diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -542,12 +542,16 @@ llvm::SplitAllCriticalEdges(Function &F,
   return NumBroken;
 }
 
-BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt,
-                             DominatorTree *DT, LoopInfo *LI,
-                             MemorySSAUpdater *MSSAU, const Twine &BBName,
-                             bool Before) {
-  if (Before)
-    return splitBlockBefore(Old, SplitPt, DT, LI, MSSAU, BBName);
+static BasicBlock *SplitBlockImpl(BasicBlock *Old, Instruction *SplitPt,
+                                  DomTreeUpdater *DTU, DominatorTree *DT,
+                                  LoopInfo *LI, MemorySSAUpdater *MSSAU,
+                                  const Twine &BBName, bool Before) {
+  if (Before) {
+    DomTreeUpdater LocalDTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
+    return splitBlockBefore(Old, SplitPt,
+                            DTU ? DTU : (DT ? &LocalDTU : nullptr), LI, MSSAU,
+                            BBName);
+  }
   BasicBlock::iterator SplitIt = SplitPt->getIterator();
   while (isa<PHINode>(SplitIt) || SplitIt->isEHPad())
     ++SplitIt;
@@ -561,7 +565,20 @@ BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt,
     if (Loop *L = LI->getLoopFor(Old))
       L->addBasicBlockToLoop(New, *LI);
 
-  if (DT)
+  if (DTU) {
+    SmallVector<DominatorTree::UpdateType, 8> Updates;
+    // Old dominates New. New node dominates all other nodes dominated by Old.
+    SmallSetVector<BasicBlock *, 8> UniqueSuccessorsOfOld(succ_begin(New),
+                                                          succ_end(New));
+    Updates.push_back({DominatorTree::Insert, Old, New});
+    Updates.reserve(Updates.size() + 2 * UniqueSuccessorsOfOld.size());
+    for (BasicBlock *UniqueSuccessorOfOld : UniqueSuccessorsOfOld) {
+      Updates.push_back({DominatorTree::Insert, New, UniqueSuccessorOfOld});
+      Updates.push_back({DominatorTree::Delete, Old, UniqueSuccessorOfOld});
+    }
+
+    DTU->applyUpdates(Updates);
+  } else if (DT)
     // Old dominates New. New node dominates all other nodes dominated by Old.
     if (DomTreeNode *OldNode = DT->getNode(Old)) {
       std::vector<DomTreeNode *> Children(OldNode->begin(), OldNode->end());
@@ -579,8 +596,23 @@ BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt,
   return New;
 }
 
+BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt,
+                             DominatorTree *DT, LoopInfo *LI,
+                             MemorySSAUpdater *MSSAU, const Twine &BBName,
+                             bool Before) {
+  return SplitBlockImpl(Old, SplitPt, /*DTU=*/nullptr, DT, LI, MSSAU, BBName,
+                        Before);
+}
+BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt,
+                             DomTreeUpdater *DTU, LoopInfo *LI,
+                             MemorySSAUpdater *MSSAU, const Twine &BBName,
+                             bool Before) {
+  return SplitBlockImpl(Old, SplitPt, DTU, /*DT=*/nullptr, LI, MSSAU, BBName,
+                        Before);
+}
+
 BasicBlock *llvm::splitBlockBefore(BasicBlock *Old, Instruction *SplitPt,
-                                   DominatorTree *DT, LoopInfo *LI,
+                                   DomTreeUpdater *DTU, LoopInfo *LI,
                                    MemorySSAUpdater *MSSAU,
                                    const Twine &BBName) {
 
@@ -598,25 +630,25 @@ BasicBlock *llvm::splitBlockBefore(BasicBlock *Old, Instruction *SplitPt,
     if (Loop *L = LI->getLoopFor(Old))
       L->addBasicBlockToLoop(New, *LI);
 
