Index: lib/Transforms/Utils/LowerSwitch.cpp
===================================================================
--- lib/Transforms/Utils/LowerSwitch.cpp
+++ lib/Transforms/Utils/LowerSwitch.cpp
@@ -387,8 +387,51 @@
     return;
   }
 
-  const bool DefaultIsUnreachable =
-      Default->size() == 1 && isa<UnreachableInst>(Default->getTerminator());
+  bool DefaultIsUnreachable =
+      isa<UnreachableInst>(Default->getFirstNonPHIOrDbg());
+
+  ConstantInt *LowerBound = nullptr;
+  ConstantInt *UpperBound = nullptr;
+
+  if (DefaultIsUnreachable) {
+    // Find the most popular successor and determine min and max case value.
+    DenseMap<BasicBlock *, unsigned> Popularity;
+    unsigned MaxPop = 0;
+    BasicBlock *PopSucc = nullptr;
+
+    assert(SI->case_begin() != SI->case_end());
+    ConstantInt *MinCase = SI->case_begin().getCaseValue();
+    ConstantInt *MaxCase = SI->case_begin().getCaseValue();
+
+    for (auto I : SI->cases()) {
+      BasicBlock *Succ = I.getCaseSuccessor();
+      if (++Popularity[Succ] > MaxPop) {
+        MaxPop = Popularity[Succ];
+        PopSucc = Succ;
+      }
+      if (I.getCaseValue()->getValue().slt(MinCase->getValue()))
+        MinCase = I.getCaseValue();
+      if (I.getCaseValue()->getValue().sgt(MaxCase->getValue()))
+        MaxCase = I.getCaseValue();
+    }
+
+    // Use the most popular block as the new default, reducing the number of cases.
+    Default = PopSucc;
+    DefaultIsUnreachable = false;
+    for (auto I = SI->case_begin(); I != SI->case_end();) {
+      if (I.getCaseSuccessor() == PopSucc)
+        SI->removeCase(I);
+      else
+        ++I;
+    }
+
+    // Make the bounds tightly fitted around the case value range, becase we
+    // know that the value passed to the switch must be exactly one of the case
+    // values.
+    LowerBound = MinCase;
+    UpperBound = MaxCase;
+  }
+
   // 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
@@ -418,18 +461,6 @@
   DEBUG(dbgs() << "Cases: " << Cases << "\n");
   (void)numCmps;
   
-  ConstantInt *UpperBound = nullptr;
-  ConstantInt *LowerBound = nullptr;
-
-  // Optimize the condition where Default is an unreachable block. In this case
-  // we can make the bounds tightly fitted around the case value ranges,
-  // because we know that the value passed to the switch should always be
-  // exactly one of the case values.
-  if (DefaultIsUnreachable) {
-    CaseItr LastCase = Cases.begin() + Cases.size() - 1;
-    UpperBound = cast<ConstantInt>(LastCase->High);
-    LowerBound = cast<ConstantInt>(Cases.begin()->Low);
-  }
   BasicBlock *SwitchBlock =
       switchConvert(Cases.begin(), Cases.end(), LowerBound, UpperBound, Val,
                     OrigBlock, OrigBlock, NewDefault);
@@ -438,11 +469,10 @@
   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/2014-06-11-SwitchDefaultUnreachableOpt.ll
===================================================================
--- test/Transforms/LowerSwitch/2014-06-11-SwitchDefaultUnreachableOpt.ll
+++ test/Transforms/LowerSwitch/2014-06-11-SwitchDefaultUnreachableOpt.ll
@@ -1,5 +1,8 @@
 ; RUN: opt < %s -lowerswitch -S | FileCheck %s
-; CHECK-NOT: {{.*}}icmp eq{{.*}}
+;
+; The switch is lowered with a single icmp.
+; CHECK: icmp
+; CHECK-NOT: icmp
 ;
 ;int foo(int a) {
 ;
@@ -14,7 +17,7 @@
 ;
 ;}
 
-define i32 @foo(i32 %a) nounwind ssp uwtable {
+define i32 @foo(i32 %a) {
   %1 = alloca i32, align 4
   %2 = alloca i32, align 4
   store i32 %a, i32* %2, align 4
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 +1,32 @@
+; RUN: opt %s -lowerswitch -S | FileCheck %s
+
+define void @foo(i32 %x, i32* %p) {
+; Cases 2 and 4 are removed and become the new default case.
+; It is now enough to use two icmps to lower the switch.
+;
+; CHECK-LABEL: @foo
+; CHECK:       icmp slt i32 %x, 5
+; CHECK:       icmp eq i32 %x, 1
+; CHECK-NOT:   icmp
+;
+entry:
+  switch i32 %x, label %default [
+    i32 1, label %bb0
+    i32 2, label %popular
+    i32 4, label %popular
+    i32 5, label %bb1
+  ]
+bb0:
+  store i32 0, i32* %p
+  br label %exit
+bb1:
+  store i32 1, i32* %p
+  br label %exit
+popular:
+  store i32 2, i32* %p
+  br label %exit
+exit:
+  ret void
+default:
+  unreachable
+}