Optimizing code‐copying JIT compilers for virtual stack machines

Abstract

AbstractJust‐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. Copyright © 2006 John Wiley & Sons, Ltd.