Index: lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
===================================================================
--- lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
+++ lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
@@ -370,7 +370,7 @@
   /// Lower W.
   void lowerWorkItem(SwitchWorkListItem W, Value *Cond,
                      MachineBasicBlock *SwitchMBB,
-                     MachineBasicBlock *DefaultMBB);
+                     MachineBasicBlock *DefaultMBB, const SwitchInst &SI);
 
   /// Peel the top probability case if it exceeds the threshold
   MachineBasicBlock *peelDominantCaseCluster(const SwitchInst &SI,
@@ -847,7 +847,8 @@
                         MachineBasicBlock *SwitchBB);
   void visitJumpTable(JumpTable &JT);
   void visitJumpTableHeader(JumpTable &JT, JumpTableHeader &JTH,
-                            MachineBasicBlock *SwitchBB);
+                            MachineBasicBlock *SwitchBB,
+                            bool OmitDefaultBranch = false);
 
 private:
   // These all get lowered before this pass.
Index: lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -2153,7 +2153,8 @@
 /// in the JumpTable from switch case.
 void SelectionDAGBuilder::visitJumpTableHeader(JumpTable &JT,
                                                JumpTableHeader &JTH,
-                                               MachineBasicBlock *SwitchBB) {
+                                               MachineBasicBlock *SwitchBB,
+                                               bool OmitDefaultBranch) {
   SDLoc dl = getCurSDLoc();
 
   // Subtract the lowest switch case value from the value being switched on and
@@ -2178,24 +2179,30 @@
                                     JumpTableReg, SwitchOp);
   JT.Reg = JumpTableReg;
 
-  // Emit the range check for the jump table, and branch to the default block
-  // for the switch statement if the value being switched on exceeds the largest
-  // case in the switch.
-  SDValue CMP = DAG.getSetCC(
-      dl, TLI.getSetCCResultType(DAG.getDataLayout(), *DAG.getContext(),
-                                 Sub.getValueType()),
-      Sub, DAG.getConstant(JTH.Last - JTH.First, dl, VT), ISD::SETUGT);
-
-  SDValue BrCond = DAG.getNode(ISD::BRCOND, dl,
-                               MVT::Other, CopyTo, CMP,
-                               DAG.getBasicBlock(JT.Default));
-
-  // Avoid emitting unnecessary branches to the next block.
-  if (JT.MBB != NextBlock(SwitchBB))
-    BrCond = DAG.getNode(ISD::BR, dl, MVT::Other, BrCond,
-                         DAG.getBasicBlock(JT.MBB));
-
-  DAG.setRoot(BrCond);
+  if (!OmitDefaultBranch) {
+    // Emit the range check for the jump table, and branch to the default block
+    // for the switch statement if the value being switched on exceeds the
+    // largest case in the switch.
+    SDValue CMP = DAG.getSetCC(
+        dl, TLI.getSetCCResultType(DAG.getDataLayout(), *DAG.getContext(),
+                                   Sub.getValueType()),
+        Sub, DAG.getConstant(JTH.Last - JTH.First, dl, VT), ISD::SETUGT);
+
+    SDValue BrCond = DAG.getNode(ISD::BRCOND, dl,
+                                 MVT::Other, CopyTo, CMP,
+                                 DAG.getBasicBlock(JT.Default));
+
+    // Avoid emitting unnecessary branches to the next block.
+    if (JT.MBB != NextBlock(SwitchBB))
+      BrCond = DAG.getNode(ISD::BR, dl, MVT::Other, BrCond,
+                           DAG.getBasicBlock(JT.MBB));
+
+    DAG.setRoot(BrCond);
+  } else {
+    SDValue BrCond = DAG.getNode(ISD::BR, dl, MVT::Other, CopyTo,
+                                 DAG.getBasicBlock(JT.MBB));
+    DAG.setRoot(BrCond);
+  }
 }
 
 /// Create a LOAD_STACK_GUARD node, and let it carry the target specific global
@@ -9797,7 +9804,8 @@
 
 void SelectionDAGBuilder::lowerWorkItem(SwitchWorkListItem W, Value *Cond,
                                         MachineBasicBlock *SwitchMBB,
-                                        MachineBasicBlock *DefaultMBB) {
+                                        MachineBasicBlock *DefaultMBB,
+                                        const SwitchInst &SI) {
   MachineFunction *CurMF = FuncInfo.MF;
   MachineBasicBlock *NextMBB = nullptr;
   MachineFunction::iterator BBI(W.MBB);
@@ -9947,7 +9955,10 @@
 
