Branch history table prediction of moving target branches due to subroutine returns

Abstract

Ideally, a pipeline processor can run at a rate that is limited by its slowest stage. Branches in the instruction stream disrupt the pipeIine, and reduce processor performance to well below ideal. Since workloads contain a high percentage of taken branches, techniques are needed to reduce or eliminate thk degradation. A Branch History Table (BHT) stores past action and target for branches, and predicts that future behavior will repeat. Although past action is a good indicator of future action, the subroutine CALL/RETURN paradigm makes correct prediction of the branch target dlfflcult. We propose a new stack mechanism for reducing this type of mispredlction. Using traces of the SPEC benchmark suite running on an RS/6000, we provide an analysis of the performance enhancements possible using a BHT. We show that the proposed mechanism can reduce the number of branch wrong guesses by 18.2°/0 on average.