A study of errant pipeline flushes caused by value misspeculation

Abstract

Value speculation has been proposed as a technique that can overcome true data dependencies, hide memory latencies, and expose higher degrees of instruction level parallelism (ILP). Branch direction prediction and target address prediction are two widely used control speculation techniques aimed at providing a steady stream of instructions to the instruction window. In this paper we consider a load value predictor used together with an aggressive branch predictor microarchitecture and investigate the effects of load value misspeculations on branch resolution. We study the performance impact of the interaction of these mechanisms and characterize the occurence of these events in a multiple issue, out-of-order, superscalar pipeline. We perform execution-driven studies using integer benchmarks taken from the SPECint2000, SPECint95 and Olden suites. We show that IPC can deteriorate by as much as 4.7% due to unnecessary pipeline flushes caused by branch resolutions that use speculative data. This paper also proposes a mechanism that can prevent these unnecessary squashes from occurring.