llvm · fhahn · Jun 8, 2025
diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp
@@ -798,6 +798,7 @@ bool LoopVectorizationLegality::canVectorizeInstrs() {
   // For each block in the loop.
   for (BasicBlock *BB : TheLoop->blocks()) {
     // Scan the instructions in the block and look for hazards.
+    PHINode *UnclassifiedPhi = nullptr;
     for (Instruction &I : *BB) {
       if (auto *Phi = dyn_cast<PHINode>(&I)) {
         Type *PhiTy = Phi->getType();
@@ -887,12 +888,7 @@ bool LoopVectorizationLegality::canVectorizeInstrs() {
           addInductionPhi(Phi, ID, AllowedExit);
           continue;
         }
-
-        reportVectorizationFailure("Found an unidentified PHI",
-            "value that could not be identified as "
-            "reduction is used outside the loop",
-            "NonReductionValueUsedOutsideLoop", ORE, TheLoop, Phi);
-        return false;
+        UnclassifiedPhi = Phi;
       } // end of PHI handling
 
       // We handle calls that:
@@ -1043,6 +1039,19 @@ bool LoopVectorizationLegality::canVectorizeInstrs() {
         return false;
       }
     } // next instr.
+    if (UnclassifiedPhi && none_of(BB->phis(), [this](PHINode &P) {
+          auto I = Reductions.find(&P);
+          return I != Reductions.end() &&
+                 RecurrenceDescriptor::isFindLastIVRecurrenceKind(
+                     I->second.getRecurrenceKind());
+        })) {
+      reportVectorizationFailure("Found an unidentified PHI",
+                                 "value that could not be identified as "
+                                 "reduction is used outside the loop",
+                                 "NonReductionValueUsedOutsideLoop", ORE,
+                                 TheLoop, UnclassifiedPhi);
+      return false;
+    }
   }
 
   if (!PrimaryInduction) {

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -7177,6 +7177,9 @@ static void fixReductionScalarResumeWhenVectorizingEpilog(
     Value *StartV = getStartValueFromReductionResult(EpiRedResult);
     Value *SentinelV = EpiRedResult->getOperand(2)->getLiveInIRValue();
     using namespace llvm::PatternMatch;
+    MainResumeValue = cast<VPInstruction>(EpiRedHeaderPhi->getStartValue())
+                          ->getOperand(0)
+                          ->getUnderlyingValue();
     Value *Cmp, *OrigResumeV, *CmpOp;
     [[maybe_unused]] bool IsExpectedPattern =
         match(MainResumeValue,
@@ -7187,7 +7190,11 @@ static void fixReductionScalarResumeWhenVectorizingEpilog(
          ((CmpOp == StartV && isGuaranteedNotToBeUndefOrPoison(CmpOp))));
     assert(IsExpectedPattern && "Unexpected reduction resume pattern");
     MainResumeValue = OrigResumeV;
+  } else {
+    if (auto *VPI = dyn_cast<VPInstruction>(EpiRedHeaderPhi->getStartValue()))
+      MainResumeValue = VPI->getOperand(0)->getUnderlyingValue();
   }
+
   PHINode *MainResumePhi = cast<PHINode>(MainResumeValue);
 
   // When fixing reductions in the epilogue loop we should already have
@@ -8193,9 +8200,6 @@ VPRecipeBase *VPRecipeBuilder::tryToCreateWidenRecipe(VPSingleDefRecipe *R,
       return Recipe;
 
