[PATCH] Break dependencies in large loops containing reductions (LoopVectorize)

[Olivier Sallenave]

Let me try to explain the rationale below the proposed cost function: UF = UF * CriticalPathLength / LoopLength

I assume CriticalPathLength is the number of reductions in the loop (or more precisely, the length of the longest reduction chain found in the loop, which would be 3 for examples above as the innermost loop is inlined).

(CriticalPathLength / LoopLength) gives us the distance between the reductions. Typically, the distance is very short for the examples above, so CriticalPathLength / LoopLength = 3 / 3 = 1, and UF remains unchanged. If the distance is long, it means there are potentially many FP instructions between each reductions, therefore an OoO pipeline will be able to exploit ILP, and we don't need to add much interleaving — UF will be diminished. And if there is no OoO, UF will be short anyways.


http://reviews.llvm.org/D7514

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