Integrating Program Optimizations and Transformations with the Scheduling of Instruction Level Parallelism

Code optimizations and restructuring transformations are typically applied before scheduling to improve the quality of generated code. However, in some cases, the optimizations and transformations do not lead to a better schedule or may even adversely affect the schedule. In particular, optimizations for redundancy elimination and restructuring transformations for increasing parallelism are often accompanied with an increase in register pressure. Therefore their application in situations where register pressure is already too high may result in the generation of additional spill code. In this paper we present an integrated approach to scheduling that enables the selective application of optimizations and restructuring transformations by the scheduler when it determines their application to be beneficial. The integration is necessary because information that is used to determine the effects of optimizations and transformations on the schedule is only available during instruction scheduling. Our integrated scheduling approach is applicable to various types of global scheduling techniques; in this paper we present an integrated algorithm for scheduling superblocks.