[PATCH] D138872: [InstCombine] canonicalize trunc + insert as bitcast + shuffle, part 1

[Sanjay Patel via Phabricator via llvm-commits]

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This is the main patch for converting a truncated scalar that is inserted into a vector to bitcast+shuffle. We could go either way on patterns like this, but this direction will allow collapsing a pair of these sequences on the motivating example from issue #17113 <https://github.com/llvm/llvm-project/issues/17113>.

The patch is split into 3 parts to make it easier to see the progression of tests diffs. We allow inserting/shuffling into a different size vector for flexibility, so there are several test variations. The length-changing is handled by shortening/padding the shuffle mask with undef elements.

In part 1, handle the basic pattern:

  inselt undef, (trunc T), IndexC --> shuffle (bitcast T), IdentityMask

Proof for the endian-dependency behaving as expected:
https://alive2.llvm.org/ce/z/BsA7yC

The TODO items for handling shifts and insert into an arbitrary base vector value are implemented as follow-ups.


https://reviews.llvm.org/D138872

Files:
  llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
  llvm/test/Transforms/InstCombine/insert-trunc.ll
  llvm/test/Transforms/InstCombine/vec_phi_extract-inseltpoison.ll
  llvm/test/Transforms/InstCombine/vec_phi_extract.ll
  llvm/test/Transforms/LoopVectorize/ARM/pointer_iv.ll
  llvm/test/Transforms/PhaseOrdering/X86/vec-load-combine.ll

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