[VPlan] Add canonical IV during construction (NFC).

This addresses an existing TODO and simply moves the current code to add
canonical IV recipes to the initial skeleton construction, at the same
place where the corresponding region will be introduced.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -9163,31 +9163,6 @@ void LoopVectorizationPlanner::buildVPlansWithVPRecipes(ElementCount MinVF,
   }
 }
 
-// Add the necessary canonical IV and branch recipes required to control the
-// loop.
-static void addCanonicalIVRecipes(VPlan &Plan, Type *IdxTy, bool HasNUW,
-                                  DebugLoc DL) {
-  Value *StartIdx = ConstantInt::get(IdxTy, 0);
-  auto *StartV = Plan.getOrAddLiveIn(StartIdx);
-
-  // Add a VPCanonicalIVPHIRecipe starting at 0 to the header.
-  auto *CanonicalIVPHI = new VPCanonicalIVPHIRecipe(StartV, DL);
-  VPRegionBlock *TopRegion = Plan.getVectorLoopRegion();
-  VPBasicBlock *Header = TopRegion->getEntryBasicBlock();
-  Header->insert(CanonicalIVPHI, Header->begin());
-
-  VPBuilder Builder(TopRegion->getExitingBasicBlock());
-  // Add a VPInstruction to increment the scalar canonical IV by VF * UF.
-  auto *CanonicalIVIncrement = Builder.createOverflowingOp(
-      Instruction::Add, {CanonicalIVPHI, &Plan.getVFxUF()}, {HasNUW, false}, DL,
-      "index.next");
-  CanonicalIVPHI->addOperand(CanonicalIVIncrement);
-
-  // Add the BranchOnCount VPInstruction to the latch.
-  Builder.createNaryOp(VPInstruction::BranchOnCount,
-                       {CanonicalIVIncrement, &Plan.getVectorTripCount()}, DL);
-}
-
 /// Create and return a ResumePhi for \p WideIV, unless it is truncated. If the
 /// induction recipe is not canonical, creates a VPDerivedIVRecipe to compute
 /// the end value of the induction.
@@ -9459,7 +9434,8 @@ LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(VFRange &Range) {
   auto Plan = VPlanTransforms::buildPlainCFG(OrigLoop, *LI, VPB2IRBB);
   VPlanTransforms::prepareForVectorization(
       *Plan, Legal->getWidestInductionType(), PSE, RequiresScalarEpilogueCheck,
-      CM.foldTailByMasking(), OrigLoop);
+      CM.foldTailByMasking(), OrigLoop,
+      getDebugLocFromInstOrOperands(Legal->getPrimaryInduction()));
   VPlanTransforms::createLoopRegions(*Plan);
 
   // Don't use getDecisionAndClampRange here, because we don't know the UF
@@ -9470,14 +9446,22 @@ LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(VFRange &Range) {
   for (ElementCount VF : Range)
     IVUpdateMayOverflow |= !isIndvarOverflowCheckKnownFalse(&CM, VF);
 
-  DebugLoc DL = getDebugLocFromInstOrOperands(Legal->getPrimaryInduction());
   TailFoldingStyle Style = CM.getTailFoldingStyle(IVUpdateMayOverflow);
   // Use NUW for the induction increment if we proved that it won't overflow in
   // the vector loop or when not folding the tail. In the later case, we know
   // that the canonical induction increment will not overflow as the vector trip
   // count is >= increment and a multiple of the increment.
   bool HasNUW = !IVUpdateMayOverflow || Style == TailFoldingStyle::None;
-  addCanonicalIVRecipes(*Plan, Legal->getWidestInductionType(), HasNUW, DL);
+  if (!HasNUW) {
+    auto *IVInc = Plan->getVectorLoopRegion()
+                      ->getExitingBasicBlock()
+                      ->getTerminator()
+                      ->getOperand(0);
+    assert(match(IVInc, m_VPInstruction<Instruction::Add>(
+                            m_Specific(Plan->getCanonicalIV()), m_VPValue())) &&
+           "Did not find the canonical IV increment");
+    cast<VPRecipeWithIRFlags>(IVInc)->dropPoisonGeneratingFlags();
+  }
 
   VPRecipeBuilder RecipeBuilder(*Plan, OrigLoop, TLI, &TTI, Legal, CM, PSE,
                                 Builder);
@@ -9751,19 +9735,13 @@ VPlanPtr LoopVectorizationPlanner::tryToBuildVPlan(VFRange &Range) {
   DenseMap<VPBlockBase *, BasicBlock *> VPB2IRBB;
   auto Plan = VPlanTransforms::buildPlainCFG(OrigLoop, *LI, VPB2IRBB);
   VPlanTransforms::prepareForVectorization(
-      *Plan, Legal->getWidestInductionType(), PSE, true, false, OrigLoop);
+      *Plan, Legal->getWidestInductionType(), PSE, true, false, OrigLoop,
+      getDebugLocFromInstOrOperands(Legal->getPrimaryInduction()));
   VPlanTransforms::createLoopRegions(*Plan);
 
   for (ElementCount VF : Range)
     Plan->addVF(VF);
 
-  // Tail folding is not supported for outer loops, so the induction increment
-  // is guaranteed to not wrap.
-  bool HasNUW = true;
-  addCanonicalIVRecipes(
-      *Plan, Legal->getWidestInductionType(), HasNUW,
-      getDebugLocFromInstOrOperands(Legal->getPrimaryInduction()));
-
   if (!VPlanTransforms::tryToConvertVPInstructionsToVPRecipes(
           Plan,
           [this](PHINode *P) {

