The Effect of Speculatively Updating Branch History on Branch
Prediction Accuracy, Revisited
Eric Hao, Po-Yung Chang, and Yale N. Patt
patt@eecs.umich.edu
Abstract
Recent research (6) 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.
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