EOLE: Combining Static and Dynamic Scheduling through Value Prediction to Reduce Complexity and Increase Performance

Arthur Perais 1 André Seznec 1
1 ALF - Amdahl's Law is Forever
Inria Rennes – Bretagne Atlantique , IRISA-D3 - ARCHITECTURE
Abstract : Recent work in the field of VP has shown that given an efficient confidence estimation mechanism, prediction validation could be removed from the out-of-order engine and delayed until commit time. As a result, a simple recovery mechanism - pipeline squashing - can be used, while the out-of-order engine remains mostly unmodified. Yet, VP and validation at commit time require additional ports on the Physical Register File, potentially rendering the overall number of ports unbearable. Fortunately, VP also implies that many single-cycle ALU instructions have their operands predicted in the front-end and can be executed in-place, in-order. Similarly, the execution of single-cycle instructions whose result has been predicted can be delayed until commit time since predictions are validated at commit time. Consequently, a significant number of instructions -- 10% to 70% in our experiments -- can bypass the out-of-order engine, allowing for a reduction of the issue width. This reduction paves the way for a truly practical implementation of Value Prediction. Furthermore, since Value Prediction in itself usually increases performance, our resulting {Early | Out-of-Order | Late} Execution architecture, EOLE, is often more efficient than a baseline VP-augmented 6-issue superscalar while having a significantly narrower 4-issue out-of-order engine.
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Arthur Perais, André Seznec. EOLE: Combining Static and Dynamic Scheduling through Value Prediction to Reduce Complexity and Increase Performance. ACM Transactions on Computer Systems, Association for Computing Machinery, 2016, pp.34. ⟨http://tocs.acm.org/⟩. ⟨hal-01259139⟩

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