A METHOD FOR PERFORMING GLOBAL COMMON SUBEXPRESSION ELIMINATION AND CODE MOTION IN AN OPTIMIZING COMPILER

A method for use during the optimization phase of an optimizing compiler for performing global common subexpression elimination and code motion which comprises: Determining the code basis for the object program which includes examining each basic block of code and determining the basis items on which each computation depends wherein basis items are defined as operands which are referenced in a basic block before being computed. The method next determines the "kill set" for each basis item. A kill set for one basis item is defined as the set of items comprising all non basis items which depends on the one basis item for its value. Following this UEX, DEX, and THRU are determined for each basic bloc using the previously determined basis and "kill set" information. A UEX is defined as a set of upward exposed computations, the set of computations which if executed at the beginning of a basic block give the same result as when executed in the original place in the block, wherein DEX is defined as a similar set of downward expressions and wherein THRU is defined as a set of computations which if computed at the beginning or end of the basic block give the same result. AVAIL, the set of computations whose results are valid when the basic block is entered, and INSERT, the set of computations which will be inserted at the end of the basic block, are computed from UEX, DEX, and THRU, and appropriate code insertions are made at those locations indicated by the preceding step, and finally redundant code is removed using the AVAIL set.