# pcre2jit

## NAME



           PCRE2 - Perl-compatible regular expressions (revised API)
    


## PCRE2 JUST-IN-TIME COMPILER SUPPORT



    
           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, so it is of
           most benefit when the same pattern is 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 even for one-off matches. JIT support
           is available for all of the 8-bit, 16-bit and 32-bit PCRE2
           libraries.
    
           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 JIT support was written by
           Zoltan Herczeg.
    


## AVAILABILITY OF JIT SUPPORT



    
           JIT support is an optional feature of PCRE2. The "configure"
           option --enable-jit (or equivalent CMake option) must be set when
           PCRE2 is built if you want to use JIT. The support is limited to
           the following hardware platforms:
    
             ARM 32-bit (v7, and Thumb2)
             ARM 64-bit
             IBM s390x 64 bit
             Intel x86 32-bit and 64-bit
             LoongArch 64 bit
             MIPS 32-bit and 64-bit
             Power PC 32-bit and 64-bit
             RISC-V 32-bit and 64-bit
    
           If --enable-jit is set on an unsupported platform, compilation
           fails.
    
           A client program can tell if JIT support has been compiled by
           calling **pcre2_config()** with the PCRE2_CONFIG_JIT option. The
           result is one if PCRE2 was built with JIT support, and zero
           otherwise. However, having the JIT code available does not
           guarantee that it will be used for any particular match. One
           reason for this is that there are a number of options and pattern
           items that are not supported by JIT (see below). Another reason is
           that in some environments JIT is unable to get executable memory
           in which to build its compiled code. The only guarantee from
           **pcre2_config()** is that if it returns zero, JIT will definitely _not_
           be used.
    
           As of release 10.45 there is a more informative way to test for
           JIT support. If **pcre2_compile_jit()** is called with the single
           option PCRE2_JIT_TEST_ALLOC it returns zero if JIT is available
           and has a working allocator. Otherwise it returns
           PCRE2_ERROR_NOMEMORY if JIT is available but cannot allocate
           executable memory, or PCRE2_ERROR_JIT_UNSUPPORTED if JIT support
           is not compiled. The code argument is ignored, so it can be a NULL
           value.
    
           A simple program does not need to check availability in order to
           use JIT when possible. The API is implemented in a way that falls
           back to the interpretive code if JIT is not available or cannot be
           used for a given match. For programs that need the best possible
           performance, there is a "fast path" API that is JIT-specific.
    


## SIMPLE USE OF JIT



    
           To make use of the JIT support in the simplest way, all you have
           to do is to call **pcre2_jit_compile()** after successfully compiling
           a pattern with **pcre2_compile()**. This function has two arguments:
           the first is the compiled pattern pointer that was returned by
           **pcre2_compile()** , and the second is zero or more of the following
           option bits: PCRE2_JIT_COMPLETE, PCRE2_JIT_PARTIAL_HARD, or
           PCRE2_JIT_PARTIAL_SOFT.
    
           If JIT support is not available, a call to **pcre2_jit_compile()**
           does nothing and returns PCRE2_ERROR_JIT_BADOPTION. 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, but yields exactly the same results. The
           returned value from **pcre2_jit_compile()** is zero on success, or a
           negative error code.
    
           There is a limit to the size of pattern that JIT supports, imposed
           by the size of machine stack that it uses. The exact rules are not
           documented because they may change at any time, in particular,
           when new optimizations are introduced.  If a pattern is too big, a
           call to **pcre2_jit_compile()** returns PCRE2_ERROR_NOMEMORY.
    
           PCRE2_JIT_COMPLETE requests the JIT compiler to generate code for
           complete matches. If you want to run partial matches using the
           PCRE2_PARTIAL_HARD or PCRE2_PARTIAL_SOFT options of **pcre2_match()** ,
           you should set one or both of the other options as well as, or
           instead of PCRE2_JIT_COMPLETE. The JIT compiler generates
           different optimized code for each of the three modes (normal, soft
           partial, hard partial). When **pcre2_match()** is called, the
           appropriate code is run if it is available. Otherwise, the pattern
           is matched using interpretive code.
    
