A comparison of dynamic branch predictors that use two levels of branch history

Recent attention to speculative execution as a mechanism for increasing performance of single instruction streams has demanded substantially better branch prediction than what has been previously available. We [1, 2] and Pan, So, and Rahmeh [4] have both proposed variations of the same aggressive dynamic branch predictor for handling those needs. We call the basic model Two-Level Adaptive Branch Prediction � Pan, So, and Rahmeh call it Correlation Branch Prediction. In this paper, we adopt the terminology of [2] and show that there arereally nine variations of the same basic model. We compare the nine variations with respect to the amount of history information kept. We study the e ects of di erent branch history lengths and pattern history table con gurations. Finally, we evaluate the cost e ectiveness of the nine variations.