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 Permission to make digital/hard copy of all or part of this material without fee for personal or classroom use provided that the copies are not made or distributed for profit or commercial advantage, the ACM copyright/server notice, the title of the publication, and its date appear, and notice is given that copying is by permission of the ACM, Inc. To copy otherwise, to republish, to post on servers, or to redistribute to lists requires prior specific permission and/or a fee.  • 181 decaying branch-predictor structures with dynamic-appropriately, decaying-cells rather than the standard static SRAM cell. Overall, this paper demonstrates that decay techniques can apply to more than just caches, with the branch predictor and BTB as an example. We show decay can either be implemented at the architectural level, or with a wholesale replacement of static storage cells with quasi-static storage cells, which naturally implement decay. More importantly, decay techniques can be applied and should be applied to other such transient and/or predictive structures.
In REF , it was proposed to apply decay techniques to branch predictors by using 4T cells.
Implementing branch-predictor decay using quasi-static memory cells
TACO
Computer Science