[LV] Remove legacy selectVectorizationFactor and assert (NFCI)#190838
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[LV] Remove legacy selectVectorizationFactor and assert (NFCI)#190838
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Almost all recipes now go through ::computeCost to properly compute their costs using the VPlan-based cost model. There are currently no known cases where the VPlan-based cost model returns an incorrect cost vs the legacy cost model. I check the remaining open issues with reports of the assertion triggering and in all cases the VPlan-based cost model is more accurate, which is causing the divergence. There are still some fall-back paths, mostly via precomputeCosts, but those cannot be easily removed without triggering the assert, as the VPlan-based cost model is more accurate for those cases. An example of this is llvm#187056. Fixes llvm#38575. Fixes llvm#149651. Fixes llvm#182646. Fixes llvm#183739. Fixes llvm#187523.
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@llvm/pr-subscribers-vectorizers @llvm/pr-subscribers-llvm-transforms Author: Florian Hahn (fhahn) ChangesAlmost all recipes now go through ::computeCost to properly compute their costs using the VPlan-based cost model. There are currently no known cases where the VPlan-based cost model returns an incorrect cost vs the legacy cost model. I check the remaining open issues with reports of the assertion triggering and in all cases the VPlan-based cost model is more accurate, which is causing the divergence. There are still some fall-back paths, mostly via precomputeCosts, but those cannot be easily removed without triggering the assert, as the VPlan-based cost model is more accurate for those cases. An example of this is #187056. Fixes #38575. Full diff: https://github.com/llvm/llvm-project/pull/190838.diff 4 Files Affected:
diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorizationPlanner.h b/llvm/lib/Transforms/Vectorize/LoopVectorizationPlanner.h
index b0cf7823ce9f8..f2e820b28e9d7 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorizationPlanner.h
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorizationPlanner.h
@@ -682,15 +682,6 @@ class LoopVectorizationPlanner {
VPRecipeBuilder &RecipeBuilder,
ElementCount MinVF);
-#ifndef NDEBUG
- /// \return The most profitable vectorization factor for the available VPlans
- /// and the cost of that VF.
- /// This is now only used to verify the decisions by the new VPlan-based
- /// cost-model and will be retired once the VPlan-based cost-model is
- /// stabilized.
- VectorizationFactor selectVectorizationFactor();
-#endif
-
/// Returns true if the per-lane cost of VectorizationFactor A is lower than
/// that of B.
bool isMoreProfitable(const VectorizationFactor &A,
diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index e907b5621b817..15bd0c15455d5 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -330,10 +330,6 @@ static cl::opt<bool> EnableIndVarRegisterHeur(
"enable-ind-var-reg-heur", cl::init(true), cl::Hidden,
cl::desc("Count the induction variable only once when interleaving"));
-static cl::opt<bool> EnableCondStoresVectorization(
- "enable-cond-stores-vec", cl::init(true), cl::Hidden,
- cl::desc("Enable if predication of stores during vectorization."));
-
static cl::opt<unsigned> MaxNestedScalarReductionIC(
"max-nested-scalar-reduction-interleave", cl::init(2), cl::Hidden,
cl::desc("The maximum interleave count to use when interleaving a scalar "
@@ -1454,8 +1450,6 @@ class LoopVectorizationCostModel {
/// the factor width.
InstructionCost expectedCost(ElementCount VF);
- bool hasPredStores() const { return NumPredStores > 0; }
-
/// Returns true if epilogue vectorization is considered profitable, and
/// false otherwise.
/// \p VF is the vectorization factor chosen for the original loop.
