Dynamic Scheduling with Narrow Operand Values

Tomasulo’s algorithm creates a dynamic execution order that extracts a high degree of instruction-level parallelism from a sequential program. Modern processors create this schedule early in the pipeline, before operand values have been computed, since present-day cycle-time demands preclude inclusion of a full ALU and bypass network delay in the instruction scheduling loop. Hence, modern schedulers must predict the latency of load instructions, since load latency cannot be determined within the scheduling pipeline. Whenever load latency is mispredicted due to an unanticipated cache miss or store alias, a significant amount of power is wasted due to incorrectly issued dependent instructions that are already traversing the execution pipeline. This paper exploits the prevalence of narrow operand values (i.e. ones with fewer signficant bits) to solve this problem, by placing a fast, narrow ALU and datapath within the scheduling loop. Virtually all load latency mispredictions can be accurately anticipated with this narrow data path, and little power is wasted on executing incorrectly scheduled instructions. We show that such a narrow data-path design, coupled with a novel partitioned store queue and pipelined data cache, can achieve a cycle time comparable to conventional approaches, while dramatically reducing misspeculation, saving power, and improving per-cycle performance. Finally, we show that due to the rarity of misspeculation in our architecture, a less-complex flush-based recovery scheme suffices for high performance.