Loose loops sink chips

Abstract

This paper explores the concept of micro-architectural loops and discusses their impact on processor pipelines. In particular, we establish the relationship between loose loops and pipeline length and configuration, and show their impact on performance. We then evaluate the load resolution loop in detail and propose the distributed register algorithm (DRA) as a way of reducing this loop. It decreases the performance loss due to load mis-speculations by reducing the issue-to-execute latency in the pipeline. A new loose loop is introduced into the pipeline by the DRA, but the frequency of mis-speculations is very low. The reduction in latency from issue to execute, along with a low mis-speculation rate in the DRA result in up to a 4% to 15% improvement in performance using a detailed architectural simulator.