[PATCH] D147868: [SCEV] Strengthen huge constant trip multiples.

[Joshua Cao via Phabricator via llvm-commits]

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code cleanup


caojoshua added a comment.

We could technically find the largest multiple under the threshold that is not limited to powers of 2, but that is much more computation heavy and the benefits are questionable.

An interesting case where this is beneficial is in the LoopUnroll test. Before, we returned a trip multiple of 1, but this change returns a trip multiple of 2^32. Since its trying to unroll by a factor of 8, and 2^32 is divisible by eight, LoopUnroll knows that the loop epilogue is not required anymore.


SCEV determines that loops with trip count >=2^32 have a trip multiple
of 1 to guard against huge multiples. This patch stregthens this to
instead find the greatest power of 2 divisor that is less than the
threshold.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D147868

Files:
  llvm/lib/Analysis/ScalarEvolution.cpp
  llvm/test/Analysis/ScalarEvolution/huge-trip-multiple.ll
  llvm/test/Transforms/LoopUnroll/X86/mmx.ll

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