Efficacy and Performance Impact of Value Prediction

Value prediction is a technique that bypasses inter-instruction data dependencies by speculating on the outcomes of producer instructions, thereby allowing dependent consumer instructions to execute in parallel. This work makes several contributions in value prediction research. A hybrid value predictor that achieves an overall prediction rate of up to 83% is presented. The design of a value-predicting eight-wide superscalar machine with its speculative execution core is described. This design is able to achieve 8.6% to 23% IPC improvements on the SPEC benchmarks. Furthermore, it is shown that prediction rate is not a good indicator of speedup because over 40% of predictions made may not be useful in enhancing performance, and a simple hardware mechanism that eliminates many of these useless predictions is introduced.