[PATCH] D157462: LoopRotate: Add code to update branch weights

[Matthias Braun via Phabricator via llvm-commits]

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This adds code to the loop rotation transformation to ensure that the computed block execution counts for the loop bodies are the same before and after the transformation. This isn't always true in practice, but I believe this is because of numeric inaccuracies in the BlockFrequency computation.

The invariants this is modeled on and heuristic choice of 0-trip loop amount is explained in a lenghty comment in the new `updateBranchWeights()` function.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D157462

Files:
  llvm/include/llvm/IR/ProfDataUtils.h
  llvm/lib/IR/ProfDataUtils.cpp
  llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
  llvm/test/Transforms/LoopRotate/update-branch-weights.ll
  llvm/test/Transforms/LoopSimplify/merge-exits.ll

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