Value Speculation through Equality Prediction

Abstract

Modern context-based value predictors tightly associate recurring values with instructions and contexts by building confidence upon them. 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 paper, we address this challenge by introducing the notion of Equality Prediction (EP), which represents the binary facet of value prediction. Following a twofold decision scheme (similar to branch prediction), EP makes use of control-flow history to determine equality between the last committed result read at fetch time, and the result of the fetched occurrence. When equality is predicted with high confidence, the read value is used. Our experiments show that this technique obtains the same level of performance as previously proposed state-of-the-art context-based predictors. However, by virtue of better exploiting patterns of interval equality, our design complements the established way that value prediction is performed, and when combined with contemporary prediction models, improves the delivered speedup by 19% on average.