Zero-Cycle Loads: Microarchitecture Support for Reducing Load Latency
Todd M. Austin, Gurindar S. Sohi · 1995
Untolerated load instruction latencies often have a significant impact on overall program performance. As one means of miti-gating this effect, we present an aggressive hardware-based mech-anism that provides effective support for reducing the latency of load instructions. Through the judicious use of instruction predecode, base regis-ter caching, and fast address calculation, it becomes possible to complete load instructions up to two cycles earlier than traditional pipeline designs. For a pipeline with one cycle data cache access, this results in what we term a zero-cycle load. A zero-cycle load produces a result prior to reaching the execute stage of the pipeline, allowing subsequent dependent instructions to issue unfettered by load dependencies. Programs executing on processors with sup-port for zero-cycle loads experience significantly fewer pipeline stalls due to load instructions and increased overall performance. We present two pipeline designs supporting zero-cycle loads: one for pipelines with a single stage of instruction decode, and another for pipelines with multiple decode stages. We evaluate these designs in a number of contexts: with and without software support, in-order vs. out-of-order issue, and on architectures with many and few registers. We find that our approach is quite ef-fective at reducing the impact of load latency, even more so on architectures with in-order issue and few registers. 1