Extending Java virtual machine with integer-reference conversion

Java virtual machine (JVM) is an architecture-independent code execution en vironment. It has recently been used not only for the Java language but als o for other languages such as Scheme and ML. On JVM, however, all values ar e statically typed as either immediate or reference, and types are checked before the execution of a program to prove that invalid memory access will never occur. This property sometimes makes implementation of other language s on JVM inefficient, In particular, implementation of a dynamically typed language is very inefficient because all possible values including frequent ly used ones such as integers must be represented by instances of a class, In this paper, we introduce a new type into JVM, which is a supertype of re ference types and a tagged integer type. This allows a more efficient imple mentation of dynamically typed language on JVM, It does not require any new instruction, maintains binary-compatibility of existing bytecode, and reta ins the safety of the original JVM, We modified an existing Scheme system r unning on JVM to exploit this extension and got a factor of 20 speedup for simple integer functions. Our extension imposes little performance penalty on existing JVM code generated from Java; we observed essentially no penalt y for Spec JVM benchmarks. Copyright (C) 2000 John Wiley & Sons, Ltd.