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[LV] Use original trip-count as the vector-trip-count if use predicated EVL instructions for tail-folding. #132675

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[LV] Use original trip-count as the vector-trip-count if use predicated EVL instructions for tail-folding. #132675

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b34c7c0
[LV] Use original trip count as the vector trip count if use predicat…
NexMing Mar 24, 2025
c331045
[NFC] Simplify code
NexMing Mar 24, 2025
ef2e776
Update llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
NexMing Mar 24, 2025
738264e
Remove unnecessary VPEVLBasedIVPHIRecipe.
NexMing Mar 25, 2025
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10 changes: 9 additions & 1 deletion llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
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Expand Up @@ -2427,6 +2427,15 @@ InnerLoopVectorizer::getOrCreateVectorTripCount(BasicBlock *InsertBlock) {
Value *TC = getTripCount();
IRBuilder<> Builder(InsertBlock->getTerminator());

// Use original trip count as the vector trip count if use predicated EVL
// instructions for tail-folding.
if (VF.isVector() && Cost->foldTailWithEVL()) {
assert(!Cost->requiresScalarEpilogue(true) &&
"Use predicated EVL instructions for tail-folding does not allow "
"scalar epilogue");
return VectorTripCount = TC;
}

Comment on lines +2430 to +2438
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This doesn't look like it is the right place to update. The vector trip count is guaranteed to be a multiple of VF.

The caller would need updating to not use it/request when folding the tail with EVL

Type *Ty = TC->getType();
// This is where we can make the step a runtime constant.
Value *Step = createStepForVF(Builder, Ty, VF, UF);
Expand Down Expand Up @@ -4468,7 +4477,6 @@ static bool willGenerateVectors(VPlan &Plan, ElementCount VF,
case VPDef::VPVectorPointerSC:
case VPDef::VPVectorEndPointerSC:
case VPDef::VPExpandSCEVSC:
case VPDef::VPEVLBasedIVPHISC:
case VPDef::VPPredInstPHISC:
case VPDef::VPBranchOnMaskSC:
continue;
Expand Down
44 changes: 0 additions & 44 deletions llvm/lib/Transforms/Vectorize/VPlan.h
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Expand Up @@ -506,7 +506,6 @@ class VPSingleDefRecipe : public VPRecipeBase, public VPValue {
static inline bool classof(const VPRecipeBase *R) {
switch (R->getVPDefID()) {
case VPRecipeBase::VPDerivedIVSC:
case VPRecipeBase::VPEVLBasedIVPHISC:
case VPRecipeBase::VPExpandSCEVSC:
case VPRecipeBase::VPInstructionSC:
case VPRecipeBase::VPReductionEVLSC:
Expand Down Expand Up @@ -2919,49 +2918,6 @@ class VPActiveLaneMaskPHIRecipe : public VPHeaderPHIRecipe {
#endif
};

/// A recipe for generating the phi node for the current index of elements,
/// adjusted in accordance with EVL value. It starts at the start value of the
/// canonical induction and gets incremented by EVL in each iteration of the
/// vector loop.
class VPEVLBasedIVPHIRecipe : public VPHeaderPHIRecipe {
public:
VPEVLBasedIVPHIRecipe(VPValue *StartIV, DebugLoc DL)
: VPHeaderPHIRecipe(VPDef::VPEVLBasedIVPHISC, nullptr, StartIV, DL) {}

~VPEVLBasedIVPHIRecipe() override = default;

VPEVLBasedIVPHIRecipe *clone() override {
llvm_unreachable("cloning not implemented yet");
}

VP_CLASSOF_IMPL(VPDef::VPEVLBasedIVPHISC)

void execute(VPTransformState &State) override {
llvm_unreachable("cannot execute this recipe, should be replaced by a "
"scalar phi recipe");
}

/// Return the cost of this VPEVLBasedIVPHIRecipe.
InstructionCost computeCost(ElementCount VF,
VPCostContext &Ctx) const override {
// For now, match the behavior of the legacy cost model.
return 0;
}

