[VPlan] Hook IR blocks into VPlan during skeleton creation (NFC)

As a first step to move towards modeling the full skeleton in VPlan,
start by wrapping IR blocks created during legacy skeleton creation in
VPIRBasicBlocks and hook them into the VPlan. This means the skeleton
CFG is represented in VPlan, just before execute. This allows moving
parts of skeleton creation into recipes in the VPBBs gradually.

Note that this allows retiring some manual DT updates, as this will be
handled automatically during VPlan execution.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -2426,6 +2426,26 @@ InnerLoopVectorizer::getOrCreateVectorTripCount(BasicBlock *InsertBlock) {
   return VectorTripCount;
 }
 
+static void connectScalarPreheaderInVPlan(VPlan &Plan) {
+  VPBlockBase *VectorPH = Plan.getVectorPreheader();
+  VPBlockBase *ScalarPH = Plan.getScalarPreheader();
+  VPBlockBase *PredVPB = VectorPH->getSinglePredecessor();
+  VPBlockUtils::disconnectBlocks(Plan.getEntry(), VectorPH);
+  VPBlockUtils::connectBlocks(PredVPB, ScalarPH);
+  VPBlockUtils::connectBlocks(PredVPB, VectorPH);
+}
+
+static void connectCheckBlockInVPlan(VPlan &Plan, BasicBlock *CheckIRBB) {
+  VPBlockBase *ScalarPH = Plan.getScalarPreheader();
+  VPBlockBase *VectorPH = Plan.getVectorPreheader();
+  VPBlockBase *PredVPB = VectorPH->getSinglePredecessor();
+  VPBlockUtils::disconnectBlocks(PredVPB, VectorPH);
+  VPIRBasicBlock *CheckVPIRBB = VPIRBasicBlock::fromBasicBlock(CheckIRBB);
+  VPBlockUtils::connectBlocks(PredVPB, CheckVPIRBB);
+  VPBlockUtils::connectBlocks(CheckVPIRBB, ScalarPH);
+  VPBlockUtils::connectBlocks(CheckVPIRBB, VectorPH);
+}
+
 void InnerLoopVectorizer::emitIterationCountCheck(BasicBlock *Bypass) {
   Value *Count = getTripCount();
   // Reuse existing vector loop preheader for TC checks.
@@ -2511,13 +2531,14 @@ void InnerLoopVectorizer::emitIterationCountCheck(BasicBlock *Bypass) {
          "TC check is expected to dominate Bypass");
 
   // Update dominator for Bypass & LoopExit (if needed).
-  DT->changeImmediateDominator(Bypass, TCCheckBlock);
   BranchInst &BI =
       *BranchInst::Create(Bypass, LoopVectorPreHeader, CheckMinIters);
   if (hasBranchWeightMD(*OrigLoop->getLoopLatch()->getTerminator()))
     setBranchWeights(BI, MinItersBypassWeights, /*IsExpected=*/false);
   ReplaceInstWithInst(TCCheckBlock->getTerminator(), &BI);
   LoopBypassBlocks.push_back(TCCheckBlock);
+
+  connectScalarPreheaderInVPlan(Plan);
 }
 
 BasicBlock *InnerLoopVectorizer::emitSCEVChecks(BasicBlock *Bypass) {
@@ -2534,6 +2555,8 @@ BasicBlock *InnerLoopVectorizer::emitSCEVChecks(BasicBlock *Bypass) {
          "Should already be a bypass block due to iteration count check");
   LoopBypassBlocks.push_back(SCEVCheckBlock);
   AddedSafetyChecks = true;
+
+  connectCheckBlockInVPlan(Plan, SCEVCheckBlock);
   return SCEVCheckBlock;
 }
 
@@ -2570,6 +2593,7 @@ BasicBlock *InnerLoopVectorizer::emitMemRuntimeChecks(BasicBlock *Bypass) {
 
   AddedSafetyChecks = true;
 
