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        Cached Two-Level Adaptive Branch Predictors with Multiple Stages

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        901720.pdf (PDF, 78Kb)
        Author
        Egan, C.
        Steven, G.B.
        Vintan, L.
        Attention
        2299/2171
        Abstract
        During the last decade, the accuracy of branch predictors was significantly improved by the development of Two-Level Adaptive Branch Predictors. However, although these predictors deliver very high prediction rates, they have several disadvantages. Firstly, the size of the secondlevel Pattern History Table (PHT) increases exponentially as a function of history register length and therefore becomes very costly if a large amount of branch history is exploited. Secondly, many of the prediction counters in the PHT are never used. Thirdly, predictions are frequently generated from non-initialised counters. Finally, several branches may update the same counter, resulting in interference between branch predictions. In this paper, we quantify the performance of a novel family of multi-stage Two-Level Adaptive Predictors. In each two-level predictor, the PHT is replaced by a Prediction Cache. Unlike a PHT, a Prediction Cache saves only relevant branch prediction information. Furthermore, predictions are never based on uninitialised entries and interference between branches is eliminated. In the case of a Prediction Cache miss in the first stage, our two-stage predictors uses a default two-bit prediction counter stored in a second stage. We demonstrate that a two-stage Cached Predictor is more accurate than a conventional two-level predictor and quantify the crucial contribution made by the second prediction stage in achieving this high accuracy. We then extend our Cached Predictor by adding a third stage and demonstrate that a Three-Stage Cached Predictor further improves the accuracy of cached predictors.
        Publication date
        2002
        Published in
        Lecture Notes in Computer Science
        Published version
        https://doi.org/10.1007/3-540-45997-9
        Other links
        http://hdl.handle.net/2299/2171
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