[PATCH] D148609: [InstCombine] Fix buggy `(mul X, Y)` -> `(shl X, Log2(Y))` transform PR62175

[Noah Goldstein via Phabricator via llvm-commits]

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Bug was because we recognized patterns like `(shl 4, Z)` as a power of
2 we could take Log2 of (`2 + Z`), but doing `(shl X, (2 + Z))` can
cause a poison shift.

  https://alive2.llvm.org/ce/z/yuJm_k

The fix is to verify that `Log2(Y)` will be a non-poisonous shift
amount. We can do this with:

  `nsw` flag:
      - https://alive2.llvm.org/ce/z/yyyJBr
      - https://alive2.llvm.org/ce/z/YgubD_
  `nuw` flag:
      - https://alive2.llvm.org/ce/z/-4mpyV
      - https://alive2.llvm.org/ce/z/a6ik6r
  Prove `Y != 0`:
      - https://alive2.llvm.org/ce/z/ced4su
      - https://alive2.llvm.org/ce/z/X-JJHb


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D148609

Files:
  llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
  llvm/test/Transforms/InstCombine/mul-pow2.ll

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