Correlated load-address predictors

摘要

As microprocessors become faster, the relative performance cost of memory accesses increases. Bigger and faster caches significantly reduce the absolute load-to-use time delay. However, increase in processor operational frequencies impairs the relative load-to-use latency, measured in processor cycles (e.g. from two cycles on the Pentium processor to three cycles of more in current designs). Load-address prediction techniques were introduced to partially cut the load-to-use latency. This paper focuses on advanced address-prediction schemes to further shorten program execution time. Existing address prediction schemes are capable of predicting simple address patterns, consisting mainly of constant addresses or stride-based addresses. This paper explores the characteristics of the remaining loads and suggests new enhanced techniques to improve prediction effectiveness: Context-based prediction to tackle part of the remaining, difficult-to-predict, load instructions. New prediction algorithms to take advantage of global correlation among different static loads. New confidence mechanisms to increase the correct prediction rate and to eliminate costly mispredictions. Mechanisms to prevent long or random address sequences from polluting the predictor data structures while providing some hysteresis behavior to the predictions. Such an enhanced address predictor accurately predicts 67% of all loads, while keeping the misprediction rate close to 1% We further prove that the proposed predictor works reasonably well in a deep pipelined architecture where the predict-to-update delay may significantly impair both prediction rate and accuracy.