@@ -4109,147 +4103,6 @@ static bool hasReplicatorRegion(VPlan &Plan) {
[](auto *VPRB) { return VPRB->isReplicator(); });
}
-#ifndef NDEBUG
-VectorizationFactor LoopVectorizationPlanner::selectVectorizationFactor() {
- InstructionCost ExpectedCost = CM.expectedCost(ElementCount::getFixed(1));
- LLVM_DEBUG(dbgs() << "LV: Scalar loop costs: " << ExpectedCost << ".\n");
- assert(ExpectedCost.isValid() && "Unexpected invalid cost for scalar loop");
- assert(
- any_of(VPlans,
- [](std::unique_ptr<VPlan> &P) { return P->hasScalarVFOnly(); }) &&
- "Expected Scalar VF to be a candidate");
-
- const VectorizationFactor ScalarCost(ElementCount::getFixed(1), ExpectedCost,
- ExpectedCost);
- VectorizationFactor ChosenFactor = ScalarCost;
-
- bool ForceVectorization = Hints.getForce() == LoopVectorizeHints::FK_Enabled;
- if (ForceVectorization &&
- (VPlans.size() > 1 || !VPlans[0]->hasScalarVFOnly())) {
- // Ignore scalar width, because the user explicitly wants vectorization.
- // Initialize cost to max so that VF = 2 is, at least, chosen during cost
- // evaluation.
- ChosenFactor.Cost = InstructionCost::getMax();
- }
-
- for (auto &P : VPlans) {
- ArrayRef<ElementCount> VFs(P->vectorFactors().begin(),
- P->vectorFactors().end());
-
- SmallVector<VPRegisterUsage, 8> RUs;
- if (any_of(VFs, [this](ElementCount VF) {
- return CM.shouldConsiderRegPressureForVF(VF);
- }))
- RUs = calculateRegisterUsageForPlan(*P, VFs, TTI, CM.ValuesToIgnore);
-
- for (unsigned I = 0; I < VFs.size(); I++) {
- ElementCount VF = VFs[I];
- // The cost for scalar VF=1 is already calculated, so ignore it.
- if (VF.isScalar())
- continue;
-
- InstructionCost C = CM.expectedCost(VF);
-
- // Add on other costs that are modelled in VPlan, but not in the legacy
- // cost model.
- VPCostContext CostCtx(CM.TTI, *CM.TLI, *P, CM, CM.CostKind, CM.PSE,
- OrigLoop);
- VPRegionBlock *VectorRegion = P->getVectorLoopRegion();
- assert(VectorRegion && "Expected to have a vector region!");
- for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
- vp_depth_first_shallow(VectorRegion->getEntry()))) {
- for (VPRecipeBase &R : *VPBB) {
- auto *VPI = dyn_cast<VPInstruction>(&R);
- if (!VPI)
- continue;
- switch (VPI->getOpcode()) {
- // Selects are only modelled in the legacy cost model for safe
- // divisors.
- case Instruction::Select: {
- if (auto *WR =
- dyn_cast_or_null<VPWidenRecipe>(VPI->getSingleUser())) {
- switch (WR->getOpcode()) {
- case Instruction::UDiv:
- case Instruction::SDiv:
- case Instruction::URem:
- case Instruction::SRem:
- continue;
- default:
- break;
- }
- }
- C += VPI->cost(VF, CostCtx);
- break;
- }
- case VPInstruction::ActiveLaneMask: {
- unsigned Multiplier =
- cast<VPConstantInt>(VPI->getOperand(2))->getZExtValue();
- C += VPI->cost(VF * Multiplier, CostCtx);
- break;
- }
- case VPInstruction::ExplicitVectorLength:
- case VPInstruction::AnyOf:
- C += VPI->cost(VF, CostCtx);
- break;
- default:
- break;
- }
- }
- }
-
- // Add the cost of any spills due to excess register usage
- if (CM.shouldConsiderRegPressureForVF(VF))
- C += RUs[I].spillCost(CostCtx, ForceTargetNumVectorRegs);
-
- VectorizationFactor Candidate(VF, C, ScalarCost.ScalarCost);
- unsigned Width =
- estimateElementCount(Candidate.Width, CM.getVScaleForTuning());
- LLVM_DEBUG(dbgs() << "LV: Vector loop of width " << VF
- << " costs: " << (Candidate.Cost / Width));
- if (VF.isScalable())
- LLVM_DEBUG(dbgs() << " (assuming a minimum vscale of "
- << CM.getVScaleForTuning().value_or(1) << ")");
- LLVM_DEBUG(dbgs() << ".\n");
-
- if (!ForceVectorization && !willGenerateVectors(*P, VF, TTI)) {
- LLVM_DEBUG(
- dbgs()
- << "LV: Not considering vector loop of width " << VF
- << " because it will not generate any vector instructions.\n");
