[VPlan] Connect Entry to scalar preheader during initial construction.

Update initial construction to connect the Plan's entry to the scalar
preheader during initial construction. This moves a small part of the
 skeleton creation out of ILV and will also enable replacing
 VPInstruction::ResumePhi with regular VPPhi recipes.

Resume phis need 2 incoming values to start with, the second being the
bypass value from the scalar ph (and used to replicate the incoming
value for other bypass blocks). Adding the extra edge ensures we
incoming values for resume phis match the incoming blocks.

Still needs various VPlan printing tests to be updated.
vplan-printing.ll shows the changes.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -490,7 +490,7 @@ class InnerLoopVectorizer {
         MinProfitableTripCount(MinProfitableTripCount), UF(UnrollFactor),
         Builder(PSE.getSE()->getContext()), Cost(CM), BFI(BFI), PSI(PSI),
         RTChecks(RTChecks), Plan(Plan),
-        VectorPHVPB(Plan.getEntry()->getSingleSuccessor()) {}
+        VectorPHVPB(Plan.getEntry()->getSuccessors()[1]) {}
 
   virtual ~InnerLoopVectorizer() = default;
 
@@ -2365,14 +2365,11 @@ InnerLoopVectorizer::getOrCreateVectorTripCount(BasicBlock *InsertBlock) {
 void InnerLoopVectorizer::introduceCheckBlockInVPlan(BasicBlock *CheckIRBB) {
   VPBlockBase *ScalarPH = Plan.getScalarPreheader();
   VPBlockBase *PreVectorPH = VectorPHVPB->getSinglePredecessor();
-  if (PreVectorPH->getNumSuccessors() != 1) {
-    assert(PreVectorPH->getNumSuccessors() == 2 && "Expected 2 successors");
-    assert(PreVectorPH->getSuccessors()[0] == ScalarPH &&
-           "Unexpected successor");
-    VPIRBasicBlock *CheckVPIRBB = Plan.createVPIRBasicBlock(CheckIRBB);
-    VPBlockUtils::insertOnEdge(PreVectorPH, VectorPHVPB, CheckVPIRBB);
-    PreVectorPH = CheckVPIRBB;
-  }
+  assert(PreVectorPH->getNumSuccessors() == 2 && "Expected 2 successors");
+  assert(PreVectorPH->getSuccessors()[0] == ScalarPH && "Unexpected successor");
+  VPIRBasicBlock *CheckVPIRBB = Plan.createVPIRBasicBlock(CheckIRBB);
+  VPBlockUtils::insertOnEdge(PreVectorPH, VectorPHVPB, CheckVPIRBB);
+  PreVectorPH = CheckVPIRBB;
   VPBlockUtils::connectBlocks(PreVectorPH, ScalarPH);
   PreVectorPH->swapSuccessors();
 
@@ -2463,9 +2460,6 @@ void InnerLoopVectorizer::emitIterationCountCheck(BasicBlock *Bypass) {
     setBranchWeights(BI, MinItersBypassWeights, /*IsExpected=*/false);
   ReplaceInstWithInst(TCCheckBlock->getTerminator(), &BI);
   LoopBypassBlocks.push_back(TCCheckBlock);
-
-  // TODO: Wrap LoopVectorPreHeader in VPIRBasicBlock here.
-  introduceCheckBlockInVPlan(TCCheckBlock);
 }
 
 BasicBlock *InnerLoopVectorizer::emitSCEVChecks(BasicBlock *Bypass) {
@@ -7837,7 +7831,7 @@ DenseMap<const SCEV *, Value *> LoopVectorizationPlanner::executePlan(
 
   // 1. Set up the skeleton for vectorization, including vector pre-header and
   // middle block. The vector loop is created during VPlan execution.
-  VPBasicBlock *VectorPH = cast<VPBasicBlock>(Entry->getSingleSuccessor());
+  VPBasicBlock *VectorPH = cast<VPBasicBlock>(Entry->getSuccessors()[1]);
   State.CFG.PrevBB = ILV.createVectorizedLoopSkeleton();
   if (VectorizingEpilogue)
     VPlanTransforms::removeDeadRecipes(BestVPlan);
@@ -8070,7 +8064,8 @@ EpilogueVectorizerMainLoop::emitIterationCountCheck(BasicBlock *Bypass,
     setBranchWeights(BI, MinItersBypassWeights, /*IsExpected=*/false);
   ReplaceInstWithInst(TCCheckBlock->getTerminator(), &BI);
 