+  connectCheckBlockInVPlan(Plan, MemCheckBlock);
   return MemCheckBlock;
 }
 
@@ -7648,10 +7672,10 @@ DenseMap<const SCEV *, Value *> LoopVectorizationPlanner::executePlan(
 
   // 0. Generate SCEV-dependent code into the preheader, including TripCount,
   // before making any changes to the CFG.
-  if (!BestVPlan.getPreheader()->empty()) {
+  if (!BestVPlan.getEntry()->empty()) {
     State.CFG.PrevBB = OrigLoop->getLoopPreheader();
     State.Builder.SetInsertPoint(OrigLoop->getLoopPreheader()->getTerminator());
-    BestVPlan.getPreheader()->execute(&State);
+    BestVPlan.getEntry()->execute(&State);
   }
   if (!ILV.getTripCount())
     ILV.setTripCount(State.get(BestVPlan.getTripCount(), VPLane(0)));
@@ -7859,8 +7883,6 @@ EpilogueVectorizerMainLoop::emitIterationCountCheck(BasicBlock *Bypass,
                                  DT->getNode(Bypass)->getIDom()) &&
            "TC check is expected to dominate Bypass");
 
-    // Update dominator for Bypass.
-    DT->changeImmediateDominator(Bypass, TCCheckBlock);
     LoopBypassBlocks.push_back(TCCheckBlock);
 
     // Save the trip count so we don't have to regenerate it in the
@@ -7875,6 +7897,12 @@ EpilogueVectorizerMainLoop::emitIterationCountCheck(BasicBlock *Bypass,
     setBranchWeights(BI, MinItersBypassWeights, /*IsExpected=*/false);
   ReplaceInstWithInst(TCCheckBlock->getTerminator(), &BI);
 
+  VPBlockBase *VectorPH = Plan.getVectorPreheader();
+  VPBlockBase *PredVPB = VectorPH->getSinglePredecessor();
+  if (PredVPB->getNumSuccessors() == 1)
+    connectScalarPreheaderInVPlan(Plan);
+  else
+    connectCheckBlockInVPlan(Plan, TCCheckBlock);
   return TCCheckBlock;
 }
 
@@ -7905,32 +7933,19 @@ EpilogueVectorizerEpilogueLoop::createEpilogueVectorizedLoopSkeleton(
   EPI.MainLoopIterationCountCheck->getTerminator()->replaceUsesOfWith(
       VecEpilogueIterationCountCheck, LoopVectorPreHeader);
 
-  DT->changeImmediateDominator(LoopVectorPreHeader,
-                               EPI.MainLoopIterationCountCheck);
-
   EPI.EpilogueIterationCountCheck->getTerminator()->replaceUsesOfWith(
       VecEpilogueIterationCountCheck, LoopScalarPreHeader);
 
   if (EPI.SCEVSafetyCheck)
     EPI.SCEVSafetyCheck->getTerminator()->replaceUsesOfWith(
         VecEpilogueIterationCountCheck, LoopScalarPreHeader);
-  if (EPI.MemSafetyCheck)
+  if (EPI.MemSafetyCheck) {
     EPI.MemSafetyCheck->getTerminator()->replaceUsesOfWith(
         VecEpilogueIterationCountCheck, LoopScalarPreHeader);
-
-  DT->changeImmediateDominator(
-      VecEpilogueIterationCountCheck,
-      VecEpilogueIterationCountCheck->getSinglePredecessor());
+  }
 