           You can call **pcre2_jit_compile()** multiple times for the same
           compiled pattern. It does nothing if it has previously compiled
           code for any of the option bits. For example, you can call it once
           with PCRE2_JIT_COMPLETE and (perhaps later, when you find you need
           partial matching) again with PCRE2_JIT_COMPLETE and
           PCRE2_JIT_PARTIAL_HARD. This time it will ignore
           PCRE2_JIT_COMPLETE and just compile code for partial matching. If
           **pcre2_jit_compile()** is called with no option bits set, it
           immediately returns zero. This is an alternative way of testing
           whether JIT support has been compiled.
    
           At present, it is not possible to free JIT compiled code except
           when the entire compiled pattern is freed by calling
           **pcre2_code_free()**.
    
           In some circumstances you may need to call additional functions.
           These are described in the section entitled "Controlling the JIT
           stack" below.
    
           There are some **pcre2_match()** options that are not supported by
           JIT, and there are also some pattern items that JIT cannot handle.
           Details are given below.  In both cases, matching automatically
           falls back to the 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
           match-time 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 compilation was successful for
           a compiled pattern by calling **pcre2_pattern_info()** with the
           PCRE2_INFO_JITSIZE option. A non-zero result means that JIT
           compilation was successful. A result of 0 means that JIT support
           is not available, or the pattern was not processed by
           **pcre2_jit_compile()** , or the JIT compiler was not able to handle
           the pattern. Successful JIT compilation does not, however,
           guarantee the use of JIT at match time because there are some
           match time options that are not supported by JIT.
    


## MATCHING SUBJECTS CONTAINING INVALID UTF



    
           When a pattern is compiled with the PCRE2_UTF option, subject
           strings are normally expected to be a valid sequence of UTF code
           units. By default, this is checked at the start of matching and an
           error is generated if invalid UTF is detected. The
           PCRE2_NO_UTF_CHECK option can be passed to **pcre2_match()** to skip
           the check (for improved performance) if you are sure that a
           subject string is valid. If this option is used with an invalid
           string, the result is undefined. The calling program may crash or
           loop or otherwise misbehave.
    
           However, a way of running matches on strings that may contain
           invalid UTF sequences is available. Calling **pcre2_compile()** with
           the PCRE2_MATCH_INVALID_UTF option has two effects: it tells the
           interpreter in **pcre2_match()** to support invalid UTF, and, if
           **pcre2_jit_compile()** is subsequently called, the compiled JIT code
           also supports invalid UTF.  Details of how this support works, in
           both the JIT and the interpretive cases, is given in the
           **pcre2unicode** documentation.
    
           There is also an obsolete option for **pcre2_jit_compile()** called
           PCRE2_JIT_INVALID_UTF, which currently exists only for backward
           compatibility.  It is superseded by the **pcre2_compile()** option
           PCRE2_MATCH_INVALID_UTF and should no longer be used. It may be
           removed in future.
    


## UNSUPPORTED OPTIONS AND PATTERN ITEMS



    
           The **pcre2_match()** options that are supported for JIT matching are
           PCRE2_COPY_MATCHED_SUBJECT, PCRE2_NOTBOL, PCRE2_NOTEOL,
           PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART, PCRE2_NO_UTF_CHECK,
           PCRE2_PARTIAL_HARD, and PCRE2_PARTIAL_SOFT. The PCRE2_ANCHORED and
           PCRE2_ENDANCHORED options are not supported at match time.
    
           If the PCRE2_NO_JIT option is passed to **pcre2_match()** it disables
           the use of JIT, forcing matching by the interpreter code.
    
           The only unsupported pattern items are \C (match a single data
           unit) when running in a UTF mode, and a callout immediately before
           an assertion condition in a conditional group.
    


## RETURN VALUES FROM JIT MATCHING



    
           When a pattern is matched using JIT, the return values are the
           same as those given by the interpretive **pcre2_match()** code, with
           the addition of one new error code: PCRE2_ERROR_JIT_STACKLIMIT.
           This means that the memory used for the JIT stack was
           insufficient. See "Controlling the JIT stack" below for a
           discussion of JIT stack usage.
    
           The error code PCRE2_ERROR_MATCHLIMIT is returned by the JIT code
           if searching a very large pattern tree goes on for too long, as it
           is in the same circumstance when JIT is not used, but the details
           of exactly what is counted are not the same. The
           PCRE2_ERROR_DEPTHLIMIT error code is never returned when JIT
           matching is used.
    


