Optimizing code-copying JIT compilers for virtual stack machines

摘要

Just-in-time (JIT) compilers are widely used to implement stack-based virtual machines, such as the Java and .NET virtual machines. One disadvantage of most JIT compilers is that they are unportable; much of the back-end is specific to the target machine. An alternative to machine-specific code generation methods is to define a routine in a high-level language for each virtual machine instruction. These can be compiled to native code using a normal C compiler. The native code for these routines can then be strung together, allowing very simple, unoptimized code to be produced just in time. In this paper we present such a system based on an existing implementation of the Forth language. We present a novel system of optimizations for the system based on exploiting common sequences of virtual machine instructions. We use a small domain specific language and tool to generate stack-optimized code for sequences of virtual machine instructions, and for choosing the most useful sequences for a code-copying compiler. By measuring the length of the resulting executable code, we allow machine-specific sequences to be chosen without any machine-dependent code in our system. Experimental results show that best (average) speedups of 47.2% (15.75%) are possible on a Pentium 4 machine, and even higher an a PowerPC based machine. Furthermore, our optimizations allow the size of the generated code to be reduced by an average of 17.9% on the Pentium 4, and 20.5% on the PowerPC over a wide range of programs.