[PATCH] D101900: [InstCombine] Fold extractelement + vector GEP with one use

[David Sherwood via Phabricator via llvm-commits]

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We sometimes see code like this:

Case 1:

  %gep = getelementptr i32, i32* %a, <2 x i64> %splat
  %ext = extractelement <2 x i32*> %gep, i32 0

or this:

Case 2:

  %gep = getelementptr i32, <4 x i32*> %a, i64 1
  %ext = extractelement <4 x i32*> %gep, i32 0

where there is only one use of the GEP. In such cases it makes
sense to fold the two together such that we create a scalar GEP:

Case 1:

  %ext = extractelement <2 x i64> %splat, i32 0
  %gep = getelementptr i32, i32* %a, i64 %ext

Case 2:

  %ext = extractelement <2 x i32*> %a, i32 0
  %gep = getelementptr i32, i32* %ext, i64 1

This may create further folding opportunities as a result, i.e.
the extract of a splat vector can be completely eliminated. I
haven't added folds for the case when we have both a vector of
pointers and a vector of indices, since this would require
generating an additional extractelement operation.

Tests have been added here:

  Transforms/InstCombine/gep-vector-indices.ll


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D101900

Files:
  llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
  llvm/test/Transforms/InstCombine/gep-vector-indices.ll
  llvm/test/Transforms/InstCombine/vec_demanded_elts-inseltpoison.ll
  llvm/test/Transforms/InstCombine/vec_demanded_elts.ll

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