[all-commits] [llvm/llvm-project] 4cc8cc: [VPlan] Hoist predicated loads with complementary ...

[Florian Hahn via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 4cc8cc81e3aa10e01ce6d54321cbddbc64f1bd15
      https://github.com/llvm/llvm-project/commit/4cc8cc81e3aa10e01ce6d54321cbddbc64f1bd15
  Author: Florian Hahn <flo at fhahn.com>
  Date:   2025-11-26 (Wed, 26 Nov 2025)

  Changed paths:
    M llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
    M llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
    M llvm/lib/Transforms/Vectorize/VPlanTransforms.h
    M llvm/test/Transforms/LoopVectorize/hoist-predicated-loads-with-predicated-stores.ll
    M llvm/test/Transforms/LoopVectorize/hoist-predicated-loads.ll

  Log Message:
  -----------
  [VPlan] Hoist predicated loads with complementary masks. (#168373)

This patch adds a new VPlan transformation to hoist predicated loads, if
we can prove they execute unconditionally, i.e. there are 2 predicated
loads to the same address with complementary masks. Then we are
guaranteed to execute one of them on each iteration, allowing us to
remove the mask.

The transform groups masked replicating loads by their address SCEV,
then checks if there are 2 loads with complementary mask. If that is the
case, we check if there are any writes that may alias the load address
in the blocks between the first and last load with the same address.
The transforms operates after linearizing the CFG, but before
introducing replicate regions, which means this is just checking a chain
of consecutive blocks.

Currently this only uses noalias metadata to check for no-alias (using
the helpers added in https://github.com/llvm/llvm-project/pull/166247).

Then we create an unpredicated VPReplicateRecipe at the position of the
first load, then replace all users of the grouped loads with it.

Small Alive2 proof for hoisting with complementary masks:
https://alive2.llvm.org/ce/z/kUx742

PR: https://github.com/llvm/llvm-project/pull/168373



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