Analysis of the TRIPS prototype block predictor

摘要

This paper analyzes the performance of the TRIPS prototype chip's block predictor. The prototype is the first implementation of the block-atomic TRIPS architecture, wherein the unit of execution is a TRIPS hyperblock. The TRIPS prototype predictor uses a two-step prediction process: it first predicts the exit from the current hyperblock and uses the predicted exit in conjunction with the current hyperblock's address to predict the next hyperblock. SPECint2000 and SPECfp2000 benchmarks record average misprediction rates of 11.5% and 4.3%, respectively, on the prototype chip. Simulation-driven analysis identifies short history lengths, inadequate offset bits in the branch target buffers, and aliasing in the exit and target predictors as the main reasons for the predictor inefficiency. If the above issues are addressed, block misprediction rates can be reduced by 15% for SPECint2000 and 22% for SPECfp2000. Using a perceptron-based analysis, we show that there is significant loss in correlation in our current hyperblocks. We conclude that while carefully tuned block predictors can achieve relatively lower misprediction rates, new predictor designs and correlation-aware hyperblock formation are necessary to bridge the gap between block prediction accuracies and branch prediction accuracies.