- continue;
- }
-
- if (CM.OptForSize && !ForceVectorization && hasReplicatorRegion(*P)) {
- LLVM_DEBUG(
- dbgs()
- << "LV: Not considering vector loop of width " << VF
- << " because it would cause replicated blocks to be generated,"
- << " which isn't allowed when optimizing for size.\n");
- continue;
- }
-
- if (isMoreProfitable(Candidate, ChosenFactor, P->hasScalarTail()))
- ChosenFactor = Candidate;
- }
- }
-
- if (!EnableCondStoresVectorization && CM.hasPredStores()) {
- reportVectorizationFailure(
- "There are conditional stores.",
- "store that is conditionally executed prevents vectorization",
- "ConditionalStore", ORE, OrigLoop);
- ChosenFactor = ScalarCost;
- }
-
- LLVM_DEBUG(if (ForceVectorization && !ChosenFactor.Width.isScalar() &&
- !isMoreProfitable(ChosenFactor, ScalarCost,
- !CM.foldTailByMasking())) dbgs()
- << "LV: Vectorization seems to be not beneficial, "
- << "but was forced by a user.\n");
- return ChosenFactor;
-}
-#endif
-
/// Returns true if the VPlan contains a VPReductionPHIRecipe with
/// FindLast recurrence kind.
static bool hasFindLastReductionPhi(VPlan &Plan) {
@@ -6831,11 +6684,6 @@ InstructionCost VPCostContext::getLegacyCost(Instruction *UI,
return Cost;
}
-bool VPCostContext::isLegacyUniformAfterVectorization(Instruction *I,
- ElementCount VF) const {
- return CM.isUniformAfterVectorization(I, VF);
-}
-
bool VPCostContext::skipCostComputation(Instruction *UI, bool IsVector) const {
return CM.ValuesToIgnore.contains(UI) ||
(IsVector && CM.VecValuesToIgnore.contains(UI)) ||
@@ -7018,162 +6866,6 @@ InstructionCost LoopVectorizationPlanner::cost(VPlan &Plan, ElementCount VF,
return Cost;
}
-#ifndef NDEBUG
-/// Return true if the original loop \ TheLoop contains any instructions that do
-/// not have corresponding recipes in \p Plan and are not marked to be ignored
-/// in \p CostCtx. This means the VPlan contains simplification that the legacy
-/// cost-model did not account for.
-static bool planContainsAdditionalSimplifications(VPlan &Plan,
- VPCostContext &CostCtx,
- Loop *TheLoop,
- ElementCount VF) {
- using namespace VPlanPatternMatch;
- // First collect all instructions for the recipes in Plan.
- auto GetInstructionForCost = [](const VPRecipeBase *R) -> Instruction * {
- if (auto *S = dyn_cast<VPSingleDefRecipe>(R))
- return dyn_cast_or_null<Instruction>(S->getUnderlyingValue());
- if (auto *WidenMem = dyn_cast<VPWidenMemoryRecipe>(R))
- return &WidenMem->getIngredient();
- return nullptr;
- };
-
- // Check if a select for a safe divisor was hoisted to the pre-header. If so,
- // the select doesn't need to be considered for the vector loop cost; go with
- // the more accurate VPlan-based cost model.
- for (VPRecipeBase &R : *Plan.getVectorPreheader()) {
- auto *VPI = dyn_cast<VPInstruction>(&R);
- if (!VPI || VPI->getOpcode() != Instruction::Select)
- continue;
-
- if (auto *WR = dyn_cast_or_null<VPWidenRecipe>(VPI->getSingleUser())) {
- switch (WR->getOpcode()) {
- case Instruction::UDiv:
- case Instruction::SDiv:
- case Instruction::URem:
- case Instruction::SRem:
- return true;
- default:
- break;
- }
- }
- }
-
- DenseSet<Instruction *> SeenInstrs;
- auto Iter = vp_depth_first_deep(Plan.getVectorLoopRegion()->getEntry());
- for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(Iter)) {
- for (VPRecipeBase &R : *VPBB) {
- if (auto *IR = dyn_cast<VPInterleaveRecipe>(&R)) {
- auto *IG = IR->getInterleaveGroup();
- unsigned NumMembers = IG->getNumMembers();
- for (unsigned I = 0; I != NumMembers; ++I) {
- if (Instruction *M = IG->getMember(I))
- SeenInstrs.insert(M);
- }
- continue;
- }
- // Unused FOR splices are removed by VPlan transforms, so the VPlan-based
- // cost model won't cost it whilst the legacy will.
