Modeling value speculation

Abstract

Several studies of speculative execution based on values have reported promising performance potential. However, virtually all microarchitectures in these studies were described in an ambiguous manner, mainly due to the lack of formalization that defines the effects of value-speculation on a microarchitecture. In particular, the manifestations of value-speculation on the latency of microarchitectural operations, such as releasing resources and reissuing, was at best partially addressed. This may be problematic since results obtained in these studies can be difficult to reproduce and/or appreciate their contribution. This paper introduces a model for a methodical description of dynamically-scheduled microarchitectures that use value-speculation. The model isolates the parts of a microarchitecture that may be influenced by value-speculation in terms of various variables and latency events. This provides systematic means for describing, evaluating and comparing the,performance of value-speculative microarchitectures. The model parameters are integrated in a simulator to investigate the performance of several value-speculation related events. Among other, the results show value-speculation performance to have non-uniform sensitivity to changes in the latency of these events. For example, fast verification latency is found to be essential, but when mis-speculation is infrequent slow invalidation may be acceptable.