Analyzing the working set characteristics of branch execution

Abstract

To achieve highly accurate branch prediction, it is necessary not only to allocate more resources to branch prediction hardware but also to improve the understanding of branch execution characteristics. In this paper, we present a new profile-based conditional branch analysis technique called branch working set analysis to provide additional information about control flow behavior of general purpose applications. This analysis evaluates the dynamic behavior of branch execution by partitioning either individual branches or pre-classified branch groups into sets based on temporal locality and ordering information. We refer to these sets as the working sets of branches. To demonstrate the usefulness of this form of analysis, we examine the efficiency of current allocation techniques for branch history table (BHT) space and propose a new solution to this allocation process that improves the performance of these tables. In our approach the mapping between branch instructions and BHT entries is specified during compilation to reduce table contention-leading to more relevant histories and improved predictor performance. As a result, even for programs with a large number of static branches, only 100 to 200 history entries are needed to approximate the performance of larger 1024-entry BHT. Furthermore, when the technique is applied to a predictor with 1024-entry BHT its prediction accuracy is improved by 16%-comparable with the performance of a BHT of infinite capacity.