Register renaming and dynamic speculation: an alternative approach
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The late release policy of conventional renaming keeps many registers in the register file assigned in spite of containing values that will never be read in the future. In this work, we study the potential of a novel scheme that speculatively releases a physical register as soon as it has been read by a predicted last instruction that references its value. An auxiliary register file placed outside the critical paths of the processor pipeline holds the early released values just in case they are unexpectedly referenced by some instruction. In addition to demonstrate the feasibility of a last-use predictor, this paper also analyzes the auxiliary register file (latency and size) required to support a speculative early release mechanism that uses a perfect predictor. The obtained results set the performance bound that any real speculative early release implementation is able to reach. We show that in a processor with a 64int+64fp register file, a perfect early release supported by an unbounded auxiliary register file has the potential of speeding up computations up to 23% and 47% for SPECint2000 and SPECfp2000 benchmarks, respectively. Speculative early release can also be used to reduce register file size without losing performance. For instance, a processor with a conventionally managed 96int+96fp register file could be replaced for equal IPC with a 64int+64fp register file managed with perfect early register release and backed with a 64int+64fp auxiliary register file, this representing a 12% IPS (Instructions Per Second) increase if the processor frequency were constrained by the register file access time.