Branch runahead: An alternative to branch prediction for impossible to predict branches

Abstract

High performance microprocessors require high levels of instruction supply. Branch prediction has been the most important driver of this for nearly 30 years. Unfortunately, modern predictors are increasingly bottlenecked by hard-to-predict data-dependent branches that fundamentally cannot be predicted via a history based approach. Pre-computation of branch instructions has been suggested as a solution, but such schemes require a careful trade-off between timeliness and complexity. This paper introduces Branch Runahead: a low-cost, hardware-only solution that achieves high accuracy while only performing lightweight pre-computation. The result: a reduction in branch MPKI of 47.5% and an average improvement in IPC of 16.9%.