Microarchitectural support for speculative register renaming

Abstract

This paper proposes and evaluates a new microarchitecture for out-of-order processors that supports speculative renaming. We call speculative renaming to the speculative omission of physical register allocation along with the speculative early release of physical registers. These renaming policies may cause a register operand not to be kept in the physical register file (PRF). Thus, we add a low-ported auxiliary register file (XRF) located outside the processor core that keeps the values absent in PRF and supplies them at higher latency. To support the location of register operands being either in PRF or XRF, we use virtual registers. We consider omission and release policies directed by hardware prediction. Namely, we use a single last-use predictor that directs both speculative omission and release. We call this mechanism SR-LUP (speculative renaming based on last-use prediction). Two last-use predictor designs of incremental complexity and performance are analyzed. In a 256-ROB, 8-way processor with an 80int+80fp PRF, SR-LUP with an 11-port 256int+256fp XRF, speeds up computations up to 11.5% and 29% for INT and FP SPEC2K benchmarks, respectively. For FP benchmarks, if the PRF limits the clock frequency, a conventionally managed 128int+128fp PRF can be replaced using SR-LUP by a 64int+64fp PRF backed up with a 10-port 224int+224fp XRF, showing 19% IPS gain.