Transparent control independence (TCI)

Abstract

Superscalar architectures have been proposed that exploit control independence, reducing the performance penalty of branch mispredictions by preserving the work of future misprediction-independent instructions. The essential goal of exploiting control independence is to completely decouple future misprediction-independent instructions from deferred misprediction-dependent instructions. Current implementations fall short of this goal because they explicitly maintain program order among misprediction-independent and misprediction-dependent instructions. Explicit approaches sacrifice design efficiency and ultimately performance. We observe it is sufficient to emulate program order. Potential misprediction-dependent instructions are singled out a priori and their unchanging source values are checkpointed. These instructions and values are set aside as a "recovery program". Checkpointed source values break the data dependencies with co-mingled misprediction-independent instructions - now long since gone from the pipeline - achieving the essential decoupling objective. When the mispredicted branch resolves, recovery is achieved by fetching the self-sufficient, condensed recovery program. Recovery is effectively transparent to the pipeline, in that speculative state is not rolled back and recovery appears as a jump to code. A coarse-grain retirement substrate permits the relaxed order between the decoupled programs. Transparent control independence (TCI) yields a highly streamlined pipeline that quickly recycles resources based on conventional speculation, enabling a large window with small cycle-critical resources, and prevents many mispredictions from disrupting this large window. TCI achieves speedups as high as 64% (16% average) and 88% (22% average) for 4-issue and 8-issue pipelines, respectively, among 15 SPEC integer benchmarks. Factors that limit the performance of explicitly ordered approaches are quantified.