Index: llvm/test/tools/llvm-reduce/remove-bb-switch-default.ll
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-reduce/remove-bb-switch-default.ll
@@ -0,0 +1,67 @@
+; RUN: llvm-reduce -abort-on-invalid-reduction --delta-passes=basic-blocks --test FileCheck --test-arg --check-prefix=CHECK-INTERESTINGNESS0 --test-arg %s --test-arg --input-file %s -o %t.0
+; RUN: FileCheck -check-prefix=RESULT0 %s < %t.0
+
+; RUN: llvm-reduce -abort-on-invalid-reduction --delta-passes=basic-blocks --test FileCheck --test-arg --check-prefix=CHECK-INTERESTINGNESS1 --test-arg %s --test-arg --input-file %s -o %t.1
+; RUN: FileCheck -check-prefix=RESULT1 %s < %t.1
+
+; CHECK-INTERESTINGNESS0: store i32 1,
+; CHECK-INTERESTINGNESS0: store i32 2,
+
+; CHECK-INTERESTINGNESS1: store i32 2,
+
+
+; RESULT0: bb:
+; RESULT0-NEXT: %bb.load = load i32, ptr null, align 4
+; RESULT0-NEXT: store i32 0, ptr null, align 4
+; RESULT0-NEXT: br i1 %arg0, label %bb1, label %bb2
+
+; RESULT0: bb1:
+; RESULT0-NEXT: %bb1.phi = phi i32 [ %bb.load, %bb ], [ %bb2.phi, %bb2 ], [ %bb2.phi, %bb2 ]
+; RESULT0-NEXT: store i32 1, ptr null, align 4
+; RESULT0-NEXT: ret void
+
+; RESULT0: bb2: ; preds = %bb
+; RESULT0-NEXT: %bb2.phi = phi i32 [ %bb.load, %bb ]
+; RESULT0-NEXT: store i32 2, ptr null, align 4
+; RESULT0-NEXT: switch i32 %bb2.phi, label %bb1 [
+; RESULT0-NEXT: i32 0, label %bb1
+; RESULT0-NEXT: ]
+
+
+; RESULT1: bb:
+; RESULT1-NEXT: %bb.load = load i32, ptr null, align 4
+; RESULT1-NEXT: store i32 0, ptr null, align 4
+; RESULT1-NEXT: br label %bb2
+
+; RESULT1: bb2:
+; RESULT1-NEXT: %bb2.phi = phi i32 [ %bb.load, %bb ]
+; RESULT1-NEXT: store i32 2, ptr null, align 4
+; RESULT1-NEXT: ret void
+define void @main(i1 %arg0) {
+bb:
+  %bb.load = load i32, ptr null
+  store i32 0, ptr null
+  br i1 %arg0, label %bb1, label %bb2
+
+bb1:
+  %bb1.phi = phi i32 [%bb.load, %bb], [9, %bb3], [%bb2.phi, %bb2]
+  store i32 1, ptr null
+  ret void
+
+bb2:
+  %bb2.phi = phi i32 [%bb.load, %bb], [%bb3.load, %bb3]
+  store i32 2, ptr null
+  switch i32 %bb2.phi, label %bb3 [
+    i32 0, label %bb1
+    i32 1, label %bb4
+  ]
+
+bb3:
+  %bb3.load = load i32, ptr null
+  store i32 3, ptr null
+  br i1 true, label %bb2, label %bb1
+
+bb4:
+  store i32 4, ptr null
+  ret void
+}
Index: llvm/tools/llvm-reduce/deltas/ReduceBasicBlocks.cpp
===================================================================
--- llvm/tools/llvm-reduce/deltas/ReduceBasicBlocks.cpp
+++ llvm/tools/llvm-reduce/deltas/ReduceBasicBlocks.cpp
@@ -85,21 +85,36 @@
 static void
 removeUninterestingBBsFromSwitch(SwitchInst &SwInst,
                                  const DenseSet<BasicBlock *> &BBsToKeep) {
+  for (int I = 0, E = SwInst.getNumCases(); I != E; ++I) {
+    auto Case = SwInst.case_begin() + I;
+    if (!BBsToKeep.count(Case->getCaseSuccessor())) {
+      SwInst.removeCase(Case);
+      --I;
+      --E;
+    }
+  }
+
   if (!BBsToKeep.count(SwInst.getDefaultDest())) {
-    auto *FnRetTy = SwInst.getParent()->getParent()->getReturnType();
-    ReturnInst::Create(SwInst.getContext(),
-                       FnRetTy->isVoidTy() ? nullptr : getDefaultValue(FnRetTy),
-                       SwInst.getParent());
-    SwInst.eraseFromParent();
-  } else
-    for (int I = 0, E = SwInst.getNumCases(); I != E; ++I) {
-      auto Case = SwInst.case_begin() + I;
-      if (!BBsToKeep.count(Case->getCaseSuccessor())) {
-        SwInst.removeCase(Case);
-        --I;
-        --E;
-      }
+    if (SwInst.getNumCases() == 0) {
+      auto *FnRetTy = SwInst.getParent()->getParent()->getReturnType();
+      Value *RetValue =
+          FnRetTy->isVoidTy() ? nullptr : getDefaultValue(FnRetTy);
+      ReturnInst::Create(SwInst.getContext(), RetValue, SwInst.getParent());
+      SwInst.eraseFromParent();
+      return;
     }
+
+    // Replace the default dest with one of the other cases
+    auto Case = SwInst.case_begin();
+
+    BasicBlock *NewDefault = Case->getCaseSuccessor();
+    SwInst.setDefaultDest(NewDefault);
+
+    for (PHINode &SuccPHI : NewDefault->phis()) {
+      SuccPHI.addIncoming(SuccPHI.getIncomingValueForBlock(SwInst.getParent()),
+                          SwInst.getParent());
+    }
+  }
 }
 
 /// Removes out-of-chunk arguments from functions, and modifies their calls
@@ -120,12 +135,6 @@
     }
   }
 
-  // Remove out-of-chunk BB from successor phi nodes
-  for (auto &BB : BBsToDelete) {
-    for (auto *Succ : successors(BB))
-      Succ->removePredecessor(BB, /*KeepOneInputPHIs=*/true);
-  }
-
   // Replace terminators that reference out-of-chunk BBs
   for (auto &F : Program)
     for (auto &BB : F) {
@@ -135,6 +144,12 @@
         replaceBranchTerminator(BB, BBsToKeep);
     }
 
+  // Remove out-of-chunk BB from successor phi nodes
+  for (auto &BB : BBsToDelete) {
+    for (auto *Succ : successors(BB))
+      Succ->removePredecessor(BB, /*KeepOneInputPHIs=*/true);
+  }
+
   // Replace out-of-chunk switch uses
   for (auto &BB : BBsToDelete) {
     // Instructions might be referenced in other BBs