/// Returns true if the recipe only uses the first lane of operand \p Op.
bool onlyFirstLaneUsed(const VPValue *Op) const override {
assert(is_contained(operands(), Op) &&
"Op must be an operand of the recipe");
return true;
}

#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
/// Print the recipe.
void print(raw_ostream &O, const Twine &Indent,
VPSlotTracker &SlotTracker) const override;
#endif
};

/// A Recipe for widening the canonical induction variable of the vector loop.
class VPWidenCanonicalIVRecipe : public VPSingleDefRecipe,
public VPUnrollPartAccessor<1> {
Expand Down
15 changes: 7 additions & 8 deletions llvm/lib/Transforms/Vectorize/VPlanAnalysis.cpp
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Expand Up @@ -245,14 +245,13 @@ Type *VPTypeAnalysis::inferScalarType(const VPValue *V) {
TypeSwitch<const VPRecipeBase *, Type *>(V->getDefiningRecipe())
.Case<VPActiveLaneMaskPHIRecipe, VPCanonicalIVPHIRecipe,
VPFirstOrderRecurrencePHIRecipe, VPReductionPHIRecipe,
VPWidenPointerInductionRecipe, VPEVLBasedIVPHIRecipe>(
[this](const auto *R) {
// Handle header phi recipes, except VPWidenIntOrFpInduction
// which needs special handling due it being possibly truncated.
// TODO: consider inferring/caching type of siblings, e.g.,
// backedge value, here and in cases below.
return inferScalarType(R->getStartValue());
})
VPWidenPointerInductionRecipe>([this](const auto *R) {
// Handle header phi recipes, except VPWidenIntOrFpInduction which
// needs special handling due it being possibly truncated.
// TODO: consider inferring/caching type of siblings, e.g., backedge
// value, here and in cases below.
return inferScalarType(R->getStartValue());
})
.Case<VPWidenIntOrFpInductionRecipe, VPDerivedIVRecipe>(
[](const auto *R) { return R->getScalarType(); })
.Case<VPReductionRecipe, VPPredInstPHIRecipe, VPWidenPHIRecipe,
Expand Down
11 changes: 0 additions & 11 deletions llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
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Expand Up @@ -3749,14 +3749,3 @@ void VPActiveLaneMaskPHIRecipe::print(raw_ostream &O, const Twine &Indent,
printOperands(O, SlotTracker);
}
#endif

#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
void VPEVLBasedIVPHIRecipe::print(raw_ostream &O, const Twine &Indent,
VPSlotTracker &SlotTracker) const {
O << Indent << "EXPLICIT-VECTOR-LENGTH-BASED-IV-PHI ";

printAsOperand(O, SlotTracker);
O << " = phi ";
printOperands(O, SlotTracker);
}
#endif
61 changes: 21 additions & 40 deletions llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp
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Expand Up @@ -2021,10 +2021,9 @@ static void transformRecipestoEVLRecipes(VPlan &Plan, VPValue &EVL) {
}
}

