Memory Dependence Speculation Tradeoffs in Centralized, Continuous-Window Superscalar Processors

We consider a variety of dynamic, hardware-based methods for exploiting load/store parallelism, including mechanisms that use memory dependence speculation. While previous work has also investigated such methods [19,4], this has been done primarily for split, distributed window processor models. We focus on centralized, continuous-window processor models (the common configuration today). We confirm that exploiting load/ store parallelism can greatly improve performance. Moreover, we show that much of this performance potential can be captured if addresses of the memory locations accessed by both loads and stores can be used to schedule loads. However, using addresses to schedule load execution may not always be an option due to complexity, latency, and cost considerations. For this reason, we also consider configurations that use just memory dependence speculation to guide load execution. We consider a variety of methods and show that speculation/synchronization can be used to effectively exploit virtually all load/store parallelism. We demonstrate that this technique is competitive to or better than the one that uses addresses for scheduling loads. We conclude by discussing why our findings differ, in part, from those reported for split, distributed window processor models.