Mechanism for Return Stack and Branch History Corrections under Misprediction in Deep Pipeline Design

Abstract

Return stack may be popped due to branch misprediction, corrupting its contents. Meanwhile, erroneous branch history is also recorded for upcoming branch predictions. These errors are more likely in deep pipelines, and their handling affects performance seriously. We study these issues, and propose solutions with two virtues: low hardware overhead, and high branch prediction accuracy comparable with that of a shallow pipeline design. To deal with return stack corruption, any push and pop after any mispredicted branch should be counted and recorded. These simple rules get overly complicated when multiple unresolved branches exist, and anyone may be erroneous, as is common in deeper pipelines. Next, to deal with branch history contamination, extra history bits, plus a branch confirmation pointer are needed. The experiment result shows that our design is effective, about 4%similar to 9% performance improvement in MiBench.