Incorporating predicate information into branch predictors

Abstract

Predicated execution can be used to alleviate the costs associated with frequently mispredicted branches. This is accomplished by trading the cost of a mispredicted branch for execution of both paths following the conditional branch. In this paper we examine two enhancements for branch prediction in the presence of predicated code. Both of the techniques use recently calculated predicate definitions to provide a more intelligent branch prediction. The first branch predictor, called the squash false path filter, recognizes fetched branches known to be guarded with a false predicate and predicts them as not-taken with 100% accuracy. The second technique, called the predicate global update branch predictor, improves prediction by incorporating recent predicate information into the branch predictor. We use these techniques to aid the prediction of region-based branches. A region-based branch is a branch that is left in a predicated region of code. A region-based branch may be correlated with predicate definitions in the region in addition to those that define the branch's guarding predicate.