[PATCH] Optimize unrolled reductions in LoopStrengthReduce

Olivier Sallenave ohsallen at us.ibm.com
Tue Feb 3 15:54:34 PST 2015

As explained in my last email, the regular loop unroller (LoopUnroll.cpp) does not break dependencies in reduction chains. Only the loop vectorizer/unroller (LoopVectorize.cpp) does. Problem with the latter is that, the code which breaks dependencies and the one which performs unrolling is tightly coupled. So, if the loop was already unrolled by the first unrolling pass, then reductions aren't optimized by the loop vectorizer/unroller.

To reuse the existing code in LoopVectorize.cpp (instead of my patch), we could choose in LoopUnroll.cpp to not unroll loops which contain reductions. Then the vectorizer would see the opportunity to unroll and perform the optimization. There would be a few cosmetic changes to do in the loop vectorizer, which I can detail if you think this is reasonable.




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