     VPHeaderPHIRecipe *PhiRecipe = nullptr;
-    assert((Legal->isReductionVariable(Phi) ||
-            Legal->isFixedOrderRecurrence(Phi)) &&
-           "can only widen reductions and fixed-order recurrences here");
     VPValue *StartV = Operands[0];
     if (Legal->isReductionVariable(Phi)) {
       const RecurrenceDescriptor &RdxDesc = Legal->getRecurrenceDescriptor(Phi);
@@ -8208,12 +8212,17 @@ VPRecipeBase *VPRecipeBuilder::tryToCreateWidenRecipe(VPSingleDefRecipe *R,
       PhiRecipe = new VPReductionPHIRecipe(
           Phi, RdxDesc.getRecurrenceKind(), *StartV, CM.isInLoopReduction(Phi),
           CM.useOrderedReductions(RdxDesc), ScaleFactor);
-    } else {
+    } else if (Legal->isFixedOrderRecurrence(Phi)) {
       // TODO: Currently fixed-order recurrences are modeled as chains of
       // first-order recurrences. If there are no users of the intermediate
       // recurrences in the chain, the fixed order recurrence should be modeled
       // directly, enabling more efficient codegen.
       PhiRecipe = new VPFirstOrderRecurrencePHIRecipe(Phi, *StartV);
+    } else {
+      // Failed to identify phi as reduction or fixed-order recurrence. Keep the
+      // original VPWidenPHIRecipe for now, to be legalized later if possible.
+      setRecipe(Phi, R);
+      return nullptr;
     }
     // Add backedge value.
     PhiRecipe->addOperand(Operands[1]);
@@ -8398,7 +8407,7 @@ static void addScalarResumePhis(VPRecipeBuilder &Builder, VPlan &Plan,
     // TODO: Extract final value from induction recipe initially, optimize to
     // pre-computed end value together in optimizeInductionExitUsers.
     auto *VectorPhiR =
-        cast<VPHeaderPHIRecipe>(Builder.getRecipe(&ScalarPhiIRI->getIRPhi()));
+        cast<VPSingleDefRecipe>(Builder.getRecipe(&ScalarPhiIRI->getIRPhi()));
     if (auto *WideIVR = dyn_cast<VPWidenInductionRecipe>(VectorPhiR)) {
       if (VPInstruction *ResumePhi = addResumePhiRecipeForInduction(
               WideIVR, VectorPHBuilder, ScalarPHBuilder, TypeInfo,
@@ -8420,7 +8429,7 @@ static void addScalarResumePhis(VPRecipeBuilder &Builder, VPlan &Plan,
     // which for FORs is a vector whose last element needs to be extracted. The
     // start value provides the value if the loop is bypassed.
     bool IsFOR = isa<VPFirstOrderRecurrencePHIRecipe>(VectorPhiR);
-    auto *ResumeFromVectorLoop = VectorPhiR->getBackedgeValue();
+    auto *ResumeFromVectorLoop = VectorPhiR->getOperand(1);
     assert(VectorRegion->getSingleSuccessor() == Plan.getMiddleBlock() &&
            "Cannot handle loops with uncountable early exits");
     if (IsFOR)
@@ -8429,7 +8438,7 @@ static void addScalarResumePhis(VPRecipeBuilder &Builder, VPlan &Plan,
           "vector.recur.extract");
     StringRef Name = IsFOR ? "scalar.recur.init" : "bc.merge.rdx";
     auto *ResumePhiR = ScalarPHBuilder.createScalarPhi(
-        {ResumeFromVectorLoop, VectorPhiR->getStartValue()}, {}, Name);
+        {ResumeFromVectorLoop, VectorPhiR->getOperand(0)}, {}, Name);
     ScalarPhiIRI->addOperand(ResumePhiR);
   }
 }
@@ -8744,6 +8753,8 @@ VPlanPtr LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(
       VPRecipeBase *Recipe =
           RecipeBuilder.tryToCreateWidenRecipe(SingleDef, Range);
       if (!Recipe) {
+        if (isa<VPWidenPHIRecipe>(SingleDef))
+          continue;
         SmallVector<VPValue *, 4> Operands(R.operands());
         Recipe = RecipeBuilder.handleReplication(Instr, Operands, Range);
       }
@@ -8809,6 +8820,11 @@ VPlanPtr LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(
   // Adjust the recipes for any inloop reductions.
   adjustRecipesForReductions(Plan, RecipeBuilder, Range.Start);
 
+  // Try to convert remaining VPWidenPHIRecipes to reduction recipes.
+  if (!VPlanTransforms::runPass(VPlanTransforms::legalizeUnclassifiedPhis,
+                                *Plan))
+    return nullptr;
+
   // Transform recipes to abstract recipes if it is legal and beneficial and
   // clamp the range for better cost estimation.
   // TODO: Enable following transform when the EVL-version of extended-reduction
@@ -9275,6 +9291,7 @@ void LoopVectorizationPlanner::adjustRecipesForReductions(
       PhiR->setOperand(0, StartV);
     }
   }
+
   for (VPRecipeBase *R : ToDelete)
     R->eraseFromParent();
 

diff --git a/llvm/lib/Transforms/Vectorize/VPlan.h b/llvm/lib/Transforms/Vectorize/VPlan.h
@@ -1854,7 +1854,8 @@ class LLVM_ABI_FOR_TEST VPHeaderPHIRecipe : public VPSingleDefRecipe,
   ~VPHeaderPHIRecipe() override = default;
 
   /// Method to support type inquiry through isa, cast, and dyn_cast.
-  static inline bool classof(const VPRecipeBase *B) {
+  static inline bool classof(const VPUser *U) {
+    auto *B = cast<VPRecipeBase>(U);
     return B->getVPDefID() >= VPDef::VPFirstHeaderPHISC &&
            B->getVPDefID() <= VPDef::VPLastHeaderPHISC;
   }
@@ -1863,6 +1864,10 @@ class LLVM_ABI_FOR_TEST VPHeaderPHIRecipe : public VPSingleDefRecipe,
     return B && B->getVPDefID() >= VPRecipeBase::VPFirstHeaderPHISC &&
            B->getVPDefID() <= VPRecipeBase::VPLastHeaderPHISC;
   }
+  static inline bool classof(const VPSingleDefRecipe *B) {
+    return B->getVPDefID() >= VPDef::VPFirstHeaderPHISC &&
+           B->getVPDefID() <= VPDef::VPLastHeaderPHISC;
+  }
 
