[PATCH] D123494: [VectorCombine] Find and remove shuffles from commutative reductions

[Dave Green via Phabricator via llvm-commits]

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Given a shuffle feeding a reduction, the lane ordering of the shuffle will not alter the result. This is also true if there are a number of operations between the reduction and the shuffle, providing they only operate lane-wise. This patch searches for cases like that in Vector Combine, allowing us to check the cost of the shuffle vs an in-order identity shuffle and replace the order of possible. This only handles a single shuffle at the moment to keep things simple, and is able to ignore splats that produce results where every result is the same.

This is a more powerful version of a combine that already happens in instrcombine, capable of optimizing more cases by looking through more instructions and being able to cost the shuffle.


https://reviews.llvm.org/D123494

Files:
  llvm/lib/Transforms/Vectorize/VectorCombine.cpp
  llvm/test/Transforms/VectorCombine/AArch64/vecreduce-shuffle.ll

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