diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 0e56904ebde46ceedbf43e98b1825363c1722450..ad3b92a71476790ca8332d2941f5a330acad989d 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -3734,14 +3734,30 @@ static bool SwitchToLookupTable(SwitchInst *SI,
                                             CommonDest->getParent(),
                                             CommonDest);
 
-  // Check whether the condition value is within the case range, and branch to
-  // the new BB.
+  // Compute the table index value.
   Builder.SetInsertPoint(SI);
   Value *TableIndex = Builder.CreateSub(SI->getCondition(), MinCaseVal,
                                         "switch.tableidx");
-  Value *Cmp = Builder.CreateICmpULT(TableIndex, ConstantInt::get(
-      MinCaseVal->getType(), TableSize));
-  Builder.CreateCondBr(Cmp, LookupBB, SI->getDefaultDest());
+
+  // Compute the maximum table size representable by the integer type we are
+  // switching upon.
+  const unsigned CaseSize = MinCaseVal->getType()->getPrimitiveSizeInBits();
+  const uint64_t MaxTableSize = CaseSize > 63? UINT64_MAX : 1ULL << CaseSize;
+  assert(MaxTableSize >= TableSize &&
+         "It is impossible for a switch to have more entries than the max "
+         "representable value of its input integer type's size.");
+
+  // If we have a covered lookup table, unconditionally branch to the lookup table
+  // BB. Otherwise, check if the condition value is within the case range. If it
+  // is so, branch to the new BB. Otherwise branch to SI's default destination.
+  const bool GeneratingCoveredLookupTable = MaxTableSize == TableSize;
+  if (GeneratingCoveredLookupTable) {
+    Builder.CreateBr(LookupBB);
+  } else {
+    Value *Cmp = Builder.CreateICmpULT(TableIndex, ConstantInt::get(
+                                         MinCaseVal->getType(), TableSize));
+    Builder.CreateCondBr(Cmp, LookupBB, SI->getDefaultDest());
+  }
 
   // Populate the BB that does the lookups.
   Builder.SetInsertPoint(LookupBB);
@@ -3772,7 +3788,13 @@ static bool SwitchToLookupTable(SwitchInst *SI,
   // Remove the switch.
   for (unsigned i = 0; i < SI->getNumSuccessors(); ++i) {
     BasicBlock *Succ = SI->getSuccessor(i);
-    if (Succ == SI->getDefaultDest()) continue;
+
+    // If we are not generating a covered lookup table, we will have a
+    // conditional branch from SI's parent BB to SI's default destination if our
+    // input value lies outside of our case range. Thus in that case leave the
+    // default destination BB as a predecessor of SI's parent BB.
+    if (Succ == SI->getDefaultDest() && !GeneratingCoveredLookupTable)
+      continue;
     Succ->removePredecessor(SI->getParent());
   }
   SI->eraseFromParent();
diff --git a/llvm/test/Transforms/SimplifyCFG/CoveredLookupTable.ll b/llvm/test/Transforms/SimplifyCFG/CoveredLookupTable.ll
new file mode 100644
index 0000000000000000000000000000000000000000..8b45a590bb1fb40c60c1071441176033bfa0596c
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/CoveredLookupTable.ll
@@ -0,0 +1,48 @@
+; RUN: opt -simplifycfg -S %s | FileCheck %s
+; rdar://15268442
+
+target datalayout = "e-p:64:64:64-S128-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f16:16:16-f32:32:32-f64:64:64-f128:128:128-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
+target triple = "x86_64-apple-darwin12.0.0"
+
+; CHECK-LABEL: define i3 @coveredswitch_test(
+; CHECK: entry:
+; CHECK-NEXT: sub i3 %input, -4
+; CHECK-NEXT: zext i3 %switch.tableidx to i24
+; CHECK-NEXT: mul i24 %switch.cast, 3
+; CHECK-NEXT: lshr i24 7507338, %switch.shiftamt
+; CHECK-NEXT: trunc i24 %switch.downshift to i3
+; CHECK-NEXT: ret i3 %switch.masked
+
+define i3 @coveredswitch_test(i3 %input) {
+entry:
+  switch i3 %input, label %bb8 [
+    i3 0, label %bb7
+    i3 1, label %bb
+    i3 2, label %bb3
+    i3 3, label %bb4
+    i3 4, label %bb5
+    i3 5, label %bb6
+  ]
+
+bb:                                               ; preds = %entry
+  br label %bb8
+
+bb3:                                              ; preds = %entry
+  br label %bb8
+
+bb4:                                              ; preds = %entry
+  br label %bb8
+
+bb5:                                              ; preds = %entry
+  br label %bb8
+
+bb6:                                              ; preds = %entry
+  br label %bb8
+
+bb7:                                              ; preds = %entry
+  br label %bb8
+
+bb8:                                              ; preds = %bb7, %bb6, %bb5, %bb4, %bb3, %bb, %entry
+  %result = phi i3 [ 0, %bb7 ], [ 1, %bb6 ], [ 2, %bb5 ], [ 3, %bb4 ], [ 4, %bb3 ], [ 5, %bb ], [ 6, %entry ]
+  ret i3 %result
+}