[PATCH] D103788: [InstCombine] Eliminate casts to optimize ctlz operation

[Datta Nagraj via Phabricator via llvm-commits]

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If a ctlz operation is performed on higher datatype and then
downcasted, then this can be optimized by doing a ctlz operation
on a lower datatype and adding the difference bitsize to the result
of ctlz to provide the same output:

https://alive2.llvm.org/ce/z/8uup9M

The original problem is shown in
https://llvm.org/PR50173


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D103788

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
  llvm/test/Transforms/InstCombine/zext-ctlz-trunc-to-ctlz-add.ll

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