A Comparative Survery of Load Speculation Architectures

Brad Calder and Glenn Reinman

Journal of Instruction Level Parallelism, May 2000.

Abstract:

Load latency remains a significant bottleneck in dynamically scheduled pipelined processors. Load speculation techniques have been proposed to reduce this latency. Dependence Prediction can be used to allow loads to be issued before all prior store addresses are known, and to predict exactly which store a load should wait upon. Address Prediction can be used to allow a load to bypass the calculation of its effective address and speculatively issue. Value Prediction can be used to bypass the load forward latency and avoid cache misses. Memory Renaming has been proposed to communicate stored values directly to aliased loads.

In this paper we examine in detail the interaction and performance tradeoffs of these four load speculation techniques in the presence of two mispeculation recovery architectures -- reexecution and squash. We examine the performance of combining these techniques to create a load speculation chooser which provides performance improvement over using any one technique in isolation. We also examine the accuracy of these load speculation techniques for predicting data cache misses.