-  if (DT) {
-    DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
+  if (DTU) {
     SmallVector<DominatorTree::UpdateType, 8> DTUpdates;
     // New dominates Old. The predecessor nodes of the Old node dominate
     // New node.
+    SmallSetVector<BasicBlock *, 8> UniquePredecessorsOfOld(pred_begin(New),
+                                                            pred_end(New));
     DTUpdates.push_back({DominatorTree::Insert, New, Old});
-    for (BasicBlock *Pred : predecessors(New))
-      if (DT->getNode(Pred)) {
-        DTUpdates.push_back({DominatorTree::Insert, Pred, New});
-        DTUpdates.push_back({DominatorTree::Delete, Pred, Old});
-      }
+    DTUpdates.reserve(DTUpdates.size() + 2 * UniquePredecessorsOfOld.size());
+    for (BasicBlock *UniquePredecessorOfOld : UniquePredecessorsOfOld) {
+      DTUpdates.push_back({DominatorTree::Insert, UniquePredecessorOfOld, New});
+      DTUpdates.push_back({DominatorTree::Delete, UniquePredecessorOfOld, Old});
+    }
 
-    DTU.applyUpdates(DTUpdates);
-    DTU.flush();
+    DTU->applyUpdates(DTUpdates);
 
     // Move MemoryAccesses still tracked in Old, but part of New now.
     // Update accesses in successor blocks accordingly.
     if (MSSAU) {
-      MSSAU->applyUpdates(DTUpdates, *DT);
+      MSSAU->applyUpdates(DTUpdates, DTU->getDomTree());
       if (VerifyMemorySSA)
         MSSAU->getMemorySSA()->verifyMemorySSA();
     }
@@ -627,11 +659,31 @@ BasicBlock *llvm::splitBlockBefore(BasicBlock *Old, Instruction *SplitPt,
 /// Update DominatorTree, LoopInfo, and LCCSA analysis information.
 static void UpdateAnalysisInformation(BasicBlock *OldBB, BasicBlock *NewBB,
                                       ArrayRef<BasicBlock *> Preds,
-                                      DominatorTree *DT, LoopInfo *LI,
-                                      MemorySSAUpdater *MSSAU,
+                                      DomTreeUpdater *DTU, DominatorTree *DT,
+                                      LoopInfo *LI, MemorySSAUpdater *MSSAU,
                                       bool PreserveLCSSA, bool &HasLoopExit) {
   // Update dominator tree if available.
-  if (DT) {
+  if (DTU) {
+    // Recalculation of DomTree is needed when updating a forward DomTree and
+    // the Entry BB is replaced.
+    if (NewBB == &NewBB->getParent()->getEntryBlock() && DTU->hasDomTree()) {
+      // The entry block was removed and there is no external interface for
+      // the dominator tree to be notified of this change. In this corner-case
+      // we recalculate the entire tree.
+      DTU->recalculate(*NewBB->getParent());
+    } else {
+      // Split block expects NewBB to have a non-empty set of predecessors.
+      SmallVector<DominatorTree::UpdateType, 8> Updates;
+      SmallSetVector<BasicBlock *, 8> UniquePreds(Preds.begin(), Preds.end());
+      Updates.push_back({DominatorTree::Insert, NewBB, OldBB});
+      Updates.reserve(Updates.size() + 2 * UniquePreds.size());
+      for (auto *UniquePred : UniquePreds) {
+        Updates.push_back({DominatorTree::Insert, UniquePred, NewBB});
+        Updates.push_back({DominatorTree::Delete, UniquePred, OldBB});
+      }
+      DTU->applyUpdates(Updates);
+    }
+  } else if (DT) {
     if (OldBB == DT->getRootNode()->getBlock()) {
       assert(NewBB == &NewBB->getParent()->getEntryBlock());
       DT->setNewRoot(NewBB);
@@ -649,6 +701,8 @@ static void UpdateAnalysisInformation(BasicBlock *OldBB, BasicBlock *NewBB,
   if (!LI)
     return;
 
+  if (DTU && DTU->hasDomTree())
+    DT = &DTU->getDomTree();
   assert(DT && "DT should be available to update LoopInfo!");
   Loop *L = LI->getLoopFor(OldBB);
 
@@ -782,11 +836,17 @@ static void UpdatePHINodes(BasicBlock *OrigBB, BasicBlock *NewBB,
   }
 }
 
-BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB,
-                                         ArrayRef<BasicBlock *> Preds,
-                                         const char *Suffix, DominatorTree *DT,
-                                         LoopInfo *LI, MemorySSAUpdater *MSSAU,
-                                         bool PreserveLCSSA) {
+static void SplitLandingPadPredecessorsImpl(
+    BasicBlock *OrigBB, ArrayRef<BasicBlock *> Preds, const char *Suffix1,
+    const char *Suffix2, SmallVectorImpl<BasicBlock *> &NewBBs,
+    DomTreeUpdater *DTU, DominatorTree *DT, LoopInfo *LI,
+    MemorySSAUpdater *MSSAU, bool PreserveLCSSA);
+
+static BasicBlock *
+SplitBlockPredecessorsImpl(BasicBlock *BB, ArrayRef<BasicBlock *> Preds,
+                           const char *Suffix, DomTreeUpdater *DTU,
+                           DominatorTree *DT, LoopInfo *LI,
+                           MemorySSAUpdater *MSSAU, bool PreserveLCSSA) {
   // Do not attempt to split that which cannot be split.
   if (!BB->canSplitPredecessors())
     return nullptr;
@@ -797,8 +857,8 @@ BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB,
     SmallVector<BasicBlock*, 2> NewBBs;
     std::string NewName = std::string(Suffix) + ".split-lp";
 
-    SplitLandingPadPredecessors(BB, Preds, Suffix, NewName.c_str(), NewBBs, DT,
-                                LI, MSSAU, PreserveLCSSA);
+    SplitLandingPadPredecessorsImpl(BB, Preds, Suffix, NewName.c_str(), NewBBs,
+                                    DTU, DT, LI, MSSAU, PreserveLCSSA);
     return NewBBs[0];
   }
 
@@ -850,7 +910,7 @@ BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB,
 
   // Update DominatorTree, LoopInfo, and LCCSA analysis information.
   bool HasLoopExit = false;
-  UpdateAnalysisInformation(BB, NewBB, Preds, DT, LI, MSSAU, PreserveLCSSA,
+  UpdateAnalysisInformation(BB, NewBB, Preds, DTU, DT, LI, MSSAU, PreserveLCSSA,
                             HasLoopExit);
 
   if (!Preds.empty()) {
@@ -870,13 +930,29 @@ BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB,
   return NewBB;
 }
 
-void llvm::SplitLandingPadPredecessors(BasicBlock *OrigBB,
-                                       ArrayRef<BasicBlock *> Preds,
-                                       const char *Suffix1, const char *Suffix2,
-                                       SmallVectorImpl<BasicBlock *> &NewBBs,
-                                       DominatorTree *DT, LoopInfo *LI,
-                                       MemorySSAUpdater *MSSAU,
-                                       bool PreserveLCSSA) {
+BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB,
+                                         ArrayRef<BasicBlock *> Preds,
+                                         const char *Suffix, DominatorTree *DT,
+                                         LoopInfo *LI, MemorySSAUpdater *MSSAU,
+                                         bool PreserveLCSSA) {
+  return SplitBlockPredecessorsImpl(BB, Preds, Suffix, /*DTU=*/nullptr, DT, LI,
+                                    MSSAU, PreserveLCSSA);
+}
+BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB,
+                                         ArrayRef<BasicBlock *> Preds,
+                                         const char *Suffix,
+                                         DomTreeUpdater *DTU, LoopInfo *LI,
+                                         MemorySSAUpdater *MSSAU,
+                                         bool PreserveLCSSA) {
+  return SplitBlockPredecessorsImpl(BB, Preds, Suffix, DTU,
+                                    /*DT=*/nullptr, LI, MSSAU, PreserveLCSSA);
+}
+
+static void SplitLandingPadPredecessorsImpl(
+    BasicBlock *OrigBB, ArrayRef<BasicBlock *> Preds, const char *Suffix1,
+    const char *Suffix2, SmallVectorImpl<BasicBlock *> &NewBBs,
+    DomTreeUpdater *DTU, DominatorTree *DT, LoopInfo *LI,
+    MemorySSAUpdater *MSSAU, bool PreserveLCSSA) {
   assert(OrigBB->isLandingPad() && "Trying to split a non-landing pad!");
 
   // Create a new basic block for OrigBB's predecessors listed in Preds. Insert
@@ -901,8 +977,8 @@ void llvm::SplitLandingPadPredecessors(BasicBlock *OrigBB,
   }
 
   bool HasLoopExit = false;
-  UpdateAnalysisInformation(OrigBB, NewBB1, Preds, DT, LI, MSSAU, PreserveLCSSA,
-                            HasLoopExit);
+  UpdateAnalysisInformation(OrigBB, NewBB1, Preds, DTU, DT, LI, MSSAU,
+                            PreserveLCSSA, HasLoopExit);
 
