Semantic approach to automatic program improvement

Abstract

The programs that are easiest to write and understand are often not the most efficient. This thesis gives methods of converting programs of the former type to those of the latter type; this involves converting definitions of algorithms given as recursion equations using high level primitives into lower level flow chart programs. The main activities involved are recursion removal (c.f. Strong), loop elimination, and the overwriting of shared structures. We have concentrated on the semantics, rather than the syntax, of the programs we are transforming and we have used techniques developed in work done on proving the correctness of programs. The transformations are done in a hierarchical manner and can be regarded as compiling a program defined in a structured manner (Dijkstra) to produce an efficient low level program that simulates it. We describe the implementation of a system that allows the user to specify algorithms in a simple set language and converts them to flow chart programs in either a bitstring or list processing language. Both of these lower languages allow the sharing of structures. The principles are applicable to other domains and we describe how our system can be applied more generally.