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[LV] Change getSmallBestKnownTC to return an ElementCount (NFC) #141793

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Merged
merged 2 commits into from
Jun 18, 2025
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[LV] Change getSmallBestKnownTC to return an ElementCount (NFC) #141793

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8af01ca
[NFC][LLVM][LoopVectorize] Change getSmallBestKnownTC to return an El…
paulwalker-arm May 16, 2025
29a79f1
Make getSmallConstantTripCount a static function.
paulwalker-arm Jun 17, 2025
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49 changes: 29 additions & 20 deletions llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -419,6 +419,12 @@ static bool hasIrregularType(Type *Ty, const DataLayout &DL) {
return DL.getTypeAllocSizeInBits(Ty) != DL.getTypeSizeInBits(Ty);
}

/// A version of ScalarEvolution::getSmallConstantTripCount that returns an
/// ElementCount to include loops whose trip count is a function of vscale.
ElementCount getSmallConstantTripCount(ScalarEvolution *SE, const Loop *L) {
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@david-arm david-arm Jun 17, 2025

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Can this be marked static or do you expect it to be called from elsewhere?

return ElementCount::getFixed(SE->getSmallConstantTripCount(L));
}

/// Returns "best known" trip count, which is either a valid positive trip count
/// or std::nullopt when an estimate cannot be made (including when the trip
/// count would overflow), for the specified loop \p L as defined by the
Expand All @@ -427,24 +433,24 @@ static bool hasIrregularType(Type *Ty, const DataLayout &DL) {
/// 2) Returns expected trip count according to profile data if any.
/// 3) Returns upper bound estimate if known, and if \p CanUseConstantMax.
/// 4) Returns std::nullopt if all of the above failed.
static std::optional<unsigned>
static std::optional<ElementCount>
getSmallBestKnownTC(PredicatedScalarEvolution &PSE, Loop *L,
bool CanUseConstantMax = true) {
Comment on lines 438 to 439
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@artagnon artagnon Jun 3, 2025

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Is it sufficient to change getSmallBestKnownTC to return ElementCount, and not touch SCEV?

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@paulwalker-arm paulwalker-arm Jun 4, 2025

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I don't think so because there are other places within LoopVectorize where getSmallConstantTripCount is called that also require the ElementCount returning variant.

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@artagnon artagnon Jun 6, 2025

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Would be good to see the follow-up patch, to see what this patch will be used?

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@paulwalker-arm paulwalker-arm Jun 6, 2025

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It's part of a chain so I'll see about pulling it out.

My objective is to allow multiples of vscale to be returned so that when vectorising a loop of the form for (int i = 0; i < llvm.vscale() * N; ++i) using a scalable VF the vectoriser can make sensible interleaving decisions. For example, today for the N=1 case LoopVectorize picks the default interleaving factor of 2 which means we never enter the vector loop.

In the same vein, representing vscale base trip counts also allows the vectoriser to better reason about whether the scalar loop becomes dead after vectorisation.

I did investigate the possibility of changing the existing interface but felt the impact was too high, especially as the majority of uses do not care about vscale based trip counts.

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@artagnon artagnon Jun 6, 2025

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Sounds good. Maybe we can move getSmallConstantRuntimeTC into LV, as nobody else cares about vscale?

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@paulwalker-arm paulwalker-arm Jun 17, 2025

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That causes a little duplication but otherwise works well so I've implemented the recommendation. As this is now LoopVectorize's version of getSmallConstantTripCount() I've adopted the same name.

// Check if exact trip count is known.
if (unsigned ExpectedTC = PSE.getSE()->getSmallConstantTripCount(L))
if (auto ExpectedTC = getSmallConstantTripCount(PSE.getSE(), L))
return ExpectedTC;

// Check if there is an expected trip count available from profile data.
if (LoopVectorizeWithBlockFrequency)
if (auto EstimatedTC = getLoopEstimatedTripCount(L))
return *EstimatedTC;
return ElementCount::getFixed(*EstimatedTC);

if (!CanUseConstantMax)
return std::nullopt;

// Check if upper bound estimate is known.
if (unsigned ExpectedTC = PSE.getSmallConstantMaxTripCount())
return ExpectedTC;
return ElementCount::getFixed(ExpectedTC);

return std::nullopt;
}
Expand Down Expand Up @@ -1960,7 +1966,8 @@ class GeneratedRTChecks {
// Get the best known TC estimate.
if (auto EstimatedTC = getSmallBestKnownTC(
PSE, OuterLoop, /* CanUseConstantMax = */ false))
BestTripCount = *EstimatedTC;
if (EstimatedTC->isFixed())
BestTripCount = EstimatedTC->getFixedValue();

InstructionCost NewMemCheckCost = MemCheckCost / BestTripCount;

Expand Down Expand Up @@ -3751,12 +3758,12 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
}