/// Add a VPEVLBasedIVPHIRecipe and related recipes to \p Plan and
/// replaces all uses except the canonical IV increment of
/// VPCanonicalIVPHIRecipe with a VPEVLBasedIVPHIRecipe. VPCanonicalIVPHIRecipe
/// is used only for loop iterations counting after this transformation.
/// Add a VPInstruction::ExplicitVectorLength and related recipes to \p Plan and
/// adjust the increment of the canonical induction variable to an explicit
/// vector length.
///
/// The function uses the following definitions:
/// %StartV is the canonical induction start value.
Expand All @@ -2035,29 +2034,25 @@ static void transformRecipestoEVLRecipes(VPlan &Plan, VPValue &EVL) {
/// ...
///
/// vector.body:
/// ...
/// %EVLPhi = EXPLICIT-VECTOR-LENGTH-BASED-IV-PHI [ %StartV, %vector.ph ],
/// [ %NextEVLIV, %vector.body ]
/// %AVL = sub original TC, %EVLPhi
/// %IVPhi = CANONICAL-INDUCTION ir<0>, %IV.NEXT
/// %AVL = sub original TC, %IVPhi
/// %VPEVL = EXPLICIT-VECTOR-LENGTH %AVL
/// ...
/// %NextEVLIV = add IVSize (cast i32 %VPEVVL to IVSize), %EVLPhi
/// %IV.NEXT = add %IVPhi, IVSize (cast i32 %VPEVL to IVSize)
/// ...
///
/// If MaxSafeElements is provided, the function adds the following recipes:
/// vector.ph:
/// ...
///
/// vector.body:
/// ...
/// %EVLPhi = EXPLICIT-VECTOR-LENGTH-BASED-IV-PHI [ %StartV, %vector.ph ],
/// [ %NextEVLIV, %vector.body ]
/// %IVPhi = CANONICAL-INDUCTION ir<0>, %IV.NEXT
/// %AVL = sub original TC, %EVLPhi
/// %cmp = cmp ult %AVL, MaxSafeElements
/// %SAFE_AVL = select %cmp, %AVL, MaxSafeElements
/// %VPEVL = EXPLICIT-VECTOR-LENGTH %SAFE_AVL
/// ...
/// %NextEVLIV = add IVSize (cast i32 %VPEVL to IVSize), %EVLPhi
/// %IV.NEXT = add %IVPhi, IVSize (cast i32 %VPEVL to IVSize)
/// ...
///
bool VPlanTransforms::tryAddExplicitVectorLength(
Expand All @@ -2073,15 +2068,11 @@ bool VPlanTransforms::tryAddExplicitVectorLength(
return false;

auto *CanonicalIVPHI = Plan.getCanonicalIV();
VPValue *StartV = CanonicalIVPHI->getStartValue();

// Create the ExplicitVectorLengthPhi recipe in the main loop.
auto *EVLPhi = new VPEVLBasedIVPHIRecipe(StartV, DebugLoc());
EVLPhi->insertAfter(CanonicalIVPHI);
VPBuilder Builder(Header, Header->getFirstNonPhi());
// Compute original TC - IV as the AVL (application vector length).
VPValue *AVL = Builder.createNaryOp(
Instruction::Sub, {Plan.getTripCount(), EVLPhi}, DebugLoc(), "avl");
Instruction::Sub, {&Plan.getVectorTripCount(), CanonicalIVPHI},
DebugLoc(), "avl");
if (MaxSafeElements) {
// Support for MaxSafeDist for correct loop emission.
VPValue *AVLSafe = Plan.getOrAddLiveIn(
Expand All @@ -2094,27 +2085,20 @@ bool VPlanTransforms::tryAddExplicitVectorLength(

auto *CanonicalIVIncrement =
cast<VPInstruction>(CanonicalIVPHI->getBackedgeValue());
Builder.setInsertPoint(CanonicalIVIncrement);
VPSingleDefRecipe *OpVPEVL = VPEVL;
if (unsigned IVSize = CanonicalIVPHI->getScalarType()->getScalarSizeInBits();
IVSize != 32) {
OpVPEVL = Builder.createScalarCast(
IVSize < 32 ? Instruction::Trunc : Instruction::ZExt, OpVPEVL,
CanonicalIVPHI->getScalarType(), CanonicalIVIncrement->getDebugLoc());
}
auto *NextEVLIV = Builder.createOverflowingOp(
Instruction::Add, {OpVPEVL, EVLPhi},
{CanonicalIVIncrement->hasNoUnsignedWrap(),
CanonicalIVIncrement->hasNoSignedWrap()},
CanonicalIVIncrement->getDebugLoc(), "index.evl.next");
EVLPhi->addOperand(NextEVLIV);

transformRecipestoEVLRecipes(Plan, *VPEVL);