   // Update the PHI nodes in OrigBB with the values coming from NewBB1.
   UpdatePHINodes(OrigBB, NewBB1, Preds, BI1, HasLoopExit);
@@ -937,7 +1013,7 @@ void llvm::SplitLandingPadPredecessors(BasicBlock *OrigBB,
 
     // Update DominatorTree, LoopInfo, and LCCSA analysis information.
     HasLoopExit = false;
-    UpdateAnalysisInformation(OrigBB, NewBB2, NewBB2Preds, DT, LI, MSSAU,
+    UpdateAnalysisInformation(OrigBB, NewBB2, NewBB2Preds, DTU, DT, LI, MSSAU,
                               PreserveLCSSA, HasLoopExit);
 
     // Update the PHI nodes in OrigBB with the values coming from NewBB2.
@@ -974,6 +1050,29 @@ void llvm::SplitLandingPadPredecessors(BasicBlock *OrigBB,
   }
 }
 
+void llvm::SplitLandingPadPredecessors(BasicBlock *OrigBB,
+                                       ArrayRef<BasicBlock *> Preds,
+                                       const char *Suffix1, const char *Suffix2,
+                                       SmallVectorImpl<BasicBlock *> &NewBBs,
+                                       DominatorTree *DT, LoopInfo *LI,
+                                       MemorySSAUpdater *MSSAU,
+                                       bool PreserveLCSSA) {
+  return SplitLandingPadPredecessorsImpl(
+      OrigBB, Preds, Suffix1, Suffix2, NewBBs,
+      /*DTU=*/nullptr, DT, LI, MSSAU, PreserveLCSSA);
+}
+void llvm::SplitLandingPadPredecessors(BasicBlock *OrigBB,
+                                       ArrayRef<BasicBlock *> Preds,
+                                       const char *Suffix1, const char *Suffix2,
+                                       SmallVectorImpl<BasicBlock *> &NewBBs,
+                                       DomTreeUpdater *DTU, LoopInfo *LI,
+                                       MemorySSAUpdater *MSSAU,
+                                       bool PreserveLCSSA) {
+  return SplitLandingPadPredecessorsImpl(OrigBB, Preds, Suffix1, Suffix2,
+                                         NewBBs, DTU, /*DT=*/nullptr, LI, MSSAU,
+                                         PreserveLCSSA);
+}
+
 ReturnInst *llvm::FoldReturnIntoUncondBranch(ReturnInst *RI, BasicBlock *BB,
                                              BasicBlock *Pred,
                                              DomTreeUpdater *DTU) {
@@ -1033,14 +1132,24 @@ ReturnInst *llvm::FoldReturnIntoUncondBranch(ReturnInst *RI, BasicBlock *BB,
   return cast<ReturnInst>(NewRet);
 }
 
