[PATCH] D138873: [InstCombine] canonicalize trunc + insert as bitcast + shuffle, part 2

[Sanjay Patel via Phabricator via llvm-commits]

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This enhances the base fold from part 1 to allow mapping a right-shift to an insert index.

Example of translating a middle chunk of the scalar to vector for either endian:
https://alive2.llvm.org/ce/z/fRXCOZ

This only allows creating an identity shuffle (with optional shortening/lengthening) because that is considered the safe baseline for any target (can be inverted if needed). If we tried this fold with target-specific costs/legality, then we could do the transform more generally.


https://reviews.llvm.org/D138873

Files:
  llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
  llvm/test/Transforms/InstCombine/insert-trunc.ll

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