Leveraging Value Equality Prediction for Value Speculation

Abstract : Value Prediction (VP) has recently been gaining interest in the research community, since prior work has established practical solutions for its implementation that provide meaningful performance gains. A constant challenge of contemporary context-based value predictors is to sufficiently capture value redundancy and exploit the predictable execution paths. To do so, modern context-based VP techniques tightly associate recurring values with instructions and contexts by building confidence upon them after a plethora of repetitions. However, when execution monotony exists in the form of intervals, the potential prediction coverage is limited, since prediction confidence is reset at the beginning of each new interval. In this study, we address this challenge by introducing the notion of Equality Prediction (EP), which represents the binary facet of VP. Following a twofold decision scheme (similar to branch prediction), at fetch time, EP makes use of control-flow history to predict equality between the last committed result for this instruction and the result of the currently fetched occurrence. When equality is predicted with high confidence, the last committed value is used. Our simulation results showthat this technique obtains the same level of performance as previously proposed state-of-the-art context-based value predictors. However, by virtue of exploiting equality patterns that are not captured by previous VP schemes, our design can improve the speedup of standard VP by 19% on average, when combined with contemporary prediction models.