-Instruction *llvm::SplitBlockAndInsertIfThen(Value *Cond,
-                                             Instruction *SplitBefore,
-                                             bool Unreachable,
-                                             MDNode *BranchWeights,
-                                             DominatorTree *DT, LoopInfo *LI,
-                                             BasicBlock *ThenBlock) {
+static Instruction *
+SplitBlockAndInsertIfThenImpl(Value *Cond, Instruction *SplitBefore,
+                              bool Unreachable, MDNode *BranchWeights,
+                              DomTreeUpdater *DTU, DominatorTree *DT,
+                              LoopInfo *LI, BasicBlock *ThenBlock) {
+  SmallVector<DominatorTree::UpdateType, 8> Updates;
   BasicBlock *Head = SplitBefore->getParent();
   BasicBlock *Tail = Head->splitBasicBlock(SplitBefore->getIterator());
+  if (DTU) {
+    SmallSetVector<BasicBlock *, 8> UniqueSuccessorsOfHead(succ_begin(Tail),
+                                                           succ_end(Tail));
+    Updates.push_back({DominatorTree::Insert, Head, Tail});
+    Updates.reserve(Updates.size() + 2 * UniqueSuccessorsOfHead.size());
+    for (BasicBlock *UniqueSuccessorOfHead : UniqueSuccessorsOfHead) {
+      Updates.push_back({DominatorTree::Insert, Tail, UniqueSuccessorOfHead});
+      Updates.push_back({DominatorTree::Delete, Head, UniqueSuccessorOfHead});
+    }
+  }
   Instruction *HeadOldTerm = Head->getTerminator();
   LLVMContext &C = Head->getContext();
   Instruction *CheckTerm;
@@ -1049,17 +1158,24 @@ Instruction *llvm::SplitBlockAndInsertIfThen(Value *Cond,
     ThenBlock = BasicBlock::Create(C, "", Head->getParent(), Tail);
     if (Unreachable)
       CheckTerm = new UnreachableInst(C, ThenBlock);
-    else
+    else {
       CheckTerm = BranchInst::Create(Tail, ThenBlock);
+      if (DTU)
+        Updates.push_back({DominatorTree::Insert, ThenBlock, Tail});
+    }
     CheckTerm->setDebugLoc(SplitBefore->getDebugLoc());
   } else
     CheckTerm = ThenBlock->getTerminator();
   BranchInst *HeadNewTerm =
-    BranchInst::Create(/*ifTrue*/ThenBlock, /*ifFalse*/Tail, Cond);
+      BranchInst::Create(/*ifTrue*/ ThenBlock, /*ifFalse*/ Tail, Cond);
+  if (DTU)
+    Updates.push_back({DominatorTree::Insert, Head, ThenBlock});
   HeadNewTerm->setMetadata(LLVMContext::MD_prof, BranchWeights);
   ReplaceInstWithInst(HeadOldTerm, HeadNewTerm);
 
-  if (DT) {
+  if (DTU)
+    DTU->applyUpdates(Updates);
+  else if (DT) {
     if (DomTreeNode *OldNode = DT->getNode(Head)) {
       std::vector<DomTreeNode *> Children(OldNode->begin(), OldNode->end());
 
@@ -1085,6 +1201,27 @@ Instruction *llvm::SplitBlockAndInsertIfThen(Value *Cond,
   return CheckTerm;
 }
 
+Instruction *llvm::SplitBlockAndInsertIfThen(Value *Cond,
+                                             Instruction *SplitBefore,
+                                             bool Unreachable,
+                                             MDNode *BranchWeights,
+                                             DominatorTree *DT, LoopInfo *LI,
+                                             BasicBlock *ThenBlock) {
+  return SplitBlockAndInsertIfThenImpl(Cond, SplitBefore, Unreachable,
+                                       BranchWeights,
+                                       /*DTU=*/nullptr, DT, LI, ThenBlock);
+}
+Instruction *llvm::SplitBlockAndInsertIfThen(Value *Cond,
+                                             Instruction *SplitBefore,
+                                             bool Unreachable,
+                                             MDNode *BranchWeights,
+                                             DomTreeUpdater *DTU, LoopInfo *LI,
+                                             BasicBlock *ThenBlock) {
+  return SplitBlockAndInsertIfThenImpl(Cond, SplitBefore, Unreachable,
+                                       BranchWeights, DTU, /*DT=*/nullptr, LI,
+                                       ThenBlock);
+}
+
 void llvm::SplitBlockAndInsertIfThenElse(Value *Cond, Instruction *SplitBefore,
                                          Instruction **ThenTerm,
                                          Instruction **ElseTerm,