- if (auto *FOR = dyn_cast<VPFirstOrderRecurrencePHIRecipe>(&R)) {
- if (none_of(FOR->users(),
- match_fn(m_VPInstruction<
- VPInstruction::FirstOrderRecurrenceSplice>())))
- return true;
- }
- // The VPlan-based cost model is more accurate for partial reductions and
- // comparing against the legacy cost isn't desirable.
- if (auto *VPR = dyn_cast<VPReductionRecipe>(&R))
- if (VPR->isPartialReduction())
- return true;
-
- // The VPlan-based cost model can analyze if recipes are scalar
- // recursively, but the legacy cost model cannot.
- if (auto *WidenMemR = dyn_cast<VPWidenMemoryRecipe>(&R)) {
- auto *AddrI = dyn_cast<Instruction>(
- getLoadStorePointerOperand(&WidenMemR->getIngredient()));
- if (AddrI && vputils::isSingleScalar(WidenMemR->getAddr()) !=
- CostCtx.isLegacyUniformAfterVectorization(AddrI, VF))
- return true;
-
- if (WidenMemR->isReverse()) {
- // If the stored value of a reverse store is invariant, LICM will
- // hoist the reverse operation to the preheader. In this case, the
- // result of the VPlan-based cost model will diverge from that of
- // the legacy model.
- if (auto *StoreR = dyn_cast<VPWidenStoreRecipe>(WidenMemR))
- if (StoreR->getStoredValue()->isDefinedOutsideLoopRegions())
- return true;
-
- if (auto *StoreR = dyn_cast<VPWidenStoreEVLRecipe>(WidenMemR))
- if (StoreR->getStoredValue()->isDefinedOutsideLoopRegions())
- return true;
- }
- }
-
- // The legacy cost model costs non-header phis with a scalar VF as a phi,
- // but scalar unrolled VPlans will have VPBlendRecipes which emit selects.
- if (isa<VPBlendRecipe>(&R) &&
- vputils::onlyFirstLaneUsed(R.getVPSingleValue()))
- return true;
-
- // The legacy cost model won't calculate the cost of the LogicalAnd which
- // will be replaced with vp_merge.
- if (match(&R, m_Intrinsic<Intrinsic::vp_merge>()))
- return true;
-
- /// If a VPlan transform folded a recipe to one producing a single-scalar,
- /// but the original instruction wasn't uniform-after-vectorization in the
- /// legacy cost model, the legacy cost overestimates the actual cost.
- if (auto *RepR = dyn_cast<VPReplicateRecipe>(&R)) {
- if (RepR->isSingleScalar() &&
- !CostCtx.isLegacyUniformAfterVectorization(
- RepR->getUnderlyingInstr(), VF))
- return true;
- }
- if (Instruction *UI = GetInstructionForCost(&R)) {
- // If we adjusted the predicate of the recipe, the cost in the legacy
- // cost model may be different.
- CmpPredicate Pred;
- if (match(&R, m_Cmp(Pred, m_VPValue(), m_VPValue())) &&
- cast<VPRecipeWithIRFlags>(R).getPredicate() !=
- cast<CmpInst>(UI)->getPredicate())
- return true;
-
- // Recipes with underlying instructions being moved out of the loop
- // region by LICM may cause discrepancies between the legacy cost model
- // and the VPlan-based cost model.