         // If we're in the right place, emit the jump table header right now.
         if (CurMBB == SwitchMBB) {
-          visitJumpTableHeader(*JT, *JTH, SwitchMBB);
+          bool UnreachableDefault =
+              isa<UnreachableInst>(SI.getDefaultDest()->getFirstNonPHIOrDbg());
+          visitJumpTableHeader(*JT, *JTH, SwitchMBB,
+                               (UnreachableDefault  &&  SI.getNumCases() > 0));
           JTH->Emitted = true;
         }
         break;
@@ -10181,6 +10192,7 @@
     const SwitchInst &SI, CaseClusterVector &Clusters,
     BranchProbability &PeeledCaseProb) {
   MachineBasicBlock *SwitchMBB = FuncInfo.MBB;
+
   // Don't perform if there is only one cluster or optimizing for size.
   if (SwitchPeelThreshold > 100 || !FuncInfo.BPI || Clusters.size() < 2 ||
       TM.getOptLevel() == CodeGenOpt::None ||
@@ -10215,7 +10227,7 @@
   auto PeeledCaseIt = Clusters.begin() + PeeledCaseIndex;
   SwitchWorkListItem W = {SwitchMBB, PeeledCaseIt, PeeledCaseIt,
                           nullptr,   nullptr,      TopCaseProb.getCompl()};
-  lowerWorkItem(W, SI.getCondition(), SwitchMBB, PeeledSwitchMBB);
+  lowerWorkItem(W, SI.getCondition(), SwitchMBB, PeeledSwitchMBB, SI);
 
   Clusters.erase(PeeledCaseIt);
   for (CaseCluster &CC : Clusters) {
@@ -10233,6 +10245,7 @@
   // Extract cases from the switch.
   BranchProbabilityInfo *BPI = FuncInfo.BPI;
   CaseClusterVector Clusters;
+
   Clusters.reserve(SI.getNumCases());
   for (auto I : SI.cases()) {
     MachineBasicBlock *Succ = FuncInfo.MBBMap[I.getCaseSuccessor()];
@@ -10250,38 +10263,6 @@
   // if there are many clusters.
   sortAndRangeify(Clusters);
 
-  if (TM.getOptLevel() != CodeGenOpt::None) {
-    // Replace an unreachable default with the most popular destination.
-    // FIXME: Exploit unreachable default more aggressively.
-    bool UnreachableDefault =
-        isa<UnreachableInst>(SI.getDefaultDest()->getFirstNonPHIOrDbg());
-    if (UnreachableDefault && !Clusters.empty()) {
-      DenseMap<const BasicBlock *, unsigned> Popularity;
-      unsigned MaxPop = 0;
-      const BasicBlock *MaxBB = nullptr;
-      for (auto I : SI.cases()) {
-        const BasicBlock *BB = I.getCaseSuccessor();
-        if (++Popularity[BB] > MaxPop) {
-          MaxPop = Popularity[BB];
-          MaxBB = BB;
-        }
-      }
-      // Set new default.
-      assert(MaxPop > 0 && MaxBB);
-      DefaultMBB = FuncInfo.MBBMap[MaxBB];
-
-      // Remove cases that were pointing to the destination that is now the
-      // default.
-      CaseClusterVector New;
-      New.reserve(Clusters.size());
-      for (CaseCluster &CC : Clusters) {
-        if (CC.MBB != DefaultMBB)
-          New.push_back(CC);
-      }
-      Clusters = std::move(New);
-    }
-  }
-
   // The branch probablity of the peeled case.
   BranchProbability PeeledCaseProb = BranchProbability::getZero();
   MachineBasicBlock *PeeledSwitchMBB =
@@ -10345,6 +10326,6 @@
       continue;
     }
 
-    lowerWorkItem(W, SI.getCondition(), SwitchMBB, DefaultMBB);
+    lowerWorkItem(W, SI.getCondition(), SwitchMBB, DefaultMBB, SI);
   }
 }