ScalarEvolution *SE = PSE.getSE();
unsigned TC = SE->getSmallConstantTripCount(TheLoop);
ElementCount TC = getSmallConstantTripCount(SE, TheLoop);
unsigned MaxTC = PSE.getSmallConstantMaxTripCount();
LLVM_DEBUG(dbgs() << "LV: Found trip count: " << TC << '\n');
if (TC != MaxTC)
if (TC != ElementCount::getFixed(MaxTC))
LLVM_DEBUG(dbgs() << "LV: Found maximum trip count: " << MaxTC << '\n');
if (TC == 1) {
if (TC.isScalar()) {
reportVectorizationFailure("Single iteration (non) loop",
"loop trip count is one, irrelevant for vectorization",
"SingleIterationLoop", ORE, TheLoop);
Expand Down Expand Up @@ -3870,7 +3877,9 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
}

auto ExpectedTC = getSmallBestKnownTC(PSE, TheLoop);
if (ExpectedTC && ExpectedTC <= TTI.getMinTripCountTailFoldingThreshold()) {
if (ExpectedTC && ExpectedTC->isFixed() &&
ExpectedTC->getFixedValue() <=
TTI.getMinTripCountTailFoldingThreshold()) {
if (MaxPowerOf2RuntimeVF > 0u) {
// If we have a low-trip-count, and the fixed-width VF is known to divide
// the trip count but the scalable factor does not, use the fixed-width
Expand Down Expand Up @@ -3928,7 +3937,7 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
return FixedScalableVFPair::getNone();
}

if (TC == 0) {
if (TC.isZero()) {
reportVectorizationFailure(
"unable to calculate the loop count due to complex control flow",
"UnknownLoopCountComplexCFG", ORE, TheLoop);
Expand Down Expand Up @@ -4817,13 +4826,13 @@ LoopVectorizationCostModel::selectInterleaveCount(VPlan &Plan, ElementCount VF,
// At least one iteration must be scalar when this constraint holds. So the
// maximum available iterations for interleaving is one less.
unsigned AvailableTC = requiresScalarEpilogue(VF.isVector())
? (*BestKnownTC) - 1
: *BestKnownTC;
? BestKnownTC->getFixedValue() - 1
: BestKnownTC->getFixedValue();

unsigned InterleaveCountLB = bit_floor(std::max(
1u, std::min(AvailableTC / (EstimatedVF * 2), MaxInterleaveCount)));

if (PSE.getSE()->getSmallConstantTripCount(TheLoop) > 0) {
if (getSmallConstantTripCount(PSE.getSE(), TheLoop).isNonZero()) {
// If the best known trip count is exact, we select between two
// prospective ICs, where
//
Expand Down Expand Up @@ -5183,8 +5192,8 @@ InstructionCost LoopVectorizationCostModel::expectedCost(ElementCount VF) {
// costs of comparison and induction instructions, as they'll get simplified
// away.
SmallPtrSet<Instruction *, 2> ValuesToIgnoreForVF;
auto TC = PSE.getSE()->getSmallConstantTripCount(TheLoop);
if (VF.isFixed() && TC == VF.getFixedValue() && !foldTailByMasking())
auto TC = getSmallConstantTripCount(PSE.getSE(), TheLoop);
if (TC == VF && !foldTailByMasking())
addFullyUnrolledInstructionsToIgnore(TheLoop, Legal->getInductionVars(),
ValuesToIgnoreForVF);

Expand Down Expand Up @@ -6884,8 +6893,8 @@ LoopVectorizationPlanner::precomputeCosts(VPlan &Plan, ElementCount VF,
// simplified away.
// TODO: Remove this code after stepping away from the legacy cost model and
// adding code to simplify VPlans before calculating their costs.
auto TC = PSE.getSE()->getSmallConstantTripCount(OrigLoop);
if (VF.isFixed() && TC == VF.getFixedValue() && !CM.foldTailByMasking())
auto TC = getSmallConstantTripCount(PSE.getSE(), OrigLoop);
if (TC == VF && !CM.foldTailByMasking())
addFullyUnrolledInstructionsToIgnore(OrigLoop, Legal->getInductionVars(),
CostCtx.SkipCostComputation);

Expand Down Expand Up @@ -9641,8 +9650,7 @@ static bool isOutsideLoopWorkProfitable(GeneratedRTChecks &Checks,
// Skip vectorization if the expected trip count is less than the minimum
// required trip count.
if (auto ExpectedTC = getSmallBestKnownTC(PSE, L)) {
if (ElementCount::isKnownLT(ElementCount::getFixed(*ExpectedTC),
VF.MinProfitableTripCount)) {
if (ElementCount::isKnownLT(*ExpectedTC, VF.MinProfitableTripCount)) {
LLVM_DEBUG(dbgs() << "LV: Vectorization is not beneficial: expected "
"trip count < minimum profitable VF ("
<< *ExpectedTC << " < " << VF.MinProfitableTripCount
Expand Down Expand Up @@ -10012,7 +10020,8 @@ bool LoopVectorizePass::processLoop(Loop *L) {
// Check the loop for a trip count threshold: vectorize loops with a tiny trip
// count by optimizing for size, to minimize overheads.
auto ExpectedTC = getSmallBestKnownTC(PSE, L);
if (ExpectedTC && *ExpectedTC < TinyTripCountVectorThreshold) {
if (ExpectedTC && ExpectedTC->isFixed() &&
ExpectedTC->getFixedValue() < TinyTripCountVectorThreshold) {
LLVM_DEBUG(dbgs() << "LV: Found a loop with a very small trip count. "
<< "This loop is worth vectorizing only if no scalar "
<< "iteration overheads are incurred.");
Expand Down
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