   /// Generate the phi nodes.
   void execute(VPTransformState &State) override = 0;
@@ -1924,7 +1929,7 @@ class VPWidenInductionRecipe : public VPHeaderPHIRecipe {
     return R && classof(R);
   }
 
-  static inline bool classof(const VPHeaderPHIRecipe *R) {
+  static inline bool classof(const VPSingleDefRecipe *R) {
     return classof(static_cast<const VPRecipeBase *>(R));
   }
 

diff --git a/llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp b/llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp
@@ -628,3 +628,148 @@ void VPlanTransforms::attachCheckBlock(VPlan &Plan, Value *Cond,
     Term->addMetadata(LLVMContext::MD_prof, BranchWeights);
   }
 }
+
+bool VPlanTransforms::legalizeUnclassifiedPhis(VPlan &Plan) {
+  using namespace VPlanPatternMatch;
+  for (auto &PhiR : make_early_inc_range(
+           Plan.getVectorLoopRegion()->getEntryBasicBlock()->phis())) {
+    if (!isa<VPWidenPHIRecipe>(&PhiR))
+      continue;
+
+    // Check if PhiR is a min/max reduction that has a user inside the loop
+    // outside the min/max reduction chain. The other user must be the compare
+    // of a FindLastIV reduction chain.
+    auto *MinMaxPhiR = cast<VPWidenPHIRecipe>(&PhiR);
+    auto *MinMaxOp = dyn_cast_or_null<VPSingleDefRecipe>(
+        MinMaxPhiR->getOperand(1)->getDefiningRecipe());
+    if (!MinMaxOp)
+      return false;
+
+    // The incoming value must be a min/max instrinsic.
+    // TODO: Also handle the select variant.
+    Intrinsic::ID ID = Intrinsic::not_intrinsic;
+    if (auto *WideInt = dyn_cast<VPWidenIntrinsicRecipe>(MinMaxOp))
+      ID = WideInt->getVectorIntrinsicID();
+    else {
+      auto *RepR = dyn_cast<VPReplicateRecipe>(MinMaxOp);
+      if (!RepR || !isa<IntrinsicInst>(RepR->getUnderlyingInstr()))
+        return false;
+      ID = cast<IntrinsicInst>(RepR->getUnderlyingInstr())->getIntrinsicID();
+    }
+    RecurKind RdxKind = RecurKind::None;
+    switch (ID) {
+    case Intrinsic::umax:
+      RdxKind = RecurKind::UMax;
+      break;
+    case Intrinsic::umin:
+      RdxKind = RecurKind::UMin;
+      break;
+    case Intrinsic::smax:
+      RdxKind = RecurKind::SMax;
+      break;
+    case Intrinsic::smin:
+      RdxKind = RecurKind::SMin;
+      break;
+    default:
+      return false;
+    }
+
+    // The min/max intrinsic must use the phi and itself must only be used by
+    // the phi and a resume-phi in the scalar preheader.
+    if (MinMaxOp->getOperand(0) != MinMaxPhiR &&
+        MinMaxOp->getOperand(1) != MinMaxPhiR)
+      return false;
+    if (MinMaxPhiR->getNumUsers() != 2 ||
+        any_of(MinMaxOp->users(), [MinMaxPhiR, &Plan](VPUser *U) {
+          auto *Phi = dyn_cast<VPPhi>(U);
+          return MinMaxPhiR != U &&
+                 (!Phi || Phi->getParent() != Plan.getScalarPreheader());
+        }))
+      return false;
+
+    // One user of MinMaxPhiR is MinMaxOp, the other users must be a compare
+    // that's part of a FindLastIV chain.
+    auto MinMaxUsers = to_vector(MinMaxPhiR->users());
+    auto *Cmp = dyn_cast<VPRecipeWithIRFlags>(
+        MinMaxUsers[0] == MinMaxOp ? MinMaxUsers[1] : MinMaxUsers[0]);
+    VPValue *CmpOpA;
+    VPValue *CmpOpB;
+    if (!Cmp || Cmp->getNumUsers() != 1 ||
+        !match(Cmp, m_Binary<Instruction::ICmp>(m_VPValue(CmpOpA),
+                                                m_VPValue(CmpOpB))))
+      return false;
+
+    // Normalize the predicate so MinMaxPhiR is on the right side.
+    CmpInst::Predicate Pred = Cmp->getPredicate();
+    if (CmpOpA == MinMaxPhiR)
+      Pred = CmpInst::getSwappedPredicate(Pred);
+
+    // Determine if the predicate is not strict.
+    bool IsNonStrictPred = ICmpInst::isLE(Pred) || ICmpInst::isGE(Pred);
