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

Recent research (A.R. Talcott, W. Yamamoto, M.J. Serrano, R.C. Wood, and M. Nemirovsky, Proc. 21st Annual Int. Symposium Comput. Archit., pp. 12-21, 1994) 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 updating 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 updating perform significantly worse than branch predictors with speculative updating.