SPREPI: Selective Prediction and REplay for predicated Instructions

Nathanaël Prémillieu 1 André Seznec 1
1 ALF - Amdahl's Law is Forever
Inria Rennes – Bretagne Atlantique , IRISA-D3 - ARCHITECTURE
Abstract : ARM ISA-based processors are no longer low-cost low-power processors. Nowadays ARM ISA based processor manufacturers are struggling to implement medium-end to high-end processor cores, and this implies implementing a state-of-the-art out-of-order execution engine. Unfortunately providing efficient out-of-order execution on legacy ARM codes may be quite challenging due to predicated instructions. In this paper, we propose a new hardware solution, Selective Prediction and REplay for Predicated Instructions (SPREPI), to provide efficient out-of-order execution of codes featuring predicated instructions. Predicting the predicated instructions addresses the so-called multiple definition problem. Predicated instructions are predicted using either a global branch-and-predicate history predictor or a global history predictor. But systematic usage of predicate prediction sometimes impairs the performance dramatically. Efficient filters are proposed to disable predicate prediction uses when they are likely to be counter-productive. Moreover predicate misprediction penalty can be as high as the branch mispenalty. To reduce this penalty we introduce a specific selective replay hardware component targeting mispredicted predicated instructions. SPREPI is shown to allow high out-order execution performance on ARM codes generated even with a compiler applying if-conversion only to very short branches. Moreover since SPREPI predicts most of the predicated instructions, a relatively inefficient hardware solution is sufficient for executing the few predicated instructions on which prediction is not used.
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Nathanaël Prémillieu, André Seznec. SPREPI: Selective Prediction and REplay for predicated Instructions. [Research Report] RR-8351, INRIA. 2013, pp.25. ⟨hal-00856160⟩

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