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -736,6 +736,11 @@ class VPRecipeWithIRFlags : public VPSingleDefRecipe {
     return R && classof(R);
   }
 
+  static inline bool classof(const VPValue *V) {
+    auto *R = dyn_cast_or_null<VPRecipeBase>(V->getDefiningRecipe());
+    return R && classof(R);
+  }
+
   /// Drop all poison-generating flags.
   void dropPoisonGeneratingFlags() {
     // NOTE: This needs to be kept in-sync with

llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp

@@ -15,6 +15,7 @@
 #include "VPlan.h"
 #include "VPlanCFG.h"
 #include "VPlanDominatorTree.h"
+#include "VPlanPatternMatch.h"
 #include "VPlanTransforms.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/LoopIterator.h"
@@ -461,10 +462,44 @@ static void createLoopRegion(VPlan &Plan, VPBlockBase *HeaderVPB) {
     VPBlockUtils::connectBlocks(R, Succ);
 }
 
+// Add the necessary canonical IV and branch recipes required to control the
+// loop.
+static void addCanonicalIVRecipes(VPlan &Plan, VPBasicBlock *HeaderVPBB,
+                                  VPBasicBlock *LatchVPBB, Type *IdxTy,
+                                  DebugLoc DL) {
+  using namespace VPlanPatternMatch;
+  Value *StartIdx = ConstantInt::get(IdxTy, 0);
+  auto *StartV = Plan.getOrAddLiveIn(StartIdx);
+
+  // Add a VPCanonicalIVPHIRecipe starting at 0 to the header.
+  auto *CanonicalIVPHI = new VPCanonicalIVPHIRecipe(StartV, DL);
+  HeaderVPBB->insert(CanonicalIVPHI, HeaderVPBB->begin());
+
+  // We are about to replace the branch to exit the region. Remove the original
+  // BranchOnCond, if there is any.
+  if (!LatchVPBB->empty() &&
+      match(&LatchVPBB->back(), m_BranchOnCond(m_VPValue())))
+    LatchVPBB->getTerminator()->eraseFromParent();
+
+  VPBuilder Builder(LatchVPBB);
+  // Add a VPInstruction to increment the scalar canonical IV by VF * UF.
+  // Initially the induction increment is guaranteed to not wrap, but that may
+  // change later, e.g. when tail-folding, when the flags need to be dropped.
+  auto *CanonicalIVIncrement = Builder.createOverflowingOp(
+      Instruction::Add, {CanonicalIVPHI, &Plan.getVFxUF()}, {true, false}, DL,
+      "index.next");
+  CanonicalIVPHI->addOperand(CanonicalIVIncrement);
+
+  // Add the BranchOnCount VPInstruction to the latch.
+  Builder.createNaryOp(VPInstruction::BranchOnCount,
+                       {CanonicalIVIncrement, &Plan.getVectorTripCount()}, DL);
+}
+
 void VPlanTransforms::prepareForVectorization(VPlan &Plan, Type *InductionTy,
                                               PredicatedScalarEvolution &PSE,
                                               bool RequiresScalarEpilogueCheck,
-                                              bool TailFolded, Loop *TheLoop) {
+                                              bool TailFolded, Loop *TheLoop,
+                                              DebugLoc IVDL) {
   VPDominatorTree VPDT;
   VPDT.recalculate(Plan);
 
@@ -479,6 +514,9 @@ void VPlanTransforms::prepareForVectorization(VPlan &Plan, Type *InductionTy,
   VPBlockUtils::connectBlocks(LatchVPB, MiddleVPBB);
   LatchVPB->swapSuccessors();
 
+  addCanonicalIVRecipes(Plan, cast<VPBasicBlock>(HeaderVPB),
+                        cast<VPBasicBlock>(LatchVPB), InductionTy, IVDL);
+
   // Create SCEV and VPValue for the trip count.
   // We use the symbolic max backedge-taken-count, which works also when
   // vectorizing loops with uncountable early exits.

llvm/lib/Transforms/Vectorize/VPlanTransforms.h

@@ -68,7 +68,8 @@ struct VPlanTransforms {
   static void prepareForVectorization(VPlan &Plan, Type *InductionTy,
                                       PredicatedScalarEvolution &PSE,
                                       bool RequiresScalarEpilogueCheck,
-                                      bool TailFolded, Loop *TheLoop);
+                                      bool TailFolded, Loop *TheLoop,
+                                      DebugLoc IVDL);
 
   /// Replace loops in \p Plan's flat CFG with VPRegionBlocks, turning \p Plan's
   /// flat CFG into a hierarchical CFG.

llvm/unittests/Transforms/Vectorize/VPlanTestBase.h

@@ -73,7 +73,7 @@ class VPlanTestIRBase : public testing::Test {
     DenseMap<VPBlockBase *, BasicBlock *> VPB2IRBB;
     auto Plan = VPlanTransforms::buildPlainCFG(L, *LI, VPB2IRBB);
     VPlanTransforms::prepareForVectorization(*Plan, IntegerType::get(*Ctx, 64),
-                                             PSE, true, false, L);
+                                             PSE, true, false, L, {});
     VPlanTransforms::createLoopRegions(*Plan);
     return Plan;
   }