INFO_JIT option. A result of 1 means that 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. +available, or the pattern was not studied with PCRE_STUDY_JIT_COMPILE etc., or +the JIT compiler was not able to handle the pattern.
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.
-The only pcre_exec() options that are supported for JIT execution are -PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and -PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not -supported. +PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, +PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT.
The unsupported pattern items are:
\C match a single byte; not supported in UTF-8 mode (?Cn) callouts - (*COMMIT) ) - (*MARK) ) - (*PRUNE) ) the backtracking control verbs - (*SKIP) ) + (*PRUNE) ) + (*SKIP) ) backtracking control verbs (*THEN) )Support for some of these may be added in future. -
When a pattern is matched using JIT execution, the return values are the same as those given by the interpretive pcre_exec() code, with the addition of @@ -166,7 +193,7 @@ when JIT is not used, but the details of exactly what same. The PCRE_ERROR_RECURSIONLIMIT error code is never returned by JIT execution.
-The code that is generated by the JIT compiler is architecture-specific, and is also position dependent. For those reasons it cannot be saved (in a file or @@ -179,7 +206,7 @@ restored pattern, and thereby recreate the JIT data, b compilation uses significant resources, it is probably not worth doing this; you might as well recompile the original pattern.
-When the compiled JIT code runs, it needs a block of memory to use as a stack. By default, it uses 32K on the machine stack. However, some large or @@ -212,8 +239,8 @@ should use. Its arguments are as follows: void *data The extra argument must be the result of studying a pattern with -PCRE_STUDY_JIT_COMPILE. There are three cases for the values of the other two -options: +PCRE_STUDY_JIT_COMPILE etc. There are three cases for the values of the other +two options:
(1) If callback is NULL and data is NULL, an internal 32K block
on the machine stack is used.
@@ -221,26 +248,38 @@ options:
(2) If callback is NULL and data is not NULL, data must be
a valid JIT stack, the result of calling pcre_jit_stack_alloc().
- (3) If callback not NULL, it must point to a function that is called
- with data as an argument at the start of matching, in order to
- set up a JIT stack. If the result is NULL, the internal 32K stack
- is used; otherwise the return value must be a valid JIT stack,
- the result of calling pcre_jit_stack_alloc().
+ (3) If callback is not NULL, it must point to a function that is
+ called with data as an argument at the start of matching, in
+ order to set up a JIT stack. If the return from the callback
+ function is NULL, the internal 32K stack is used; otherwise the
+ return value must be a valid JIT stack, the result of calling
+ pcre_jit_stack_alloc().
-You may safely assign the same JIT stack to more than one pattern, as long as
-they are all matched sequentially in the same thread. In a multithread
-application, each thread must use its own JIT stack.
+A callback function is obeyed whenever JIT code is about to be run; it is not
+obeyed when pcre_exec() is called with options that are incompatible for
+JIT execution. A callback function can therefore be used to determine whether a
+match operation was executed by JIT or by the interpreter.
-Strictly speaking, even more is allowed. You can assign the same stack to any -number of patterns as long as they are not used for matching by multiple +You may safely use the same JIT stack for more than one pattern (either by +assigning directly or by callback), as long as the patterns are all matched +sequentially in the same thread. In a multithread application, if you do not +specify a JIT stack, or if you assign or pass back NULL from a callback, that +is thread-safe, because each thread has its own machine stack. However, if you +assign or pass back a non-NULL JIT stack, this must be a different stack for +each thread so that the application is thread-safe. +
++Strictly speaking, even more is allowed. You can assign the same non-NULL stack +to any number of patterns as long as they are not used for matching by multiple threads at the same time. For example, you can assign the same stack to all compiled patterns, and use a global mutex in the callback to wait until the -stack is available for use. However, this is an inefficient solution, and -not recommended. +stack is available for use. However, this is an inefficient solution, and not +recommended.
-This is a suggestion for how a typical multithreaded program might operate: +This is a suggestion for how a multithreaded program that needs to set up +non-default JIT stacks might operate:
During thread initalization
thread_local_var = pcre_jit_stack_alloc(...)
@@ -254,9 +293,9 @@ This is a suggestion for how a typical multithreaded p
All the functions described in this section do nothing if JIT is not available,
and pcre_assign_jit_stack() does nothing unless the extra argument
is non-NULL and points to a pcre_extra block that is the result of a
-successful study with PCRE_STUDY_JIT_COMPILE.
+successful study with PCRE_STUDY_JIT_COMPILE etc.
-
JIT STACK FAQ
+
JIT STACK FAQ
(1) Why do we need JIT stacks?
@@ -320,11 +359,11 @@ pattern causes stack overflow with a stack of 1M? Is t
stack is freed?
-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.
+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.
(7) This is too much of a headache. Isn't there any better solution for JIT
@@ -334,7 +373,7 @@ stack handling?
No, thanks to Windows. If POSIX threads were used everywhere, we could throw
out this complicated API.
-
EXAMPLE CODE
+
EXAMPLE CODE
This is a single-threaded example that specifies a JIT stack without using a
callback.
@@ -359,11 +398,11 @@ callback.
-pcreapi(3)
-
Philip Hazel (FAQ by Zoltan Herczeg)
@@ -372,11 +411,11 @@ University Computing Service
Cambridge CB2 3QH, England.
-Last updated: 26 November 2011
+Last updated: 04 May 2012
-Copyright © 1997-2011 University of Cambridge.
+Copyright © 1997-2012 University of Cambridge.
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