Virtual Register Renaming

Abstract

This paper presents a novel high performance substrate for building energy-efficient out-of-order superscalar cores. The architecture does not require a reorder buffer or physical registers for register renaming and instruction retirement. Instead, it uses a large number of virtual register IDs for register renaming, a physical register file of the same size as the logical register file, and checkpoints to bulk retire instructions and to recover from exceptions and branch mispredictions. By eliminating physical register renaming and the reorder buffer, the architecture not only eliminates complex power hungry hardware structures, but also reduces reorder buffer capacity stalls when execution encounters long delays from data cache misses, thus improving performance. The paper presents performance and power evaluation of this new architecture using Spec 2006 benchmarks. The performance data was collected using an x86 ASIM-based performance simulator from Intel Labs. The data shows that the new architecture improves performance of a 2-wide out-of-order x86 processor core by an average of 4.2%, while saving 43% of the energy consumption of the reorder buffer and retirement register file functional block.