Revisiting the performance impact of branch predictor latencies

Abstract

Branch predictors play a critical role in the performance of modern processors, and the prediction accuracy is known to be the most important attribute of such predictors. However, the latency of the predictor can also have a profound impact on performance as well. In past studies that have considered branch prediction latency, most only consider the latency required to make a prediction. However, in deeply pipelined processors, the latency between prediction and update can also greatly affect performance. In this study, we revisit the performance impact of both of these latencies and demonstrate that update latency can also have a significant impact on performance. We then describe two techniques, multi-overriding and hierarchical updates, to address both latencies which provide 4.4% and 5.7% IPC improvements on moderately (20-stage) and deeply (40-stage) pipelined processors, respectively, for minimal hardware complexity.