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[VPlan] Use wide IV if scalar lanes > 0 are used with scalable vectors. #169796

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Merged
fhahn merged 3 commits into from
Dec 1, 2025
Merged

[VPlan] Use wide IV if scalar lanes > 0 are used with scalable vectors. #169796

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bb81e6f
[VPlan] Use wide IV if scalar lanes > 0 are used with scalable vectors.
fhahn Nov 27, 2025
87cbef7
!fixup add assert, thanks!
fhahn Dec 1, 2025
edabe79
Merge branch 'main' into vplan-handle-scalable-users-of-ivsteps
fhahn Dec 1, 2025
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22 changes: 0 additions & 22 deletions llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2367,15 +2367,6 @@ void VPScalarIVStepsRecipe::execute(VPTransformState &State) {
// Compute the scalar steps and save the results in State.
Type *IntStepTy =
IntegerType::get(BaseIVTy->getContext(), BaseIVTy->getScalarSizeInBits());
Type *VecIVTy = nullptr;
Value *UnitStepVec = nullptr, *SplatStep = nullptr, *SplatIV = nullptr;
if (!FirstLaneOnly && State.VF.isScalable()) {
VecIVTy = VectorType::get(BaseIVTy, State.VF);
UnitStepVec =
Builder.CreateStepVector(VectorType::get(IntStepTy, State.VF));
SplatStep = Builder.CreateVectorSplat(State.VF, Step);
SplatIV = Builder.CreateVectorSplat(State.VF, BaseIV);
}

unsigned StartLane = 0;
unsigned EndLane = FirstLaneOnly ? 1 : State.VF.getKnownMinValue();
Expand All @@ -2396,19 +2387,6 @@ void VPScalarIVStepsRecipe::execute(VPTransformState &State) {
StartIdx0 = Builder.CreateSExtOrTrunc(StartIdx0, IntStepTy);
}

if (!FirstLaneOnly && State.VF.isScalable()) {
auto *SplatStartIdx = Builder.CreateVectorSplat(State.VF, StartIdx0);
auto *InitVec = Builder.CreateAdd(SplatStartIdx, UnitStepVec);
if (BaseIVTy->isFloatingPointTy())
InitVec = Builder.CreateSIToFP(InitVec, VecIVTy);
auto *Mul = Builder.CreateBinOp(MulOp, InitVec, SplatStep);
auto *Add = Builder.CreateBinOp(AddOp, SplatIV, Mul);
State.set(this, Add);
// It's useful to record the lane values too for the known minimum number
// of elements so we do those below. This improves the code quality when
// trying to extract the first element, for example.
}

if (BaseIVTy->isFloatingPointTy())
StartIdx0 = Builder.CreateSIToFP(StartIdx0, BaseIVTy);

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17 changes: 12 additions & 5 deletions llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
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Expand Up @@ -794,12 +794,19 @@ static void legalizeAndOptimizeInductions(VPlan &Plan) {
WideIV->getDebugLoc(), Builder);

// Update scalar users of IV to use Step instead.
if (!HasOnlyVectorVFs)
if (!HasOnlyVectorVFs) {
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@david-arm david-arm Dec 1, 2025

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Is it worth adding an assert here that the plan doesn't have a scalable VF? i.e.

  if (!HasOnlyVectorVFs) {
    assert(!Plan.hasScalableVF() && ...);
...

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@fhahn fhahn Dec 1, 2025

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added thanks. If the plan contains the scalar VF, it should never include scalable VFs.

assert(!Plan.hasScalableVF() &&
"plans containing a scalar VF cannot also include scalable VFs");
WideIV->replaceAllUsesWith(Steps);
else
WideIV->replaceUsesWithIf(Steps, [WideIV](VPUser &U, unsigned) {
return U.usesScalars(WideIV);
});
} else {
bool HasScalableVF = Plan.hasScalableVF();
WideIV->replaceUsesWithIf(Steps,
[WideIV, HasScalableVF](VPUser &U, unsigned) {
if (HasScalableVF)
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@david-arm david-arm Dec 1, 2025

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Are plans containing scalable VFs always guaranteed to never include a fixed-width VF? I think that's probably true in practice, but this might give strange behaviour if not.

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@fhahn fhahn Dec 1, 2025

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Yes, currently we build the plans for scalable/fixed vectorr completely separately, so they should never mix.

return U.usesFirstLaneOnly(WideIV);
return U.usesScalars(WideIV);
});
}
}
}

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