Exploring value prediction limits

International audienceIn this study we explore the performance limits of value prediction for unlimited size predictors in the context of the Championship Value Prediction evaluation framework (CVP). The CVP framework assumes a processor with a large instruction window (256-entry ROB), an aggressive instruction front-end fetching 16 instructions per cycle, an unlimited number of functional units, and a large value misprediction penalty with a complete pipeline flush at commiton a value misprediction.This framework emphasizes two major difficulties that an effective hardware implementation value prediction willface. First the prediction of a value should be forwared to the pipeline only when the potential performance benefit on a correct prediction outweigths the potential performance loss on a misprediction. Second, value prediction has to be used in the context of an out-of-order execution processor with a large instruction window. In many cases the result of an instruction has to be predicted while one or several occurrences of the same instruction are still progressing speculatively in the pipeline. In this study, we illustrate that these speculative values are not required to deliver state-of-the-art value prediction.Our proposition ES-HC-VS-VT combines four predictor components which do not use the speculative results of the inflight occurrences of the instruction to compute the prediction. The four components are respectively the E-stride predictor( ES) [13], the HCVP, Heterogeneous-Context Value, predictor (HC)[9], the VSEP, Value Speculative Equality Predictor (VS) [3] and the VTAGE predictor(VT) [6]. Prediction is computed as, first VTAGE prediction, if not high confidence: VSEP prediction, if not high confidence: HCVP prediction, if not high confidence: E-stride prediction. E-Stride computes its prediction from the last committed occurrence of the instruction and the number of speculative inflight occurrences of the instruction in the pipeline. HCVP computes the predicted value through two successive contexts; first the PC and the global history are used to read a stride history, then this stride history isused to obtain a value and a stride. On VTAGE and VSEP he predicted value is the value directly read at predictiontime on the predictor tables.As for EVES [13], for ES-HC-VS-VT, we optimize the algorithms to assign confidence to predictions on each predictor component depending on the expected benefit/loss of a prediction.On the 135 traces from CVP, ES-HC-VS-VT achieves 4.030 IPC against 3.881 IPC achieved by the previous leader of the CVP, HCVP+E-stride