[PATCH] D75145: [PassManager] adjust VectorCombine placement

[Dave Green via Phabricator via llvm-commits]

dmgreen added a comment.

Not sure about #2. Define "more effective". Creating unanalyzable loops isn't very effective ;)

I was working under the assumption that the old form was more canonical, and CSE had messed that up somehow. I might well have had that backwards though, and you might be right. Perhaps the new form would be better, if only SCEV could understand it?

I'm not sure if it's possible to fix SCEV in these cases? Any ideas? Not being able to calculate BackEdgeTakenCount's for vector loop bodies sounds like a big problem. For us ends up disabling low overhead/hardware loops, so tail predication would also be effected (if it was enabled). Loop unrolling would also be effected if it was desirable for vectorized loops (in our case it is only by accident for a few edge cases).


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https://reviews.llvm.org/D75145