+    // Account for a mis-match between RdxKind and the predicate.
+    switch (RdxKind) {
+    case RecurKind::UMin:
+    case RecurKind::SMin:
+      IsNonStrictPred |= ICmpInst::isGT(Pred);
+      break;
+    case RecurKind::UMax:
+    case RecurKind::SMax:
+      IsNonStrictPred |= ICmpInst::isLT(Pred);
+      break;
+    default:
+      llvm_unreachable("unsupported kind");
+    }
+
+    // TODO: Strict predicates need to find the first IV value for which the
+    // predicate holds, not the last.
+    if (Pred == CmpInst::ICMP_NE || !IsNonStrictPred)
+      return false;
+
+    // Cmp must be used by the select of a FindLastIV chain.
+    VPValue *Sel = dyn_cast<VPSingleDefRecipe>(*Cmp->user_begin());
+    VPValue *IVOp, *FindIV;
+    if (!Sel ||
+        !match(Sel,
+               m_Select(m_Specific(Cmp), m_VPValue(IVOp), m_VPValue(FindIV))) ||
+        Sel->getNumUsers() != 2 || !isa<VPWidenIntOrFpInductionRecipe>(IVOp))
+      return false;
+    auto *FindIVPhiR = dyn_cast<VPReductionPHIRecipe>(FindIV);
+    if (!FindIVPhiR || !RecurrenceDescriptor::isFindLastIVRecurrenceKind(
+                           FindIVPhiR->getRecurrenceKind()))
+      return false;
+
+    assert(!FindIVPhiR->isInLoop() && !FindIVPhiR->isOrdered() &&
+           "cannot handle inloop/ordered reductions yet");
+
+    auto NewPhiR = new VPReductionPHIRecipe(
+        cast<PHINode>(MinMaxPhiR->getUnderlyingInstr()), RdxKind,
+        *MinMaxPhiR->getOperand(0), false, false, 1);
+    NewPhiR->insertBefore(MinMaxPhiR);
+    MinMaxPhiR->replaceAllUsesWith(NewPhiR);
+    NewPhiR->addOperand(MinMaxPhiR->getOperand(1));
+    MinMaxPhiR->eraseFromParent();
+
+    // The reduction using MinMaxPhiR needs adjusting to compute the correct
+    // result:
+    //  1. We need to find the last IV for which the condition based on the
+    //  min/max recurrence is true,
+    //  2. Compare the partial min/max reduction result to its final value and,
+    //  3. Select the lanes of the partial FindLastIV reductions which
+    //  correspond to the lanes matching the min/max reduction result.
+    VPInstruction *FindIVResult = cast<VPInstruction>(
+        *(Sel->user_begin() + (*Sel->user_begin() == FindIVPhiR ? 1 : 0)));
+    VPBuilder B(FindIVResult);
+    VPInstruction *MinMaxResult =
+        B.createNaryOp(VPInstruction::ComputeReductionResult,
+                       {NewPhiR, NewPhiR->getBackedgeValue()}, VPIRFlags(), {});
+    NewPhiR->getBackedgeValue()->replaceUsesWithIf(
+        MinMaxResult, [](VPUser &U, unsigned) { return isa<VPPhi>(&U); });
+    auto *FinalMinMaxCmp = B.createICmp(
+        CmpInst::ICMP_EQ, MinMaxResult->getOperand(1), MinMaxResult);
+    auto *FinalIVSelect =
+        B.createSelect(FinalMinMaxCmp, FindIVResult->getOperand(3),
+                       FindIVResult->getOperand(2));
+    FindIVResult->setOperand(3, FinalIVSelect);
+  }
+  return true;
+}
diff --git a/llvm/lib/Transforms/Vectorize/VPlanTransforms.h b/llvm/lib/Transforms/Vectorize/VPlanTransforms.h
@@ -89,6 +89,11 @@ struct VPlanTransforms {
           GetIntOrFpInductionDescriptor,
       ScalarEvolution &SE, const TargetLibraryInfo &TLI);
 
+  /// Try to legalize unclassified phis by converting VPWidenPHIRecipes to
+  /// min-max reductions used by FindLastIV reductions if possible. Returns
+  /// false if the VPlan contains VPWidenPHIRecipes that cannot be legalized.
+  static bool legalizeUnclassifiedPhis(VPlan &Plan);
+
   /// Try to have all users of fixed-order recurrences appear after the recipe
   /// defining their previous value, by either sinking users or hoisting recipes
   /// defining their previous value (and its operands). Then introduce