Speculative execution and branch prediction on parallel machines

Abstract

Several recent studies on the limits of parallelism have reported that speculative execution can substantially increase the amount of exploitable parallelism in programs, especially non-numerical programs. This is true even for parallel machines models which allow multiple flows of control. However, most architectural techniques for speculation and branch prediction are geared toward conventional computers with a single flow of control, and little has been done in studying speculation models and techniques for parallel machines with multiple threads of control.This paper presents a model of speculative execution for parallel machines. We define two different types of speculation (conservative and aggressive), and define the level of speculation (how far ahead the speculation can go). We then show how to modify conventional methods of branch and jump prediction to work with this model.This paper also presents a comprehensive quantitative study of: (1) how parallelism is affected by the speculative execution and branch prediction techniques under a parallel model of execution, and (2) what speculation depth is required to get a large portion of available parallelism. We measure the parallelism limits of 5 long benchmarks (4 non-numerical) with different speculation models and branch prediction methods, and compare the results.