On reducing misspeculations on a pipelined scheduler

Abstract

Pipelining the scheduling logic, which exposes and exploits the instruction level parallelism, degrades processor performance. In a 4-issue processor, our evaluations show that pipelining the scheduling logic over two cycles degrades performance by 10% in SPEC-2000 integer benchmarks. Such a performance degradation is due to sacrificing the ability to execute dependent instructions in consecutive cycles. Speculative selection is a previously proposed technique that boosts the performance of a processor with a pipelined scheduling logic. However, this new speculation source increases the overall number of misspeculated instructions, and this unuseful work wastes energy. In this work we introduce a non-speculative mechanism named Dependence Level Scheduler (DLS)which not only tolerates the scheduling-logic latency but also reduces the number of misspeculated instructions with respect to a scheduler with speculative selection. In DLS, the selection of a group of one-cycle instructions (producer-level) is overlapped with the wake up in advance of its group of dependent instructions. DLS is not speculative because the group of woken in advance instructions will compete for selection only after issuing all producer-level instructions. On average, DLS reduces the number of misspeculated instructions with respect to a speculative scheduler by 17.9%. From the IPC point of view, the speculative scheduler outperforms DLS by 0.3%. Moreover, we propose two non-speculative improvements to DLS.