[VPlan] Introduce VPScalarPHIRecipe, use for can & EVL IV codegen (NFC).

Introduce a general recipe to generate a scalar phi. Lower
VPCanonicalIVPHIRecipe and VPEVLBasedIVRecipe to VPScalarIVPHIrecipe
before plan execution, avoiding the need for duplicated ::execute
implementations. There are other cases that could benefit, including
in-loop reduction phis.

Builds on a similar idea as
llvm#82270.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7702,6 +7702,7 @@ DenseMap<const SCEV *, Value *> LoopVectorizationPlanner::executePlan(
   BestVPlan.prepareToExecute(ILV.getTripCount(),
                              ILV.getOrCreateVectorTripCount(nullptr),
                              CanonicalIVStartValue, State);
+  VPlanTransforms::prepareToExecute(BestVPlan);
 
   BestVPlan.execute(&State);
 

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -1097,10 +1097,9 @@ void VPlan::execute(VPTransformState *State) {
     }
 
     auto *PhiR = cast<VPHeaderPHIRecipe>(&R);
-    bool NeedsScalar =
-        isa<VPCanonicalIVPHIRecipe, VPEVLBasedIVPHIRecipe>(PhiR) ||
-        (isa<VPReductionPHIRecipe>(PhiR) &&
-         cast<VPReductionPHIRecipe>(PhiR)->isInLoop());
+    bool NeedsScalar = isa<VPScalarPHIRecipe>(PhiR) ||
+                       (isa<VPReductionPHIRecipe>(PhiR) &&
+                        cast<VPReductionPHIRecipe>(PhiR)->isInLoop());
     Value *Phi = State->get(PhiR, NeedsScalar);
     Value *Val = State->get(PhiR->getBackedgeValue(), NeedsScalar);
     cast<PHINode>(Phi)->addIncoming(Val, VectorLatchBB);

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -2239,6 +2239,45 @@ class VPWidenPointerInductionRecipe : public VPHeaderPHIRecipe,
 #endif
 };
 
+/// Recipe to generate a scalar PHI. Used to generate code for recipes that
+/// produce scalar header phis, including VPCanonicalIVPHIRecipe and
+/// VPEVLBasedIVPHIRecipe.
+class VPScalarPHIRecipe : public VPHeaderPHIRecipe {
+  std::string Name;
+
+public:
+  VPScalarPHIRecipe(VPValue *Start, VPValue *BackedgeValue, DebugLoc DL,
+                    StringRef Name)
+      : VPHeaderPHIRecipe(VPDef::VPScalarPHISC, nullptr, Start, DL),
+        Name(Name.str()) {
+    addOperand(BackedgeValue);
+  }
+
+  ~VPScalarPHIRecipe() override = default;
+
+  VPScalarPHIRecipe *clone() override {
+    llvm_unreachable("cloning not implemented yet");
+  }
+
+  VP_CLASSOF_IMPL(VPDef::VPScalarPHISC)
+
+  /// Generate the phi/select nodes.
+  void execute(VPTransformState &State) override;
+
+  /// 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 handling phis that are widened in the vector loop.
 /// In the VPlan native path, all incoming VPValues & VPBasicBlock pairs are
 /// managed in the recipe directly.
@@ -3115,8 +3154,10 @@ class VPCanonicalIVPHIRecipe : public VPHeaderPHIRecipe {
     return D->getVPDefID() == VPDef::VPCanonicalIVPHISC;
   }
 
-  /// Generate the canonical scalar induction phi of the vector loop.
-  void execute(VPTransformState &State) override;
+  void execute(VPTransformState &State) override {
+    llvm_unreachable(
+        "cannot execute this recipe, should be replaced by VPScalarPHIRecipe");
+  }
 
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
   /// Print the recipe.
@@ -3212,9 +3253,10 @@ class VPEVLBasedIVPHIRecipe : public VPHeaderPHIRecipe {
     return D->getVPDefID() == VPDef::VPEVLBasedIVPHISC;
   }
 
-  /// Generate phi for handling IV based on EVL over iterations correctly.
-  /// TODO: investigate if it can share the code with VPCanonicalIVPHIRecipe.
-  void execute(VPTransformState &State) override;
+  void execute(VPTransformState &State) override {
+    llvm_unreachable(
+        "cannot execute this recipe, should be replaced by VPScalarPHIRecipe");
+  }
 
   /// Return the cost of this VPEVLBasedIVPHIRecipe.
   InstructionCost computeCost(ElementCount VF,

llvm/lib/Transforms/Vectorize/VPlanAnalysis.cpp

@@ -251,14 +251,14 @@ 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, VPEVLBasedIVPHIRecipe,
+                VPScalarPHIRecipe>([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,

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -3122,17 +3122,6 @@ InstructionCost VPInterleaveRecipe::computeCost(ElementCount VF,
                                            VectorTy, std::nullopt, CostKind, 0);
 }
 
