Branch Prediction Literature TseYu Yeh and Yale N

Branch Prediction

Literature • Tse-Yu Yeh and Yale N. Patt, “A Comparison of Dynamic Branch Predictors that use Two Levels of Branch History, ” Proceedings of the 20 th annual international symposium on computer architecture, ISCA '93 – Also important: Reference 1, 2, 3 and 4 of this paper • Tse-Yu Yeh and Yale N. Patt, “Two-level adaptive training branch “ prediction, “ Proceedings of the 24 th annual international symposium on Microarchitecture

Two-Level Adaptive Branch Prediction • First level, execution history register(s): History of the last k branches encountered. • Second level, pattern history table(s): k refers to a place in the pattern history table that contains the prediction based on the outcome of the branches at the last j occurences of k, Predictor is for instance a prediction counter.

1 st Level Branch History Register or Table (BHR or BHT) • Global History of all branches kept in a single register of length k. • Per-address Table that contains a history register of length k for each branch. • Per Set Table that contains a history register of length k for each set of branches.

2 nd level Pattern History Table (PHT) • global Table with a prediction for all possible values of k. • per-address Table with a prediction for all possible values of k for each branch address. • Per set Table with a prediction for all possible values of k for each branch set.

The 9 Variations of 2 -Level Adaptive Branch prediction

Summary • Branch prediction is a very important factor in reducing CPI in modern processors that use extensive pipelining. • A counter is often used for prediction (2 bit) • Two-Level Adaptive Dynamic Branch Prediction ‘learns’ the outcome of branches in different program states. • 9 Variations of 2 -L. A. B. P. (Global, Per-Address and Per-Set for both levels), but only 4 useful.