## CONTROLLING THE JIT STACK



    
           When the compiled JIT code runs, it needs a block of memory to use
           as a stack.  By default, it uses 32KiB on the machine stack.
           However, some large or complicated patterns need more than this.
           The error PCRE2_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. There is further discussion about
           the use of JIT stacks in the section entitled "JIT stack FAQ"
           below.
    
           The **pcre2_jit_stack_create()** function creates a JIT stack. Its
           arguments are a starting size, a maximum size, and a general
           context (for memory allocation functions, or NULL for standard
           memory allocation). It returns a pointer to an opaque structure of
           type **pcre2_jit_stack** , or NULL if there is an error. The
           **pcre2_jit_stack_free()** function is used to free a stack that is no
           longer needed. If its argument is NULL, this function returns
           immediately, without doing anything. (For the technically minded:
           the address space is allocated by mmap or VirtualAlloc.) A maximum
           stack size of 512KiB to 1MiB should be more than enough for any
           pattern.
    
           The **pcre2_jit_stack_assign()** function specifies which stack JIT
           code should use. Its arguments are as follows:
    
             pcre2_match_context  *mcontext
             pcre2_jit_callback    callback
             void                 *data
    
           The first argument is a pointer to a match context. When this is
           subsequently passed to a matching function, its information
           determines which JIT stack is used. If this argument is NULL, the
           function returns immediately, without doing anything. There are
           three cases for the values of the other two options:
    
             (1) If _callback_ is NULL and _data_ is NULL, an internal 32KiB
           block
                 on the machine stack is used. This is the default when a
           match
                 context is created.
    
             (2) If _callback_ is NULL and _data_ is not NULL, _data_ must be
                 a pointer to a valid JIT stack, the result of calling
                 **pcre2_jit_stack_create()**.
    
             (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 32KiB stack is used;
           otherwise the
                 return value must be a valid JIT stack, the result of
           calling
                 **pcre2_jit_stack_create()**.
    
           A callback function is obeyed whenever JIT code is about to be
           run; it is not obeyed when **pcre2_match()** is called with options
           that are incompatible for JIT matching. A callback function can
           therefore be used to determine whether a match operation was
           executed by JIT or by the interpreter.
    
           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 matched sequentially in the same thread. Currently,
           the only way to set up non-sequential matches in one thread is to
           use callouts: if a callout function starts another match, that
           match must use a different JIT stack to the one used for currently
           suspended match(es).
    
           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 a match context that is used by any number of
           patterns, as long as they are not used for matching by multiple
           threads at the same time. For example, you could use the same
           stack in all compiled patterns, with a global mutex in the
           callback to wait until the stack is available for use. However,
           this is an inefficient solution, and not recommended.
    
           This is a suggestion for how a multithreaded program that needs to
           set up non-default JIT stacks might operate:
    
             During thread initialization
               thread_local_var = pcre2_jit_stack_create(...)
    
             During thread exit
               pcre2_jit_stack_free(thread_local_var)
    
             Use a one-line callback function
               return thread_local_var
    
           All the functions described in this section do nothing if JIT is
           not available.
    


## JIT STACK FAQ



    
           (1) Why do we need JIT stacks?
    
           PCRE2 (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.
    
           (2) Why don't we simply allocate blocks of memory with **malloc()**?
    
           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 1MiB address space, and use only a
           single memory page (usually 4KiB) if that is enough. However, we
           can still grow up to 1MiB anytime if needed.
    
           (3) Who "owns" a JIT stack?
    
           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 being used by **pcre2_match()** , (that is, it is assigned to
           a match context that is passed 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.
    
           (4) When should a JIT stack be freed?
    
           You can free a JIT stack at any time, as long as it will not be
           used by **pcre2_match()** again. When you assign the stack to a match
           context, only a pointer is set. There is no reference counting or
           any other magic. You can free compiled patterns, contexts, and
           stacks in any order, anytime.  Just _do not_ call **pcre2_match()** with
           a match context pointing to an already freed stack, as that will
           cause SEGFAULT. (Also, do not free a stack currently used by
           **pcre2_match()** in another thread). You can also replace the stack
           in a context at any time when it is not in use. You should free
           the previous stack before assigning a replacement.
    
