Processors

EXACT: Explicit Dynamic-Branch Prediction With Active Updates

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Executive Summary

Branches that depend directly or indirectly on load instructions are a leading cause of mispredictions by state-of-the-art branch predictors. For a branch of this type, there is a unique dynamic instance of the branch for each unique combination of producer-load addresses. Based on this definition, a study of mispredictions reveals two related problems: Global branch history often fails to distinguish between different dynamic branches. In this case, the predictor is unable to specialize predictions for different dynamic branches, causing mispredictions if their outcomes differ. Ideally, the remedy is to predict a dynamic branch using its Program Counter (PC) and the addresses of its producer loads, since this context uniquely identifies the dynamic branch.

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