-  introduceCheckBlockInVPlan(TCCheckBlock);
+  if (!ForEpilogue)
+    introduceCheckBlockInVPlan(TCCheckBlock);
   return TCCheckBlock;
 }
 
@@ -8200,7 +8195,6 @@ EpilogueVectorizerEpilogueLoop::emitMinimumVectorEpilogueIterCountCheck(
   Plan.setEntry(NewEntry);
   // OldEntry is now dead and will be cleaned up when the plan gets destroyed.
 
-  introduceCheckBlockInVPlan(Insert);
   return Insert;
 }
 
@@ -9160,7 +9154,7 @@ static void addScalarResumePhis(VPRecipeBuilder &Builder, VPlan &Plan,
                                 DenseMap<VPValue *, VPValue *> &IVEndValues) {
   VPTypeAnalysis TypeInfo(Plan.getCanonicalIV()->getScalarType());
   auto *ScalarPH = Plan.getScalarPreheader();
-  auto *MiddleVPBB = cast<VPBasicBlock>(ScalarPH->getSinglePredecessor());
+  auto *MiddleVPBB = cast<VPBasicBlock>(ScalarPH->getPredecessors()[0]);
   VPRegionBlock *VectorRegion = Plan.getVectorLoopRegion();
   VPBuilder VectorPHBuilder(
       cast<VPBasicBlock>(VectorRegion->getSinglePredecessor()));

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -4246,7 +4246,8 @@ class VPlan {
   /// that this relies on unneeded branches to the scalar tail loop being
   /// removed.
   bool hasScalarTail() const {
-    return getScalarPreheader()->getNumPredecessors() != 0;
+    return getScalarPreheader()->getNumPredecessors() != 0 &&
+           getScalarPreheader()->getSinglePredecessor() != getEntry();
   }
 };
 

llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp

@@ -546,6 +546,9 @@ void VPlanTransforms::prepareForVectorization(
     if (auto *LatchExitVPB = MiddleVPBB->getSingleSuccessor())
       VPBlockUtils::disconnectBlocks(MiddleVPBB, LatchExitVPB);
     VPBlockUtils::connectBlocks(MiddleVPBB, ScalarPH);
+    VPBlockUtils::connectBlocks(Plan.getEntry(), ScalarPH);
+    Plan.getEntry()->swapSuccessors();
+
     // The exit blocks are unreachable, remove their recipes to make sure no
     // users remain that may pessimize transforms.
     for (auto *EB : Plan.getExitBlocks()) {
@@ -558,6 +561,9 @@ void VPlanTransforms::prepareForVectorization(
   // The connection order corresponds to the operands of the conditional branch,
   // with the middle block already connected to the exit block.
   VPBlockUtils::connectBlocks(MiddleVPBB, ScalarPH);
+  // Also connect the entry block to the scalar preheader.
+  VPBlockUtils::connectBlocks(Plan.getEntry(), ScalarPH);
+  Plan.getEntry()->swapSuccessors();
 
   auto *ScalarLatchTerm = TheLoop->getLoopLatch()->getTerminator();
   // Here we use the same DebugLoc as the scalar loop latch terminator instead

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -1732,7 +1732,7 @@ static void removeBranchOnCondTrue(VPlan &Plan) {
   using namespace llvm::VPlanPatternMatch;
   for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
            vp_depth_first_shallow(Plan.getEntry()))) {
-    if (VPBB->getNumSuccessors() != 2 ||
+    if (VPBB->getNumSuccessors() != 2 || isa<VPIRBasicBlock>(VPBB) ||
         !match(&VPBB->back(), m_BranchOnCond(m_True())))
       continue;