Implementing Branch-Predictor Decay Using Quasi-Static Memory Cells

With semiconductor technology advancing toward deep submicron, leakage energy is of increasing concern, especially for large on-chip array structures such as caches and branch predictors. Recent work has suggested that larger, aggressive branch predictors can and should be used in order to improve microprocessor performance. A further consideration is that more aggressive branch predictors, especially multiported predictors for multiple branch prediction, may be thermal hot spots, thus further increasing leakage. Moreover, as the branch predictor holds state that is transient and predictive, elements can be discarded without adverse effect. For these reasons, it is natural to consider applying decay techniques—already shown to reduce leakage energy for caches—to branch-prediction structures. Due to the structural difference between caches and branch predictors, applying decay techniques to branch predictors is not straightforward. This paper explores the strategies for exploiting spatial and temporal locality to make decay effective for bimodal, gshare, and hybrid predictors, as well as the branch target buffer (BTB). Furthermore, the predictive behavior of branch predictors steers them towards decay based not on state-preserving, static storage cells, but rather quasi-static, dynamic storage cells. This paper will examine the results of implementing.