A Code Motion Framework for Global Instruction Scheduling

A framework for global instruction scheduling is developed that progressively improves a given instruction schedule by eliminating delay slots in the schedule one at a time through the application of global code motions. The elimination of a delay slot is carried out in two steps: a goal oriented search which identiies a global code motion or a cascade of code motions that eliminate the delay without introducing additional delay slots in critical areas of the program; and a transformation step in which the code motion is performed along with compensation code placement and application of code optimizations enabled by code motion. The framework is powerful in that it incorporates code hoisting, including speculative hoisting, code sinking, and integrates partial redundancy elimination and partial dead code elimination optimizations with the code motion transformations. The framework is exible in that it allows formulation of new as well as existing powerful scheduling strategies, allows restrictions to be placed on the type of instructions involved in code motion, the program regions from which the instructions can be selected for code motion, and the extent to which code motion alternatives are explored. The framework is eecient because it computes the required data dependence information on demand from the program control ow graph. Thus, it neither requires exhaustive precomputation of all data dependencies and nor their representation in form of a dependence graph prior to scheduling.