[all-commits] [llvm/llvm-project] a4d102: [VectorCombine] Add foldShuffleToIdentity (#88693)

[David Green via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: a4d10266d20bfe5930dfed77e17832af341ed66e
      https://github.com/llvm/llvm-project/commit/a4d10266d20bfe5930dfed77e17832af341ed66e
  Author: David Green <david.green at arm.com>
  Date:   2024-05-03 (Fri, 03 May 2024)

  Changed paths:
    M llvm/lib/Transforms/Vectorize/VectorCombine.cpp
    M llvm/test/Transforms/PhaseOrdering/AArch64/interleavevectorization.ll
    M llvm/test/Transforms/VectorCombine/AArch64/shuffletoidentity.ll
    M llvm/test/Transforms/VectorCombine/X86/select-shuffle.ll

  Log Message:
  -----------
  [VectorCombine] Add foldShuffleToIdentity (#88693)

This patch adds a basic version of a combine that attempts to remove
shuffles that when combined simplify away to an identity shuffle. For
example:
%ab = shufflevector <8 x half> %a, <8 x half> poison, <4 x i32> <i32 3,
i32 2, i32 1, i32 0>
%at = shufflevector <8 x half> %a, <8 x half> poison, <4 x i32> <i32 7,
i32 6, i32 5, i32 4>
  %abt = fneg <4 x half> %at
  %abb = fneg <4 x half> %ab
%r = shufflevector <4 x half> %abt, <4 x half> %abb, <8 x i32> <i32 7,
i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0>
By looking through the shuffles and fneg, it can be simplified to:
  %r = fneg <8 x half> %a

The code tracks each lane starting from the original shuffle, keeping a
track of a vector of {src, idx}. As we propagate up through the
instructions we will either look through intermediate instructions
(binops and unops) or see a collections of lanes that all have the same
src and incrementing idx (an identity). We can also see a single value
with identical lanes, which we can treat like a splat.

Only the basic version is added here, handling identities, splats,
binops and unops. In follow-up patches other instructions can be added
such as constants, intrinsics, cmp/sel and zext/sext/trunc.



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