Partial Redundancy Elimination in SSA
نویسنده
چکیده
The SSAPRE algorithm for performing partial redundancy elimination based entirely on SSA form is presented. The algorithm is formulated based on a new conceptual framework, the fac-tored redundancy graph, for analyzing redundancy, and represents the rst sparse approach to the classical problem of partial redundancy elimination. At the same time, it provides new perspectives on the problem and on methods for its solution. With the algorithm description, theorems and their proofs are given showing that the algorithm produces the best possible code by the criteria of computational optimality and lifetime optimality of the introduced temporaries. In addition to the base algorithm, a practical implementation of SSAPRE that exhibits additional compile-time eeciencies is described. In closing, measurement statistics are provided that characterize the instances of the partial redundancy problem from a set of benchmark programs and compare optimization time spent by an implementation of SSAPRE against a classical partial redundancy elimination implementation. The data lend insight into the nature of partial redundancy elimination and demonstrate the expediency of this new approach. 1. INTRODUCTION Partial redundancy elimination (PRE) is a powerful optimization technique rst developed by Morel and Renvoise Morel and Renvoise 1979]. The technique removes partial redundancies in the program by performing data ow analysis that solves for code placements. Since global common subexpressions and loop-invariant
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