           (5) Should I allocate/free a stack every time before/after calling
           **pcre2_match()**?
    
           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
           achieve this without keeping a list of patterns.
    
           (6) OK, the stack is for long term memory allocation. But what
           happens if a pattern causes stack overflow with a stack of 1MiB?
           Is that 1MiB kept until the stack is freed?
    
           Especially on embedded systems, 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 stack handling?
    
           No, thanks to Windows. If POSIX threads were used everywhere, we
           could throw out this complicated API.
    


## FREEING JIT SPECULATIVE MEMORY



    
           **void pcre2_jit_free_unused_memory(pcre2_general_context ***_gcontext_**);**
    
           The JIT executable allocator does not free all memory when it is
           possible. It expects new allocations, and keeps some free memory
           around to improve allocation speed. However, in low memory
           conditions, it might be better to free all possible memory. You
           can cause this to happen by calling
           pcre2_jit_free_unused_memory(). Its argument is a general context,
           for custom memory management, or NULL for standard memory
           management.
    


## EXAMPLE CODE



    
           This is a single-threaded example that specifies a JIT stack
           without using a callback. A real program should include error
           checking after all the function calls.
    
             int rc;
             pcre2_code *re;
             pcre2_match_data *match_data;
             pcre2_match_context *mcontext;
             pcre2_jit_stack *jit_stack;
    
             re = pcre2_compile(pattern, PCRE2_ZERO_TERMINATED, 0,
               &errornumber, &erroffset, NULL);
             rc = pcre2_jit_compile(re, PCRE2_JIT_COMPLETE);
             mcontext = pcre2_match_context_create(NULL);
             jit_stack = pcre2_jit_stack_create(32*1024, 512*1024, NULL);
             pcre2_jit_stack_assign(mcontext, NULL, jit_stack);
             match_data = pcre2_match_data_create(re, 10);
             rc = pcre2_match(re, subject, length, 0, 0, match_data,
           mcontext);
             /* Process result */
    
             pcre2_code_free(re);
             pcre2_match_data_free(match_data);
             pcre2_match_context_free(mcontext);
             pcre2_jit_stack_free(jit_stack);
    


## JIT FAST PATH API



    
           Because the API described above falls back to interpreted matching
           when JIT is not available, it is convenient for programs that are
           written for general use in many environments. However, calling JIT
           via **pcre2_match()** does have a performance impact. Programs that
           are written for use where JIT is known to be available, and which
           need the best possible performance, can instead use a "fast path"
           API to call JIT matching directly instead of calling **pcre2_match()**
           (obviously only for patterns that have been successfully processed
           by **pcre2_jit_compile()**).
    
           The fast path function is called **pcre2_jit_match()** , and it takes
           exactly the same arguments as **pcre2_match()**. However, the subject
           string must be specified with a length; PCRE2_ZERO_TERMINATED is
           not supported. Unsupported option bits (for example,
           PCRE2_ANCHORED and PCRE2_ENDANCHORED) are ignored, as is the
           PCRE2_NO_JIT option. The return values are also the same as for
           **pcre2_match()** , plus PCRE2_ERROR_JIT_BADOPTION if a matching mode
           (partial or complete) is requested that was not compiled.
    
           When you call **pcre2_match()** , as well as testing for invalid
           options, a number of other sanity checks are performed on the
           arguments. For example, if the subject pointer is NULL but the
           length is non-zero, an immediate error is given. Also, unless
           PCRE2_NO_UTF_CHECK is set, a UTF subject string is tested for
           validity. In the interests of speed, these checks do not happen on
           the JIT fast path. If invalid UTF data is passed when
           PCRE2_MATCH_INVALID_UTF was not set for **pcre2_compile()** , the
           result is undefined. The program may crash or loop or give wrong
           results. In the absence of PCRE2_MATCH_INVALID_UTF you should call
           **pcre2_jit_match()** in UTF mode only if you are sure the subject is
           valid.
    
           Bypassing the sanity checks and the **pcre2_match()** wrapping can
           give speedups of more than 10%.
    


## REVISION



    
           Last updated: 22 August 2024
           Copyright (c) 1997-2024 University of Cambridge.
    


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