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -1113,7 +1113,7 @@ bool SimplifyCFGOpt::FoldValueComparisonIntoPredecessors(Instruction *TI,
       if (!SafeToMergeTerminators(TI, PTI, &FailBlocks)) {
         for (auto *Succ : FailBlocks) {
           if (!SplitBlockPredecessors(Succ, TI->getParent(), ".fold.split",
-                                      DTU ? &DTU->getDomTree() : nullptr))
+                                      DTU))
             return false;
         }
       }
@@ -1977,8 +1977,7 @@ static bool SinkCommonCodeFromPredecessors(BasicBlock *BB,
     LLVM_DEBUG(dbgs() << "SINK: Splitting edge\n");
     // We have a conditional edge and we're going to sink some instructions.
     // Insert a new block postdominating all blocks we're going to sink from.
-    if (!SplitBlockPredecessors(BB, UnconditionalPreds, ".sink.split",
-                                DTU ? &DTU->getDomTree() : nullptr))
+    if (!SplitBlockPredecessors(BB, UnconditionalPreds, ".sink.split", DTU))
       // Edges couldn't be split.
       return false;
     Changed = true;
@@ -3391,8 +3390,7 @@ static bool mergeConditionalStoreToAddress(
     // branch to QFB and PostBB.
     BasicBlock *TruePred = QTB ? QTB : QFB->getSinglePredecessor();
     BasicBlock *NewBB =
-        SplitBlockPredecessors(PostBB, {QFB, TruePred}, "condstore.split",
-                               DTU ? &DTU->getDomTree() : nullptr);
+        SplitBlockPredecessors(PostBB, {QFB, TruePred}, "condstore.split", DTU);
     if (!NewBB)
       return false;
     PostBB = NewBB;
@@ -3421,9 +3419,9 @@ static bool mergeConditionalStoreToAddress(
     QPred = QB.CreateNot(QPred);
   Value *CombinedPred = QB.CreateOr(PPred, QPred);
 
-  auto *T = SplitBlockAndInsertIfThen(
-      CombinedPred, &*QB.GetInsertPoint(), /*Unreachable=*/false,
-      /*BranchWeights=*/nullptr, DTU ? &DTU->getDomTree() : nullptr);
+  auto *T = SplitBlockAndInsertIfThen(CombinedPred, &*QB.GetInsertPoint(),
+                                      /*Unreachable=*/false,
+                                      /*BranchWeights=*/nullptr, DTU);
   QB.SetInsertPoint(T);
   StoreInst *SI = cast<StoreInst>(QB.CreateStore(QPHI, Address));
   AAMDNodes AAMD;
@@ -4187,9 +4185,8 @@ bool SimplifyCFGOpt::SimplifyBranchOnICmpChain(BranchInst *BI,
   // then we evaluate them with an explicit branch first. Split the block
   // right before the condbr to handle it.
   if (ExtraCase) {
-    BasicBlock *NewBB =
-        SplitBlock(BB, BI, DTU ? &DTU->getDomTree() : nullptr, /*LI=*/nullptr,
-                   /*MSSAU=*/nullptr, "switch.early.test");
+    BasicBlock *NewBB = SplitBlock(BB, BI, DTU, /*LI=*/nullptr,
+                                   /*MSSAU=*/nullptr, "switch.early.test");
 
     // Remove the uncond branch added to the old block.
     Instruction *OldTI = BB->getTerminator();
@@ -4828,16 +4825,14 @@ static void createUnreachableSwitchDefault(SwitchInst *Switch,
                                            DomTreeUpdater *DTU) {
   LLVM_DEBUG(dbgs() << "SimplifyCFG: switch default is dead.\n");
   auto *BB = Switch->getParent();
-  BasicBlock *NewDefaultBlock =
-      SplitBlockPredecessors(Switch->getDefaultDest(), Switch->getParent(), "",
-                             DTU ? &DTU->getDomTree() : nullptr);
+  BasicBlock *NewDefaultBlock = SplitBlockPredecessors(
+      Switch->getDefaultDest(), Switch->getParent(), "", DTU);
   auto *OrigDefaultBlock = Switch->getDefaultDest();
   Switch->setDefaultDest(&*NewDefaultBlock);
   if (DTU)
     DTU->applyUpdates({{DominatorTree::Insert, BB, &*NewDefaultBlock},
                        {DominatorTree::Delete, BB, OrigDefaultBlock}});
-  SplitBlock(&*NewDefaultBlock, &NewDefaultBlock->front(),
-             DTU ? &DTU->getDomTree() : nullptr);
+  SplitBlock(&*NewDefaultBlock, &NewDefaultBlock->front(), DTU);
   SmallVector<DominatorTree::UpdateType, 2> Updates;
   for (auto *Successor : successors(NewDefaultBlock))
     Updates.push_back({DominatorTree::Delete, NewDefaultBlock, Successor});
@@ -5399,20 +5394,25 @@ static void RemoveSwitchAfterSelectConversion(SwitchInst *SI, PHINode *PHI,
                                               Value *SelectValue,
                                               IRBuilder<> &Builder,
                                               DomTreeUpdater *DTU) {
+  std::vector<DominatorTree::UpdateType> Updates;
+
   BasicBlock *SelectBB = SI->getParent();
+  BasicBlock *DestBB = PHI->getParent();
+
+  if (!is_contained(predecessors(DestBB), SelectBB))
+    Updates.push_back({DominatorTree::Insert, SelectBB, DestBB});
+  Builder.CreateBr(DestBB);
+
+  // Remove the switch.
+
   while (PHI->getBasicBlockIndex(SelectBB) >= 0)
     PHI->removeIncomingValue(SelectBB);
   PHI->addIncoming(SelectValue, SelectBB);
 
