Alternative implementations of hybrid branch predictors

Abstract

Very accurate branch prediction is an important requirement for achieving high performance on deeply pipelined, superscalar processors. To improve on the prediction accuracy of current single-scheme branch predictors, hybrid (multiple-scheme) branch predictors have been proposed (McFarling (1993), Chang et al. (1994)). These predictors combined multiple single-scheme predictors into a single predictor. They use a selection mechanism to decide for each branch, which single-scheme predictor to use. The performance of a hybrid predictor depends on its single-scheme predictor components and its selection mechanism. Using known single-scheme predictors and selection mechanisms, this paper identifies the most effective hybrid predictor implementation. In addition, it introduces a new selection mechanism, the 2-level selector, which further improves the performance of the hybrid branch predictor.