Optimizing compilers, including those in virtual machines, commonly utilize Static Single Assignment Form as their intermediate representation, but interpreters typically implement stack-oriented virtual machines. This paper introduces an easily interpreted variant of Static Single Assignment Form. Each instruction of this Interpretable Static Single Assignment Form, including the Phi Instruction, has self-contained operational semantics facilitating efficient interpretation. Even the array manipulation instructions possess directly-executable single-assignment semantics. In addition, this paper describes the construction of a prototype virtual machine realizing Interpretable Static Single Assignment Form and reports on its performance. CONTENTS i Contents 1

Static Single Assignment Form is a common intermediate representation for optimizing compilers but several of its features are difficult to implement in a traditional imperative interpreter. An interpreter for Static Single Assignment would enable mixed-mode (interpreting and compiling) virtual machines to be developed around program representations in Static Single Assignment Form. We propose an extension to Static Single Assignment Form, Interpretable Static Single Assignment (ISSA). Each of Interpretable Static Single Assignment's instructions, including the phi instruction, has self-contained operational semantics. In addition, we describe a prototype interpreter of Interpretable Static Single Assignment and report on its performance.

Mobile programs can potentially be malicious. To protect itself, a host that receives such mobile programs from an untrusted party or via an untrusted network connection will want some kind of guarantee that the mobile code is not about to cause any damage. The traditional solution to this problem has been verification, by which the receiving host examines the mobile program to discover all its actions even before starting execution. Unfortunately, aside from consuming computing resources in itself, verification inhibits traditional compiler optimizations, making such verifiable mobile code much less efficient than native code. We have found an alternative solution...