   DT->changeImmediateDominator(LoopScalarPreHeader,
                                EPI.EpilogueIterationCountCheck);
-  if (!Cost->requiresScalarEpilogue(EPI.EpilogueVF.isVector()))
-    // If there is an epilogue which must run, there's no edge from the
-    // middle block to exit blocks  and thus no need to update the immediate
-    // dominator of the exit blocks.
-    DT->changeImmediateDominator(LoopExitBlock,
-                                 EPI.EpilogueIterationCountCheck);
-
   // Keep track of bypass blocks, as they feed start values to the induction and
   // reduction phis in the scalar loop preheader.
   if (EPI.SCEVSafetyCheck)
@@ -8033,6 +8048,20 @@ EpilogueVectorizerEpilogueLoop::emitMinimumVectorEpilogueIterCountCheck(
   }
   ReplaceInstWithInst(Insert->getTerminator(), &BI);
   LoopBypassBlocks.push_back(Insert);
+
+  // A new entry block has been created for the epilogue VPlan. Hook it in.
+  VPIRBasicBlock *NewEntry = VPIRBasicBlock::fromBasicBlock(Insert);
+  VPBasicBlock *OldEntry = Plan.getEntry();
+  VPBlockUtils::reassociateBlocks(OldEntry, NewEntry);
+  Plan.setEntry(NewEntry);
+  for (auto &R : make_early_inc_range(*NewEntry)) {
+    auto *VPIR = dyn_cast<VPIRInstruction>(&R);
+    if (!VPIR || !isa<PHINode>(VPIR->getInstruction()))
+      break;
+    VPIR->eraseFromParent();
+  }
+
+  connectScalarPreheaderInVPlan(Plan);
   return Insert;
 }
 
@@ -10256,7 +10285,7 @@ bool LoopVectorizePass::processLoop(Loop *L) {
         // should be removed once induction resume value creation is done
         // directly in VPlan.
         EpilogILV.setTripCount(MainILV.getTripCount());
-        for (auto &R : make_early_inc_range(*BestEpiPlan.getPreheader())) {
+        for (auto &R : make_early_inc_range(*BestEpiPlan.getEntry())) {
           auto *ExpandR = dyn_cast<VPExpandSCEVRecipe>(&R);
           if (!ExpandR)
             continue;
@@ -10316,8 +10345,6 @@ bool LoopVectorizePass::processLoop(Loop *L) {
           cast<VPHeaderPHIRecipe>(&R)->setStartValue(StartVal);
         }
 
-        assert(DT->verify(DominatorTree::VerificationLevel::Fast) &&
-               "DT not preserved correctly");
         LVP.executePlan(EPI.EpilogueVF, EPI.EpilogueUF, BestEpiPlan, EpilogILV,
                         DT, true, &ExpandedSCEVs);
         ++LoopsEpilogueVectorized;
@@ -10345,6 +10372,9 @@ bool LoopVectorizePass::processLoop(Loop *L) {
       checkMixedPrecision(L, ORE);
   }
 
+  assert(DT->verify(DominatorTree::VerificationLevel::Fast) &&
+         "DT not preserved correctly");
+
   std::optional<MDNode *> RemainderLoopID =
       makeFollowupLoopID(OrigLoopID, {LLVMLoopVectorizeFollowupAll,
                                       LLVMLoopVectorizeFollowupEpilogue});

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -170,9 +170,8 @@ VPBasicBlock *VPBlockBase::getEntryBasicBlock() {
 }
 
 void VPBlockBase::setPlan(VPlan *ParentPlan) {
-  assert(
-      (ParentPlan->getEntry() == this || ParentPlan->getPreheader() == this) &&
-      "Can only set plan on its entry or preheader block.");
+  assert(ParentPlan->getEntry() == this &&
+         "Can only set plan on its entry or preheader block.");
   Plan = ParentPlan;
 }
 
@@ -463,7 +462,6 @@ void VPIRBasicBlock::execute(VPTransformState *State) {
         (getNumSuccessors() == 0 || isa<BranchInst>(IRBB->getTerminator())) &&
         "other blocks must be terminated by a branch");
   }
-
   for (VPBlockBase *PredVPBlock : getHierarchicalPredecessors()) {
     VPBasicBlock *PredVPBB = PredVPBlock->getExitingBasicBlock();
     BasicBlock *PredBB = State->CFG.VPBB2IRBB[PredVPBB];
@@ -851,9 +849,6 @@ VPlan::~VPlan() {
       Block->dropAllReferences(&DummyValue);
 
