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[LV] Move VPlan-based calculateRegisterUsage to VPlanAnalysis (NFC). #135673

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Jun 2, 2025
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[LV] Move VPlan-based calculateRegisterUsage to VPlanAnalysis (NFC). #135673

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284 changes: 4 additions & 280 deletions llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -987,25 +987,6 @@ class LoopVectorizationCostModel {
/// decision in a map for use in planning and plan execution.
void setVectorizedCallDecision(ElementCount VF);

/// A struct that represents some properties of the register usage
/// of a loop.
struct RegisterUsage {
/// Holds the number of loop invariant values that are used in the loop.
/// The key is ClassID of target-provided register class.
SmallMapVector<unsigned, unsigned, 4> LoopInvariantRegs;
/// Holds the maximum number of concurrent live intervals in the loop.
/// The key is ClassID of target-provided register class.
SmallMapVector<unsigned, unsigned, 4> MaxLocalUsers;

/// Check if any of the tracked live intervals exceeds the number of
/// available registers for the target.
bool exceedsMaxNumRegs(const TargetTransformInfo &TTI) const {
return any_of(MaxLocalUsers, [&TTI](auto &LU) {
return LU.second > TTI.getNumberOfRegisters(LU.first);
});
}
};

/// Collect values we want to ignore in the cost model.
void collectValuesToIgnore();

Expand Down Expand Up @@ -4343,15 +4324,6 @@ static bool hasReplicatorRegion(VPlan &Plan) {
}

#ifndef NDEBUG
/// Estimate the register usage for \p Plan and vectorization factors in \p VFs
/// by calculating the highest number of values that are live at a single
/// location as a rough estimate. Returns the register usage for each VF in \p
/// VFs.
static SmallVector<LoopVectorizationCostModel::RegisterUsage, 8>
calculateRegisterUsage(VPlan &Plan, ArrayRef<ElementCount> VFs,
const TargetTransformInfo &TTI,
const SmallPtrSetImpl<const Value *> &ValuesToIgnore);

VectorizationFactor LoopVectorizationPlanner::selectVectorizationFactor() {
InstructionCost ExpectedCost = CM.expectedCost(ElementCount::getFixed(1));
LLVM_DEBUG(dbgs() << "LV: Scalar loop costs: " << ExpectedCost << ".\n");
Expand All @@ -4377,7 +4349,7 @@ VectorizationFactor LoopVectorizationPlanner::selectVectorizationFactor() {
for (auto &P : VPlans) {
ArrayRef<ElementCount> VFs(P->vectorFactors().begin(),
P->vectorFactors().end());
auto RUs = ::calculateRegisterUsage(*P, VFs, TTI, CM.ValuesToIgnore);
auto RUs = calculateRegisterUsageForPlan(*P, VFs, TTI, CM.ValuesToIgnore);
for (auto [VF, RU] : zip_equal(VFs, RUs)) {
// The cost for scalar VF=1 is already calculated, so ignore it.
if (VF.isScalar())
Expand Down Expand Up @@ -4704,254 +4676,6 @@ void LoopVectorizationCostModel::collectElementTypesForWidening() {
}
}

/// Get the VF scaling factor applied to the recipe's output, if the recipe has
/// one.
static unsigned getVFScaleFactor(VPRecipeBase *R) {
if (auto *RR = dyn_cast<VPReductionPHIRecipe>(R))
return RR->getVFScaleFactor();
if (auto *RR = dyn_cast<VPPartialReductionRecipe>(R))
return RR->getVFScaleFactor();
return 1;
}

/// Estimate the register usage for \p Plan and vectorization factors in \p VFs
/// by calculating the highest number of values that are live at a single
/// location as a rough estimate. Returns the register usage for each VF in \p
/// VFs.
static SmallVector<LoopVectorizationCostModel::RegisterUsage, 8>
calculateRegisterUsage(VPlan &Plan, ArrayRef<ElementCount> VFs,
const TargetTransformInfo &TTI,
const SmallPtrSetImpl<const Value *> &ValuesToIgnore) {
// Each 'key' in the map opens a new interval. The values
// of the map are the index of the 'last seen' usage of the
// recipe that is the key.
using IntervalMap = SmallDenseMap<VPRecipeBase *, unsigned, 16>;

// Maps indices to recipes.
SmallVector<VPRecipeBase *, 64> Idx2Recipe;
// Marks the end of each interval.
IntervalMap EndPoint;
// Saves the list of recipe indices that are used in the loop.
SmallPtrSet<VPRecipeBase *, 8> Ends;
// Saves the list of values that are used in the loop but are defined outside
// the loop (not including non-recipe values such as arguments and
// constants).
SmallSetVector<VPValue *, 8> LoopInvariants;
LoopInvariants.insert(&Plan.getVectorTripCount());

