Index: lib/Transforms/Utils/LowerSwitch.cpp
===================================================================
--- lib/Transforms/Utils/LowerSwitch.cpp
+++ lib/Transforms/Utils/LowerSwitch.cpp
@@ -387,8 +387,34 @@
     return;
   }
 
-  const bool DefaultIsUnreachable =
-      Default->size() == 1 && isa<UnreachableInst>(Default->getTerminator());
+  bool DefaultIsUnreachable =
+      isa<UnreachableInst>(Default->getFirstNonPHIOrDbg());
+
+  if (DefaultIsUnreachable) {
+    DenseMap<BasicBlock *, unsigned> Popularity;
+    unsigned MaxPop = 0;
+    BasicBlock *MaxBB = nullptr;
+    for (auto I : SI->cases()) {
+      BasicBlock *BB = I.getCaseSuccessor();
+      if (++Popularity[BB] > MaxPop) {
+        MaxPop = Popularity[BB];
+        MaxBB = BB;
+      }
+    }
+    if (MaxPop > 1) {
+      // If some block is extra popular, use that as the default result,
+      // reducing the number of cases.
+      Default = MaxBB;
+      DefaultIsUnreachable = false;
+      for (auto I = SI->case_begin(); I != SI->case_end();) {
+        if (I.getCaseSuccessor() == MaxBB)
+          SI->removeCase(I);
+        else
+          ++I;
+      }
+    }
+  }
+
   // Create a new, empty default block so that the new hierarchy of
   // if-then statements go to this and the PHI nodes are happy.
   // if the default block is set as an unreachable we avoid creating one
@@ -438,11 +464,10 @@
   BranchInst::Create(SwitchBlock, OrigBlock);
 
   // We are now done with the switch instruction, delete it.
+  BasicBlock *OldDefault = SI->getDefaultDest();
   CurBlock->getInstList().erase(SI);
 
-  pred_iterator PI = pred_begin(Default), E = pred_end(Default);
-  // If the Default block has no more predecessors just remove it
-  if (PI == E) {
-    DeleteDeadBlock(Default);
-  }
+  // If the Default block has no more predecessors just remove it.
+  if (pred_begin(OldDefault) == pred_end(OldDefault))
+    DeleteDeadBlock(OldDefault);
 }
Index: test/Transforms/LowerSwitch/fold-popular-case-to-unreachable-default.ll
===================================================================
--- /dev/null
+++ test/Transforms/LowerSwitch/fold-popular-case-to-unreachable-default.ll
@@ -0,0 +1,43 @@
+; RUN: opt %s -lowerswitch -S | FileCheck %s
+
+; An unreachabel default is replaced with a popular case label.
+
+define void @foo(i32 %x, i32* %p) {
+; CHECK-LABEL: @foo
+; CHECK:       icmp {{[a-z]+}} i32 %x, 3
+; CHECK-NOT:   icmp {{[a-z]+}} i32 %x, 4
+; CHECK-NOT:   icmp {{[a-z]+}} i32 %x, 5
+; CHECK-LABEL: NewDefault:
+; CHECK-NEXT:  br label %bb4
+; CHECK-NOT: default:
+; CEHCK-NOT: unreachable
+
+entry:
+  switch i32 %x, label %default [
+    i32 0, label %bb0
+    i32 1, label %bb1
+    i32 2, label %bb2
+    i32 3, label %bb3
+    i32 4, label %bb4
+    i32 5, label %bb4
+  ]
+bb0:
+  store i32 0, i32* %p
+  br label %exit
+bb1:
+  store i32 1, i32* %p
+  br label %exit
+bb2:
+  store i32 2, i32* %p
+  br label %exit
+bb3:
+  store i32 3, i32* %p
+  br label %exit
+bb4:
+  store i32 4, i32* %p
+  br label %exit
+exit:
+  ret void
+default:
+  unreachable
+}