The concept of a supercompiler

Abstract

A supercompiler is a program transformer of a certain type. It traces the possible generalized histories of computation by the original program, and compiles an equivalent program, reducing in the process the redundancy that could be present in the original program. The nature of the redundancy that can be eliminated by supercompilation may be various, e.g., some variables might have predefined values (as in partial evaluation), or the structure of control transfer could be made more efficient (as in lazy evaluation), or it could simply be the fact that the same variable is used more than once. The general principles of supercompilation are described and compared with the usual approach to program transformation as a stepwise application of a number of equivalence rules. It is argued that the language Refal serves the needs of supercompilation best. Refal is formally defined and compared with Prolog and other languages. Examples are given of the operation of a Refal supercompiler implemented at CCNY on an IBM/370.

Occam's razor in metacomputation: the notion of a perfect process tree

“…How can one construct a process graph for a program and an initial configuration? Supercompilation [22,24] uses two methods for graph development: driving and folding .…”

“…This is a general method for constructing a (potentially infinite) process tree (a process graph that happens to be a tree) by step-wise exploring all possible computations of a program p starting from an initial configuration ci [20,22,24]. At any point during driving one has a perhaps incomplete process tree and a way to extend the process tree by adding a new node.…”

“…Note the conditional in S-Graph: the test Cntr may update the environment. As in su percompila tion, we refer to such tests as contractions [24]. Two elementary contractions are sufficient for S-expressions:…”

“…If we do not want to define perfect driving, then we may approximate the sets of states using covering config ura tions that represent larger sets of states. [24]. We introduce placeholders, called configuration variables (cvariables), which range over arbitrary con stants.…”

“…The principles of driving were first formulated in the early seventies by Turchin [20,21] and further developed in the eighties [22,24]. From its very inception, supercompilation has been tied to a specific programming language, called Refal [24].…”