// We scan the loop in a topological order in order and assign a number to
// each recipe. We use RPO to ensure that defs are met before their users. We
// assume that each recipe that has in-loop users starts an interval. We
// record every time that an in-loop value is used, so we have a list of the
// first and last occurrences of each recipe.
ReversePostOrderTraversal<VPBlockDeepTraversalWrapper<VPBlockBase *>> RPOT(
Plan.getVectorLoopRegion());
for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(RPOT)) {
if (!VPBB->getParent())
break;
for (VPRecipeBase &R : *VPBB) {
Idx2Recipe.push_back(&R);

// Save the end location of each USE.
for (VPValue *U : R.operands()) {
auto *DefR = U->getDefiningRecipe();

// Ignore non-recipe values such as arguments, constants, etc.
// FIXME: Might need some motivation why these values are ignored. If
// for example an argument is used inside the loop it will increase the
// register pressure (so shouldn't we add it to LoopInvariants).
if (!DefR && (!U->getLiveInIRValue() ||
!isa<Instruction>(U->getLiveInIRValue())))
continue;

// If this recipe is outside the loop then record it and continue.
if (!DefR) {
LoopInvariants.insert(U);
continue;
}

// Overwrite previous end points.
EndPoint[DefR] = Idx2Recipe.size();
Ends.insert(DefR);
}
}
if (VPBB == Plan.getVectorLoopRegion()->getExiting()) {
// VPWidenIntOrFpInductionRecipes are used implicitly at the end of the
// exiting block, where their increment will get materialized eventually.
for (auto &R : Plan.getVectorLoopRegion()->getEntryBasicBlock()->phis()) {
if (isa<VPWidenIntOrFpInductionRecipe>(&R)) {
EndPoint[&R] = Idx2Recipe.size();
Ends.insert(&R);
}
}
}
}

// Saves the list of intervals that end with the index in 'key'.
using RecipeList = SmallVector<VPRecipeBase *, 2>;
SmallDenseMap<unsigned, RecipeList, 16> TransposeEnds;

// Next, we transpose the EndPoints into a multi map that holds the list of
// intervals that *end* at a specific location.
for (auto &Interval : EndPoint)
TransposeEnds[Interval.second].push_back(Interval.first);

SmallPtrSet<VPRecipeBase *, 8> OpenIntervals;
SmallVector<LoopVectorizationCostModel::RegisterUsage, 8> RUs(VFs.size());
SmallVector<SmallMapVector<unsigned, unsigned, 4>, 8> MaxUsages(VFs.size());

LLVM_DEBUG(dbgs() << "LV(REG): Calculating max register usage:\n");

VPTypeAnalysis TypeInfo(Plan.getCanonicalIV()->getScalarType());

const auto &TTICapture = TTI;
auto GetRegUsage = [&TTICapture](Type *Ty, ElementCount VF) -> unsigned {
if (Ty->isTokenTy() || !VectorType::isValidElementType(Ty) ||
(VF.isScalable() &&
!TTICapture.isElementTypeLegalForScalableVector(Ty)))
return 0;
return TTICapture.getRegUsageForType(VectorType::get(Ty, VF));
};

// We scan the instructions linearly and record each time that a new interval
// starts, by placing it in a set. If we find this value in TransposEnds then
// we remove it from the set. The max register usage is the maximum register
// usage of the recipes of the set.
for (unsigned int Idx = 0, Sz = Idx2Recipe.size(); Idx < Sz; ++Idx) {
VPRecipeBase *R = Idx2Recipe[Idx];

// Remove all of the recipes that end at this location.
RecipeList &List = TransposeEnds[Idx];
for (VPRecipeBase *ToRemove : List)
OpenIntervals.erase(ToRemove);

// Ignore recipes that are never used within the loop and do not have side
// effects.
if (!Ends.count(R) && !R->mayHaveSideEffects())
continue;

// Skip recipes for ignored values.
// TODO: Should mark recipes for ephemeral values that cannot be removed
// explictly in VPlan.
if (isa<VPSingleDefRecipe>(R) &&
ValuesToIgnore.contains(
cast<VPSingleDefRecipe>(R)->getUnderlyingValue()))
continue;

// For each VF find the maximum usage of registers.
for (unsigned J = 0, E = VFs.size(); J < E; ++J) {
// Count the number of registers used, per register class, given all open
// intervals.
// Note that elements in this SmallMapVector will be default constructed
// as 0. So we can use "RegUsage[ClassID] += n" in the code below even if
// there is no previous entry for ClassID.
SmallMapVector<unsigned, unsigned, 4> RegUsage;

for (auto *R : OpenIntervals) {
// Skip recipes that weren't present in the original loop.
// TODO: Remove after removing the legacy
// LoopVectorizationCostModel::calculateRegisterUsage
if (isa<VPVectorPointerRecipe, VPVectorEndPointerRecipe,
VPBranchOnMaskRecipe>(R))
continue;

