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[VPlan] Don't reset canonical IV start value. #161589

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Merged
merged 3 commits into from
Oct 11, 2025
Merged

[VPlan] Don't reset canonical IV start value. #161589

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84bd5ef
[VPlan] Don't reset canonical IV start value.
fhahn Aug 29, 2025
bb0c0ac
Merge remote-tracking branch 'origin/main' into vplan-can-iv-dont-reset
fhahn Oct 11, 2025
5cb548e
!fixup address latest comments, thanks
fhahn Oct 11, 2025
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19 changes: 12 additions & 7 deletions llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -9348,13 +9348,12 @@ static SmallVector<Instruction *> preparePlanForEpilogueVectorLoop(
VPBasicBlock *Header = VectorLoop->getEntryBasicBlock();
Header->setName("vec.epilog.vector.body");

// Ensure that the start values for all header phi recipes are updated before
// vectorizing the epilogue loop.
VPCanonicalIVPHIRecipe *IV = Plan.getCanonicalIV();
// When vectorizing the epilogue loop, the canonical induction start
// value needs to be changed from zero to the value after the main
// vector loop. Find the resume value created during execution of the main
// VPlan. It must be the first phi in the loop preheader.
// When vectorizing the epilogue loop, the canonical induction needs to be
// adjusted by the value after the main vector loop. Find the resume value
// created during execution of the main VPlan. It must be the first phi in the
// loop preheader. Use the value to increment the canonical IV, and update all
// users in the loop region to use the adjusted value.
// FIXME: Improve modeling for canonical IV start values in the epilogue
// loop.
using namespace llvm::PatternMatch;
Expand Down Expand Up @@ -9389,10 +9388,16 @@ static SmallVector<Instruction *> preparePlanForEpilogueVectorLoop(
}) &&
"the canonical IV should only be used by its increment or "
"ScalarIVSteps when resetting the start value");
IV->setOperand(0, VPV);
VPBuilder Builder(Header, Header->getFirstNonPhi());
VPInstruction *Add = Builder.createNaryOp(Instruction::Add, {IV, VPV});
IV->replaceAllUsesWith(Add);
Add->setOperand(0, IV);

DenseMap<Value *, Value *> ToFrozen;
SmallVector<Instruction *> InstsToMove;
// Ensure that the start values for all header phi recipes are updated before
// vectorizing the epilogue loop. Skip the canonical IV, which has been
// handled above.
for (VPRecipeBase &R : drop_begin(Header->phis())) {
Value *ResumeV = nullptr;
// TODO: Move setting of resume values to prepareToExecute.
Expand Down
12 changes: 12 additions & 0 deletions llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
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Expand Up @@ -1234,6 +1234,18 @@ static void simplifyRecipe(VPRecipeBase &R, VPTypeAnalysis &TypeInfo) {
if (!Plan->isUnrolled())
return;

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Suggested change
// Hoist an invariant increment Y of a canonical IV phi X, by having X start at Y.

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Added with minor edit, as this applies to any (scalar) phi

// Hoist an invariant increment Y of a phi X, by having X start at Y.
if (match(Def, m_c_Add(m_VPValue(X), m_VPValue(Y))) && Y->isLiveIn() &&
isa<VPPhi>(X)) {
auto *Phi = cast<VPPhi>(X);
if (Phi->getOperand(1) != Def && match(Phi->getOperand(0), m_ZeroInt()) &&
Phi->getNumUsers() == 1 && (*Phi->user_begin() == &R)) {
Phi->setOperand(0, Y);
Def->replaceAllUsesWith(Phi);
return;
}
}

// VPVectorPointer for part 0 can be replaced by their start pointer.
if (auto *VecPtr = dyn_cast<VPVectorPointerRecipe>(&R)) {
if (VecPtr->isFirstPart()) {
Expand Down