-void VPCanonicalIVPHIRecipe::execute(VPTransformState &State) {
-  Value *Start = getStartValue()->getLiveInIRValue();
-  PHINode *Phi = PHINode::Create(Start->getType(), 2, "index");
-  Phi->insertBefore(State.CFG.PrevBB->getFirstInsertionPt());
-
-  BasicBlock *VectorPH = State.CFG.getPreheaderBBFor(this);
-  Phi->addIncoming(Start, VectorPH);
-  Phi->setDebugLoc(getDebugLoc());
-  State.set(this, Phi, /*IsScalar*/ true);
-}
-
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
 void VPCanonicalIVPHIRecipe::print(raw_ostream &O, const Twine &Indent,
                                    VPSlotTracker &SlotTracker) const {
@@ -3174,8 +3163,6 @@ void VPWidenPointerInductionRecipe::execute(VPTransformState &State) {
   assert(!onlyScalarsGenerated(State.VF.isScalable()) &&
          "Recipe should have been replaced");
 
-  auto *IVR = getParent()->getPlan()->getCanonicalIV();
-  PHINode *CanonicalIV = cast<PHINode>(State.get(IVR, /*IsScalar*/ true));
   unsigned CurrentPart = getUnrollPart(*this);
 
   // Build a pointer phi
@@ -3185,6 +3172,12 @@ void VPWidenPointerInductionRecipe::execute(VPTransformState &State) {
   BasicBlock *VectorPH = State.CFG.getPreheaderBBFor(this);
   PHINode *NewPointerPhi = nullptr;
   if (CurrentPart == 0) {
+    auto *IVR = cast<VPHeaderPHIRecipe>(&getParent()
+                                             ->getPlan()
+                                             ->getVectorLoopRegion()
+                                             ->getEntryBasicBlock()
+                                             ->front());
+    PHINode *CanonicalIV = cast<PHINode>(State.get(IVR, /*IsScalar*/ true));
     NewPointerPhi = PHINode::Create(ScStValueType, 2, "pointer.phi",
                                     CanonicalIV->getIterator());
     NewPointerPhi->addIncoming(ScalarStartValue, VectorPH);
@@ -3495,20 +3488,30 @@ void VPActiveLaneMaskPHIRecipe::print(raw_ostream &O, const Twine &Indent,
 }
 #endif
 
-void VPEVLBasedIVPHIRecipe::execute(VPTransformState &State) {
+#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
+
+void VPScalarPHIRecipe::execute(VPTransformState &State) {
   BasicBlock *VectorPH = State.CFG.getPreheaderBBFor(this);
   Value *Start = State.get(getOperand(0), VPLane(0));
-  PHINode *Phi = State.Builder.CreatePHI(Start->getType(), 2, "evl.based.iv");
+  PHINode *Phi = State.Builder.CreatePHI(Start->getType(), 2, Name);
   Phi->addIncoming(Start, VectorPH);
   Phi->setDebugLoc(getDebugLoc());
   State.set(this, Phi, /*IsScalar=*/true);
 }
 
 #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 ";
-
+void VPScalarPHIRecipe::print(raw_ostream &O, const Twine &Indent,
+                              VPSlotTracker &SlotTracker) const {
+  O << Indent << "SCALAR-PHI";
   printAsOperand(O, SlotTracker);
   O << " = phi ";
   printOperands(O, SlotTracker);

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -1795,3 +1795,24 @@ void VPlanTransforms::createInterleaveGroups(
       }
   }
 }
+
+void VPlanTransforms::prepareToExecute(VPlan &Plan) {
+  ReversePostOrderTraversal<VPBlockDeepTraversalWrapper<VPBlockBase *>> RPOT(
+      Plan.getVectorLoopRegion());
+  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 VPScalarPHIRecipe(PhiR->getStartValue(), PhiR->getBackedgeValue(),
+                                PhiR->getDebugLoc(), Name);
+      ScalarR->insertBefore(PhiR);
+      PhiR->replaceAllUsesWith(ScalarR);
+      PhiR->eraseFromParent();
+    }
+  }
+}

llvm/lib/Transforms/Vectorize/VPlanTransforms.h

@@ -123,6 +123,9 @@ struct VPlanTransforms {
 
   /// Remove dead recipes from \p Plan.
   static void removeDeadRecipes(VPlan &Plan);
+
+  /// Lower abstract recipes to concrete ones, that can be codegen'd.
+  static void prepareToExecute(VPlan &Plan);
 };
 
 } // namespace llvm

llvm/lib/Transforms/Vectorize/VPlanValue.h

@@ -372,6 +372,7 @@ class VPDef {
     VPFirstOrderRecurrencePHISC,
     VPWidenIntOrFpInductionSC,
     VPWidenPointerInductionSC,
+    VPScalarPHISC,
     VPReductionPHISC,
     // END: SubclassID for recipes that inherit VPHeaderPHIRecipe
     // END: Phi-like recipes