if (VFs[J].isScalar() ||
isa<VPCanonicalIVPHIRecipe, VPReplicateRecipe, VPDerivedIVRecipe,
VPScalarIVStepsRecipe>(R) ||
(isa<VPInstruction>(R) &&
all_of(cast<VPSingleDefRecipe>(R)->users(),
[&](VPUser *U) {
return cast<VPRecipeBase>(U)->usesScalars(
R->getVPSingleValue());
})) ||
(isa<VPReductionPHIRecipe>(R) &&
(cast<VPReductionPHIRecipe>(R))->isInLoop())) {
unsigned ClassID = TTI.getRegisterClassForType(
false, TypeInfo.inferScalarType(R->getVPSingleValue()));
// FIXME: The target might use more than one register for the type
// even in the scalar case.
RegUsage[ClassID] += 1;
} else {
// The output from scaled phis and scaled reductions actually has
// fewer lanes than the VF.
unsigned ScaleFactor = getVFScaleFactor(R);
ElementCount VF = VFs[J].divideCoefficientBy(ScaleFactor);
LLVM_DEBUG(if (VF != VFs[J]) {
dbgs() << "LV(REG): Scaled down VF from " << VFs[J] << " to " << VF
<< " for " << *R << "\n";
});

for (VPValue *DefV : R->definedValues()) {
Type *ScalarTy = TypeInfo.inferScalarType(DefV);
unsigned ClassID = TTI.getRegisterClassForType(true, ScalarTy);
RegUsage[ClassID] += GetRegUsage(ScalarTy, VF);
}
}
}

for (const auto &Pair : RegUsage) {
auto &Entry = MaxUsages[J][Pair.first];
Entry = std::max(Entry, Pair.second);
}
}

LLVM_DEBUG(dbgs() << "LV(REG): At #" << Idx << " Interval # "
<< OpenIntervals.size() << '\n');

// Add the current recipe to the list of open intervals.
OpenIntervals.insert(R);
}

// We also search for instructions that are defined outside the loop, but are
// used inside the loop. We need this number separately from the max-interval
// usage number because when we unroll, loop-invariant values do not take
// more register.
LoopVectorizationCostModel::RegisterUsage RU;
for (unsigned Idx = 0, End = VFs.size(); Idx < End; ++Idx) {
// Note that elements in this SmallMapVector will be default constructed
// as 0. So we can use "Invariant[ClassID] += n" in the code below even if
// there is no previous entry for ClassID.
SmallMapVector<unsigned, unsigned, 4> Invariant;

for (auto *In : LoopInvariants) {
// FIXME: The target might use more than one register for the type
// even in the scalar case.
bool IsScalar = all_of(In->users(), [&](VPUser *U) {
return cast<VPRecipeBase>(U)->usesScalars(In);
});

ElementCount VF = IsScalar ? ElementCount::getFixed(1) : VFs[Idx];
unsigned ClassID = TTI.getRegisterClassForType(
VF.isVector(), TypeInfo.inferScalarType(In));
Invariant[ClassID] += GetRegUsage(TypeInfo.inferScalarType(In), VF);
}

LLVM_DEBUG({
dbgs() << "LV(REG): VF = " << VFs[Idx] << '\n';
dbgs() << "LV(REG): Found max usage: " << MaxUsages[Idx].size()
<< " item\n";
for (const auto &pair : MaxUsages[Idx]) {
dbgs() << "LV(REG): RegisterClass: "
<< TTI.getRegisterClassName(pair.first) << ", " << pair.second
<< " registers\n";
}
dbgs() << "LV(REG): Found invariant usage: " << Invariant.size()
<< " item\n";
for (const auto &pair : Invariant) {
dbgs() << "LV(REG): RegisterClass: "
<< TTI.getRegisterClassName(pair.first) << ", " << pair.second
<< " registers\n";
}
});

RU.LoopInvariantRegs = Invariant;
RU.MaxLocalUsers = MaxUsages[Idx];
RUs[Idx] = RU;
}

return RUs;
}

unsigned
LoopVectorizationCostModel::selectInterleaveCount(VPlan &Plan, ElementCount VF,
InstructionCost LoopCost) {
Expand Down Expand Up @@ -5002,8 +4726,8 @@ LoopVectorizationCostModel::selectInterleaveCount(VPlan &Plan, ElementCount VF,
return 1;
}

RegisterUsage R =
::calculateRegisterUsage(Plan, {VF}, TTI, ValuesToIgnore)[0];
VPRegisterUsage R =
calculateRegisterUsageForPlan(Plan, {VF}, TTI, ValuesToIgnore)[0];
// We divide by these constants so assume that we have at least one
// instruction that uses at least one register.
for (auto &Pair : R.MaxLocalUsers) {
Expand Down Expand Up @@ -7380,7 +7104,7 @@ VectorizationFactor LoopVectorizationPlanner::computeBestVF() {
for (auto &P : VPlans) {
ArrayRef<ElementCount> VFs(P->vectorFactors().begin(),
P->vectorFactors().end());
auto RUs = ::calculateRegisterUsage(*P, VFs, TTI, CM.ValuesToIgnore);
auto RUs = calculateRegisterUsageForPlan(*P, VFs, TTI, CM.ValuesToIgnore);
for (auto [VF, RU] : zip_equal(VFs, RUs)) {
if (VF.isScalar())
continue;
Expand Down
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