- if (!VPBB->getEnclosingLoopRegion())
- return true;
-
- SeenInstrs.insert(UI);
- }
- }
- }
-
- // If a reverse recipe has been sunk to the middle block (e.g., for a load
- // whose result is only used as a live-out), VPlan avoids the per-iteration
- // reverse shuffle cost that the legacy model accounts for.
- if (any_of(*Plan.getMiddleBlock(), [](const VPRecipeBase &R) {
- return match(&R, m_VPInstruction<VPInstruction::Reverse>());
- }))
- return true;
-
- // Return true if the loop contains any instructions that are not also part of
- // the VPlan or are skipped for VPlan-based cost computations. This indicates
- // that the VPlan contains extra simplifications.
- return any_of(TheLoop->blocks(), [&SeenInstrs, &CostCtx,
- TheLoop](BasicBlock *BB) {
- return any_of(*BB, [&SeenInstrs, &CostCtx, TheLoop, BB](Instruction &I) {
- // Skip induction phis when checking for simplifications, as they may not
- // be lowered directly be lowered to a corresponding PHI recipe.
- if (isa<PHINode>(&I) && BB == TheLoop->getHeader() &&
- CostCtx.CM.Legal->isInductionPhi(cast<PHINode>(&I)))
- return false;
- return !SeenInstrs.contains(&I) && !CostCtx.skipCostComputation(&I, true);
- });
- });
-}
-#endif
-
std::pair<VectorizationFactor, VPlan *>
LoopVectorizationPlanner::computeBestVF() {
if (VPlans.empty())
@@ -7261,46 +6953,8 @@ LoopVectorizationPlanner::computeBestVF() {
VPlan &BestPlan = *PlanForBestVF;
-#ifndef NDEBUG
- // Select the optimal vectorization factor according to the legacy cost-model.
- // This is now only used to verify the decisions by the new VPlan-based
- // cost-model and will be retired once the VPlan-based cost-model is
- // stabilized.
- VectorizationFactor LegacyVF = selectVectorizationFactor();
-
- // Pre-compute the cost and use it to check if BestPlan contains any
- // simplifications not accounted for in the legacy cost model. If that's the
- // case, don't trigger the assertion, as the extra simplifications may cause a
- // different VF to be picked by the VPlan-based cost model.
- VPCostContext CostCtx(CM.TTI, *CM.TLI, BestPlan, CM, CM.CostKind, CM.PSE,
- OrigLoop);
- precomputeCosts(BestPlan, BestFactor.Width, CostCtx);
- // Verify that the VPlan-based and legacy cost models agree, except for
- // * VPlans with early exits,
- // * VPlans with additional VPlan simplifications,
- // * EVL-based VPlans with gather/scatters (the VPlan-based cost model uses
- // vp_scatter/vp_gather).
- // The legacy cost model doesn't properly model costs for such loops.
- bool UsesEVLGatherScatter =
- any_of(VPBlockUtils::blocksOnly<VPBasicBlock>(vp_depth_first_shallow(
- BestPlan.getVectorLoopRegion()->getEntry())),
- [](VPBasicBlock *VPBB) {
- return any_of(*VPBB, [](VPRecipeBase &R) {
- return isa<VPWidenLoadEVLRecipe, VPWidenStoreEVLRecipe>(&R) &&
- !cast<VPWidenMemoryRecipe>(&R)->isConsecutive();
- });
- });
- assert((BestFactor.Width == LegacyVF.Width || BestPlan.hasEarlyExit() ||
- !Legal->getLAI()->getSymbolicStrides().empty() ||
- UsesEVLGatherScatter ||
- planContainsAdditionalSimplifications(BestPlan, CostCtx, OrigLoop,
- BestFactor.Width) ||
- planContainsAdditionalSimplifications(
- getPlanFor(LegacyVF.Width), CostCtx, OrigLoop, LegacyVF.Width)) &&
- " VPlan cost model and legacy cost model disagreed");
assert((BestFactor.Width.isScalar() || BestFactor.ScalarCost > 0) &&
"when vectorizing, the scalar cost must be computed.");
-#endif
LLVM_DEBUG(dbgs() << "LV: Selecting VF: " << BestFactor.Width << ".\n");
return {BestFactor, &BestPlan};
diff --git a/llvm/lib/Transforms/Vectorize/VPlanHelpers.h b/llvm/lib/Transforms/Vectorize/VPlanHelpers.h
index 5de7ab36a6d75..b54b80b496668 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanHelpers.h
+++ b/llvm/lib/Transforms/Vectorize/VPlanHelpers.h
@@ -367,11 +367,6 @@ struct VPCostContext {
/// Returns the OperandInfo for \p V, if it is a live-in.