     VPBlockBase::deleteCFG(Entry);
-
-    Preheader->dropAllReferences(&DummyValue);
-    delete Preheader;
   }
   for (VPValue *VPV : VPLiveInsToFree)
     delete VPV;
@@ -876,9 +871,10 @@ VPlanPtr VPlan::createInitialVPlan(Type *InductionTy,
   VPIRBasicBlock *Entry =
       VPIRBasicBlock::fromBasicBlock(TheLoop->getLoopPreheader());
   VPBasicBlock *VecPreheader = new VPBasicBlock("vector.ph");
+  VPBlockUtils::connectBlocks(Entry, VecPreheader);
   VPIRBasicBlock *ScalarHeader =
       VPIRBasicBlock::fromBasicBlock(TheLoop->getHeader());
-  auto Plan = std::make_unique<VPlan>(Entry, VecPreheader, ScalarHeader);
+  auto Plan = std::make_unique<VPlan>(Entry, ScalarHeader);
 
   // Create SCEV and VPValue for the trip count.
 
@@ -1021,8 +1017,9 @@ void VPlan::execute(VPTransformState *State) {
   BasicBlock *VectorPreHeader = State->CFG.PrevBB;
   State->Builder.SetInsertPoint(VectorPreHeader->getTerminator());
 
-  // Disconnect VectorPreHeader from ExitBB in both the CFG and DT.
-  cast<BranchInst>(VectorPreHeader->getTerminator())->setSuccessor(0, nullptr);
+  replaceVPBBWithIRVPBB(
+      cast<VPBasicBlock>(getVectorLoopRegion()->getSinglePredecessor()),
+      VectorPreHeader);
   State->CFG.DTU.applyUpdates(
       {{DominatorTree::Delete, VectorPreHeader, State->CFG.ExitBB}});
 
@@ -1049,8 +1046,10 @@ void VPlan::execute(VPTransformState *State) {
   State->CFG.DTU.applyUpdates(
       {{DominatorTree::Delete, ScalarPh, ScalarPh->getSingleSuccessor()}});
 
+  ReversePostOrderTraversal<VPBlockShallowTraversalWrapper<VPBlockBase *>> RPOT(
+      Entry);
   // Generate code in the loop pre-header and body.
-  for (VPBlockBase *Block : vp_depth_first_shallow(Entry))
+  for (VPBlockBase *Block : make_range(RPOT.begin(), RPOT.end()))
     Block->execute(State);
 
   VPBasicBlock *LatchVPBB = getVectorLoopRegion()->getExitingBasicBlock();
@@ -1101,9 +1100,6 @@ void VPlan::execute(VPTransformState *State) {
   }
 
   State->CFG.DTU.flush();
-  assert(State->CFG.DTU.getDomTree().verify(
-             DominatorTree::VerificationLevel::Fast) &&
-         "DT not preserved correctly");
 }
 
 InstructionCost VPlan::cost(ElementCount VF, VPCostContext &Ctx) {
@@ -1156,12 +1152,10 @@ void VPlan::print(raw_ostream &O) const {
 
   printLiveIns(O);
 
-  if (!getPreheader()->empty()) {
-    O << "\n";
-    getPreheader()->print(O, "", SlotTracker);
-  }
+  ReversePostOrderTraversal<VPBlockShallowTraversalWrapper<const VPBlockBase *>>
+      RPOT(getEntry());
 
-  for (const VPBlockBase *Block : vp_depth_first_shallow(getEntry())) {
+  for (const VPBlockBase *Block : RPOT) {
     O << '\n';
     Block->print(O, "", SlotTracker);
   }
@@ -1192,6 +1186,20 @@ std::string VPlan::getName() const {
   return Out;
 }
 
