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Branch Prediction

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Branch Prediction

- Tse-Yu Yeh and Yale N. Patt, “A Comparison of Dynamic Branch Predictors that use Two Levels of Branch History,” Proceedings of the 20th 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 24th 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.

1st Level Branch History Register or Table (BHR or BHT)

- Global History of all branches kept in a single register of length k.
- Per-addressTable that contains a history register of length k for each branch.
- Per SetTable that contains a history register of length k for each set of branches.

2nd levelPattern History Table (PHT)

- global Table with a prediction for all possible values of k.
- per-addressTable with a prediction for all possible values of k for each branch address.
- Per setTable with a prediction for all possible values of k for each branch set.

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.