TargetTransformInfo::OperandValueInfo getOperandInfo(VPValue *V) const;
- /// Return true if \p I is considered uniform-after-vectorization in the
- /// legacy cost model for \p VF. Only used to check for additional VPlan
- /// simplifications.
- bool isLegacyUniformAfterVectorization(Instruction *I, ElementCount VF) const;
-
/// Estimate the overhead of scalarizing a recipe with result type \p ResultTy
/// and \p Operands with \p VF. This is a convenience wrapper for the
/// type-based getScalarizationOverhead API. \p VIC provides context about
diff --git a/llvm/test/Transforms/LoopVectorize/conditional-assignment.ll b/llvm/test/Transforms/LoopVectorize/conditional-assignment.ll
index 9a2aa299e5d16..4836ffc6b293e 100644
--- a/llvm/test/Transforms/LoopVectorize/conditional-assignment.ll
+++ b/llvm/test/Transforms/LoopVectorize/conditional-assignment.ll
@@ -1,4 +1,4 @@
-; RUN: opt < %s -enable-cond-stores-vec=false -passes=loop-vectorize -S -pass-remarks-missed='loop-vectorize' -pass-remarks-analysis='loop-vectorize' 2>&1 | FileCheck %s
+; RUN: opt < %s -passes=loop-vectorize -S -pass-remarks-missed='loop-vectorize' -pass-remarks-analysis='loop-vectorize' 2>&1 | FileCheck %s
; CHECK: remark: source.c:2:8: the cost-model indicates that vectorization is not beneficial
|
🐧 Linux x64 Test Results
✅ The build succeeded and all tests passed. |
Contributor
|
Looks like there is a test failure in LLVM.tools/UpdateTestChecks/update_analyze_test_checks/loopvectorize-costmodel.test. Once that's fixed I'm happy to accept the PR! |
david-arm
reviewed
Apr 8, 2026
| "enable-ind-var-reg-heur", cl::init(true), cl::Hidden, | ||
| cl::desc("Count the induction variable only once when interleaving")); | ||
|
|
||
| static cl::opt<bool> EnableCondStoresVectorization( |
Contributor
There was a problem hiding this comment.
It looks like this option was never supported for the vplan model, but nobody has complained about it so I guess it's fine to remove it?
Contributor
Author
There was a problem hiding this comment.
Yep, it looks like there was only a single test that actually set it to false and it does not impact the test
fhahn
commented
Apr 8, 2026
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fhahn
left a comment
fhahn
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There was a problem hiding this comment.
n LLVM.tools/UpdateTestChecks/update_analyze_test_checks/loopvectorize-costmodel.test.
Should be updated now to also check some VPlan cost model prints
| "enable-ind-var-reg-heur", cl::init(true), cl::Hidden, | ||
| cl::desc("Count the induction variable only once when interleaving")); | ||
|
|
||
| static cl::opt<bool> EnableCondStoresVectorization( |
Contributor
Author
There was a problem hiding this comment.
Yep, it looks like there was only a single test that actually set it to false and it does not impact the test
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Almost all recipes now go through ::computeCost to properly compute their costs using the VPlan-based cost model. There are currently no known cases where the VPlan-based cost model returns an incorrect cost vs the legacy cost model. I check the remaining open issues with reports of the assertion triggering and in all cases the VPlan-based cost model is more accurate, which is causing the divergence.
There are still some fall-back paths, mostly via precomputeCosts, but those cannot be easily removed without triggering the assert, as the VPlan-based cost model is more accurate for those cases. An example of this is #187056.
Fixes #38575.
Fixes #149651.
Fixes #182646.
Fixes #183739.
Fixes #187523.