-  Builder.CreateBr(PHI->getParent());
-
-  std::vector<DominatorTree::UpdateType> Updates;
-
-  // Remove the switch.
   for (unsigned i = 0, e = SI->getNumSuccessors(); i < e; ++i) {
     BasicBlock *Succ = SI->getSuccessor(i);
 
-    if (Succ == PHI->getParent())
+    if (Succ == DestBB)
       continue;
     Succ->removePredecessor(SelectBB);
     Updates.push_back({DominatorTree::Delete, SelectBB, Succ});

diff --git a/llvm/test/Transforms/SimplifyCFG/switchToSelect-domtree-preservation-edgecase.ll b/llvm/test/Transforms/SimplifyCFG/switchToSelect-domtree-preservation-edgecase.ll
@@ -0,0 +1,57 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -simplifycfg -simplifycfg-require-and-preserve-domtree=1 -sink-common-insts=1 < %s | FileCheck %s
+
+declare void @widget(i8)
+
+define void @baz(i8 %arg, i8 %arg10, i1 %arg11) {
+; CHECK-LABEL: @baz(
+; CHECK-NEXT:  bb:
+; CHECK-NEXT:    br label [[BB12:%.*]]
+; CHECK:       bb12:
+; CHECK-NEXT:    [[TMP:%.*]] = icmp eq i8 [[ARG:%.*]], 0
+; CHECK-NEXT:    br i1 [[TMP]], label [[BB17:%.*]], label [[BB13:%.*]]
+; CHECK:       bb13:
+; CHECK-NEXT:    tail call void @widget(i8 11)
+; CHECK-NEXT:    [[SWITCH_SELECTCMP:%.*]] = icmp eq i8 [[ARG10:%.*]], 73
+; CHECK-NEXT:    [[SWITCH_SELECT:%.*]] = select i1 [[SWITCH_SELECTCMP]], i8 44, i8 22
+; CHECK-NEXT:    [[SWITCH_SELECTCMP1:%.*]] = icmp eq i8 [[ARG10]], 68
+; CHECK-NEXT:    [[SWITCH_SELECT2:%.*]] = select i1 [[SWITCH_SELECTCMP1]], i8 33, i8 [[SWITCH_SELECT]]
+; CHECK-NEXT:    tail call void @widget(i8 [[SWITCH_SELECT2]])
+; CHECK-NEXT:    br label [[BB17]]
+; CHECK:       bb17:
+; CHECK-NEXT:    br i1 [[ARG11:%.*]], label [[BB12]], label [[BB18:%.*]]
+; CHECK:       bb18:
+; CHECK-NEXT:    ret void
+;
+bb:
+  br label %bb12
+
+bb12:                                             ; preds = %bb17, %bb
+  %tmp = icmp eq i8 %arg, 0
+  br i1 %tmp, label %bb17, label %bb13
+
+bb13:                                             ; preds = %bb12
+  tail call void @widget(i8 11)
+  switch i8 %arg10, label %bb14 [
+  i8 68, label %bb15
+  i8 73, label %bb16
+  ]
+
+bb14:                                             ; preds = %bb13
+  tail call void @widget(i8 22)
+  br label %bb17
+
+bb15:                                             ; preds = %bb13
+  tail call void @widget(i8 33)
+  br label %bb17
+
+bb16:                                             ; preds = %bb13
+  tail call void @widget(i8 44)
+  br label %bb17
+
+bb17:                                             ; preds = %bb16, %bb15, %bb14, %bb12
+  br i1 %arg11, label %bb12, label %bb18
+
+bb18:                                             ; preds = %bb17
+  ret void
+}