Wide and efficient trace prediction using the local trace predictor

Abstract

High prediction bandwidth enables performance improvements and power reduction techniques. This paper explores a mechanism to increase prediction width (instructions per prediction) by predicting instruction traces. Our analysis shows that predicting traces including multiple branches is not significantly less accurate than predicting single branches. A novel Local Trace Predictor organization is proposed. It increases prediction width without reducing the ratio of prediction accuracy versus memory resources with respect to a Basic Block Predictor.Compared to the previously proposed Next-Trace Predictor, the Local Trace Predictor reduces memory requirements by codifying trace predictions, and by limiting the number of traces starting at the same instruction to 2 or 4. The limit lessens prediction width only slightly, and does not affect prediction accuracy. The overall result is that the Local Trace Predictor outperforms the Next-Trace Predictor for sizes higher than 12 KBytes.