An Environment for Automatic Generation of Code Optimizers

Abstract

Code optimization or code transformation is a complex function of a compiler involving analyses and modifications with the entire program as its scope. In spite of its complexity, hardly any tools exist to support this function of the compiler. This thesis presents the development of a code transformation system, specifically for scalar transformations, which can be used either as a tool to assist the generation of code transformers or as an environment for experimentation with code transformations.

The development of the code transformation system involves the formal specification of code transformations using dependence relations. We have written formal specifications for the whole class of traditional scalar transformations, including induction variable elimination - a complex transformation - for which no formal specifications are available in the literature. All transformations considered in this thesis are global. Most of the specifications given here, for which specifications are already available in the literature, are improved versions, in terms of conservativeness.The study of algorithms for code transformations, in the context of their formal specification, lead us to the development of a new algorithm for partial redundancy elimination. The basic idea behind the algorithm is the new concepts of safe partial availability and safe partial anticipability. Our algorithm is computationally and lifetime optimal. It works on flow graphs whose nodes are basic blocks, which makes it practical.In comparison with existing algorithms the new algorithm also requires four unidirectional analyses, but saves some preprocessing time. The main advantage of the algorithm is its conceptual simplicity. The code transformation system provides an environment in which one can specify a transformation using dependence relations (in the specification language we have designed), generate code for a transformer from its specification,and experiment with the generated transformers on real-world programs. The system takes a program to be transformed, in C or FORTRAN, as input,translates it into intermediate code, interacts with the user to decide the transformation to be performed, computes the necessary dependence relations using the dependence analyzer, applies the specified transformer on the intermediate code, and converts the transformed intermediate code back to high-level. The system is unique of its kind,providing a complete environment for the generation of code transformers, and allowing experimentations with them using real-world programs.