+VPRegionBlock *VPlan::getVectorLoopRegion() {
+  for (VPBlockBase *B : vp_depth_first_shallow(getEntry()))
+    if (auto *R = dyn_cast<VPRegionBlock>(B))
+      return R;
+  return nullptr;
+}
+
+const VPRegionBlock *VPlan::getVectorLoopRegion() const {
+  for (const VPBlockBase *B : vp_depth_first_shallow(getEntry()))
+    if (auto *R = dyn_cast<VPRegionBlock>(B))
+      return R;
+  return nullptr;
+}
+
 LLVM_DUMP_METHOD
 void VPlan::printDOT(raw_ostream &O) const {
   VPlanPrinter Printer(O, *this);
@@ -1242,7 +1250,6 @@ static void remapOperands(VPBlockBase *Entry, VPBlockBase *NewEntry,
 
 VPlan *VPlan::duplicate() {
   // Clone blocks.
-  VPBasicBlock *NewPreheader = Preheader->clone();
   const auto &[NewEntry, __] = cloneFrom(Entry);
 
   BasicBlock *ScalarHeaderIRBB = getScalarHeader()->getIRBasicBlock();
@@ -1252,8 +1259,7 @@ VPlan *VPlan::duplicate() {
         return VPIRBB && VPIRBB->getIRBasicBlock() == ScalarHeaderIRBB;
       }));
   // Create VPlan, clone live-ins and remap operands in the cloned blocks.
-  auto *NewPlan =
-      new VPlan(NewPreheader, cast<VPBasicBlock>(NewEntry), NewScalarHeader);
+  auto *NewPlan = new VPlan(cast<VPBasicBlock>(NewEntry), NewScalarHeader);
   DenseMap<VPValue *, VPValue *> Old2NewVPValues;
   for (VPValue *OldLiveIn : VPLiveInsToFree) {
     Old2NewVPValues[OldLiveIn] =
@@ -1273,7 +1279,6 @@ VPlan *VPlan::duplicate() {
   // else NewTripCount will be created and inserted into Old2NewVPValues when
   // TripCount is cloned. In any case NewPlan->TripCount is updated below.
 
-  remapOperands(Preheader, NewPreheader, Old2NewVPValues);
   remapOperands(Entry, NewEntry, Old2NewVPValues);
 
   // Initialize remaining fields of cloned VPlan.
@@ -1287,6 +1292,19 @@ VPlan *VPlan::duplicate() {
   return NewPlan;
 }
 
+VPBasicBlock *VPlan::getScalarPreheader() {
+  auto *MiddleVPBB =
+      cast<VPBasicBlock>(getVectorLoopRegion()->getSingleSuccessor());
+  if (MiddleVPBB->getNumSuccessors() == 2) {
+    // Order is strict: first is the exit block, second is the scalar preheader.
+    return cast<VPBasicBlock>(MiddleVPBB->getSuccessors()[1]);
+  }
+  if (auto *IRVPBB = dyn_cast<VPBasicBlock>(MiddleVPBB->getSingleSuccessor()))
+    return IRVPBB;
+
+  return nullptr;
+}
+
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
 
 Twine VPlanPrinter::getUID(const VPBlockBase *Block) {
@@ -1325,8 +1343,6 @@ void VPlanPrinter::dump() {
   OS << "edge [fontname=Courier, fontsize=30]\n";
   OS << "compound=true\n";
 
-  dumpBlock(Plan.getPreheader());
-
   for (const VPBlockBase *Block : vp_depth_first_shallow(Plan.getEntry()))
     dumpBlock(Block);
 
@@ -1587,7 +1603,6 @@ void VPSlotTracker::assignNames(const VPlan &Plan) {
     assignName(Plan.BackedgeTakenCount);
   for (VPValue *LI : Plan.VPLiveInsToFree)
     assignName(LI);
-  assignNames(Plan.getPreheader());
 
   ReversePostOrderTraversal<VPBlockDeepTraversalWrapper<const VPBlockBase *>>
       RPOT(VPBlockDeepTraversalWrapper<const VPBlockBase *>(Plan.getEntry()));