The effect of speculatively updating branch history on branch prediction accuracy, revisited

Abstract

Recent research has suggested that the branch history register need not contain the outcomes of the most recent branches in order for the Two-Level Adaptive Branch Predictor to work well. From this result, it is tempting to conclude that the branch history register need not be speculatively updated. This paper revisits this work and explains when the most recent branch outcomes can be omitted without significantly affecting performance. It also explains why this result does not imply that speculative update is not important. This paper shows that because the number of unresolved branches present in the machine varies during program execution, branch predictors without speculative update perform significantly worse than branch predictors with speculative update.