Speculative Memory Cloaking and Bypassing

• Authors: Andreas Moshovos;Gurindar S. Sohi
• Venue: International Journal of Parallel Programming - Special issue on the 30th annual ACM/IEEE international symposium on microarchitecture, part II
• Year: 1999

We revisit memory hierarchy design viewing memory as an inter-operation communication mechanism. We show how dynamically collected information about inter-operation memory communication can be used to improve memory latency. We propose two techniques: (1) Speculative Memory Cloaking, and (2) Speculative Memory Bypassing. In the first technique, we use memory dependence prediction to speculatively identify dependent loads and stores early in the pipeline. These instructions may then communicate prior to address calculation and disambiguation via a fast communication mechanism. In the second technique, we use memory dependence prediction to speculatively transform DEF-store-load-USE dependence chains within the instruction window into DEF-USE ones. As a result, dependent stores and loads are taken off the communication path resulting in further reduction in communication latency. Experimental analysis shows that our methods, on the average, correctly handle 40% (integer) and 19% (floating point) of all memory loads. Moreover, our techniques result in performance improvements of 4.28% (integer) and 3.20% (floating point) over a highly aggressive, dynamically scheduled processor implementing naive memory dependence speculation. We also study the value and address locality characteristics of the values our methods correctly handle. We demonstrate that our methods are orthogonal to both address and value prediction.