// Replace all uses of VPCanonicalIVPHIRecipe by
// VPEVLBasedIVPHIRecipe except for the canonical IV increment.
CanonicalIVPHI->replaceAllUsesWith(EVLPhi);
// Adjust the increment of the canonical induction variable to an explicit
// vector length.
CanonicalIVIncrement->setOperand(0, CanonicalIVPHI);
CanonicalIVIncrement->setOperand(1, OpVPEVL);
// TODO: support unroll factor > 1.
Plan.setUF(1);
return true;
Expand Down Expand Up @@ -2298,17 +2282,14 @@ void VPlanTransforms::convertToConcreteRecipes(VPlan &Plan) {
for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
vp_depth_first_deep(Plan.getEntry()))) {
for (VPRecipeBase &R : make_early_inc_range(VPBB->phis())) {
if (!isa<VPCanonicalIVPHIRecipe, VPEVLBasedIVPHIRecipe>(&R))
continue;
auto *PhiR = cast<VPHeaderPHIRecipe>(&R);
StringRef Name =
isa<VPCanonicalIVPHIRecipe>(PhiR) ? "index" : "evl.based.iv";
auto *ScalarR = new VPInstruction(
Instruction::PHI, {PhiR->getStartValue(), PhiR->getBackedgeValue()},
PhiR->getDebugLoc(), Name);
ScalarR->insertBefore(PhiR);
PhiR->replaceAllUsesWith(ScalarR);
PhiR->eraseFromParent();
if (auto *PhiR = dyn_cast<VPCanonicalIVPHIRecipe>(&R)) {
auto *ScalarR = new VPInstruction(
Instruction::PHI, {PhiR->getStartValue(), PhiR->getBackedgeValue()},
PhiR->getDebugLoc(), "index");
ScalarR->insertBefore(PhiR);
PhiR->replaceAllUsesWith(ScalarR);
PhiR->eraseFromParent();
}
Comment on lines +2285 to +2292
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VPCanonicalIVPHIRecipe's step must be loop invariant, otherwise it isn't an induction.

If you want to do this transform, you must replace the canonical IV with a plain scalar phi recipe, like done in convertToConcreteRecipes

}
}
}
Expand Down
8 changes: 3 additions & 5 deletions llvm/lib/Transforms/Vectorize/VPlanTransforms.h
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Expand Up @@ -143,11 +143,9 @@ struct VPlanTransforms {
VPlan &Plan,
const std::function<bool(BasicBlock *)> &BlockNeedsPredication);

/// Add a VPEVLBasedIVPHIRecipe and related recipes to \p Plan and
/// replaces all uses except the canonical IV increment of
/// VPCanonicalIVPHIRecipe with a VPEVLBasedIVPHIRecipe.
/// VPCanonicalIVPHIRecipe is only used to control the loop after
/// this transformation.
/// Add a VPInstruction::ExplicitVectorLength and related recipes to \p Plan
/// and adjust the increment of the canonical induction variable to an
/// explicit vector length.
/// \returns true if the transformation succeeds, or false if it doesn't.
static bool
tryAddExplicitVectorLength(VPlan &Plan,
Expand Down
1 change: 0 additions & 1 deletion llvm/lib/Transforms/Vectorize/VPlanValue.h
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Expand Up @@ -356,7 +356,6 @@ class VPDef {
// VPHeaderPHIRecipe need to be kept together.
VPCanonicalIVPHISC,
VPActiveLaneMaskPHISC,
VPEVLBasedIVPHISC,
VPFirstOrderRecurrencePHISC,
VPWidenIntOrFpInductionSC,
VPWidenPointerInductionSC,
Expand Down
10 changes: 0 additions & 10 deletions llvm/lib/Transforms/Vectorize/VPlanVerifier.cpp
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Expand Up @@ -156,16 +156,6 @@ bool VPlanVerifier::verifyEVLRecipe(const VPInstruction &EVL) const {
errs() << "EVL is used as an operand in non-VPInstruction::Add\n";
return false;
}
if (I->getNumUsers() != 1) {
errs() << "EVL is used in VPInstruction:Add with multiple "
"users\n";
return false;
}
if (!isa<VPEVLBasedIVPHIRecipe>(*I->users().begin())) {
errs() << "Result of VPInstruction::Add with EVL operand is "
"not used by VPEVLBasedIVPHIRecipe\n";
return false;
}
return true;
})
.Default([&](const VPUser *U) {
Expand Down
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