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sljit

What is sljit?

The sljit is a stack-less platform independent JIT compiler, or perhaps a platform independent assembler is even a better name. Since it is a stack-less implementation, the arguments of sljit LIR (low-level intermediate representation) instructions are either machine registers or memory locations. The memory addressing modes are similar to x86, but simplified a little. It can be an absolute address, a base register with an immediate offset, or a sum of a base and a left shifted offset register. This approach is very effective for byte-code interpreters since their machine independent byte code (middle level representation) typically contains instructions which either can be easly translated to machine code, or not worth to translate them at all.
    Interpreter byte-code instruction examples
      pop - pop from stack
        Very easy to implement in sljit level, since it just decrease the stack pointer by 1.
      add - add
        Fast case for integer addition, and slow case for anything else.
      resolve - resolve an identifier
        Not suitable to do it in JIT level, just call a nativ C++ helper function.

Pattern matching: JIT-ing regular expressions

    PCRE-sljit combines the well known PCRE regular expression library with the performance boost provided by sljit, creating a lightning fast, PERL compatible backtracking engine. More about this project can be found here. Comparing the PCRE-sljit with other engines can be found here.

Challenge

    Different CPU architectures are ... really different. Software ABI (function call, stack handling mechanism) could make the problem even more difficult. The goal of this poject to find a platform independent assembler, which is both platform independent (enough), and all architectures can be supported without considerable overhead (fewest instructions possible). Thus, we can expect high performance from the resulting code on all supported CPUs.

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What a JIT ...

    ... can do
      decrease the number of executed instructions, which speeds up the execution. You can embed constants and constant pointers into the JIT code, so you don't need to access them before use (eliminates several loads). However, its trade-off is the extra memory space consumed by the jitted code. On embedded systems, large amount of JIT-ed code might decrease the efficiency of the instruction cache.
    ... can't do
      miracles. JIT is a good thing if you know what you are doing.

My practical experiences

    - JIT is kind of a code inlining (static compiler optimization). It basically has the same disadvantages as well.
    - focus on the most frequently executed part of your program. (Profiling can help) Never translate complicated algorithms to machine code. Do that in C/C++ level.

Supported architectures

    Intel x86-32
    AMD x86-64
    ARM (Including ARM-v5, ARM-v7 and Thumb2 instruction sets)
    IBM PowerPC-32
    IBM PowerPC-64
    MIPS-32
    SPARC-32

SL-JIT Advantages

    The execution can be continued from any LIR instruction In other words, jump into and out of the code is safe.
    Target of (conditional) jump and call instructions can be dynamically modified during the execution of the code.
    Constants can be modified during the execution of the code.
    Fast, non-ABI compilant function call (when a JIT code calls anoher JIT code). Requires only a few machine instructions.
    Move with update instructions. It means the base register is updated before the actual load or store.

SL-JIT Disadvantages

    Limited number of registers (only 10, max 5 temporary and max 5 general)
    Limited number of float registers (only 4 double registers)

More about the project

The source package contains a readme, which describes how to add sljit to an existing project. The details about the sljit LIR (low-level intermediate representation) is found in sljitLir.h, which is the only file, you need to know to use sljit.

Help needed

I have limited access to various software tools and hardware, which makes testing difficult. You could help me by trying sljit with various compilers (ARM RVCT) and various CPUs, especially mips and sparc.

Contribution

Please join to the forums at sourceforge or send me private mail through sourceforge. My account name is dark100.

Last modification: 2.12.2012