diff --git a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index bcb0ffe..c2711c8 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -2483,7 +2483,8 @@
     if (OpOpcode == ISD::SIGN_EXTEND || OpOpcode == ISD::ZERO_EXTEND)
       return getNode(OpOpcode, DL, VT, Operand.getNode()->getOperand(0));
     else if (OpOpcode == ISD::UNDEF)
-      return getUNDEF(VT);
+      // sext(undef) = 0, because the top bits will all be the same.
+      return getConstant(0, VT);
     break;
   case ISD::ZERO_EXTEND:
     assert(VT.isInteger() && Operand.getValueType().isInteger() &&
@@ -2498,6 +2499,9 @@
     if (OpOpcode == ISD::ZERO_EXTEND)   // (zext (zext x)) -> (zext x)
       return getNode(ISD::ZERO_EXTEND, DL, VT,
                      Operand.getNode()->getOperand(0));
+    else if (OpOpcode == ISD::UNDEF)
+      // zext(undef) = 0, because the top bits will be zero.
+      return getConstant(0, VT);
     break;
   case ISD::ANY_EXTEND:
     assert(VT.isInteger() && Operand.getValueType().isInteger() &&
diff --git a/test/CodeGen/X86/machine-cse.ll b/test/CodeGen/X86/machine-cse.ll
index e284776..d819fc8 100644
--- a/test/CodeGen/X86/machine-cse.ll
+++ b/test/CodeGen/X86/machine-cse.ll
@@ -6,11 +6,11 @@
 %struct.s2 = type { i32, i8*, i8*, [256 x %struct.s1*], [8 x i32], i64, i8*, i32, i64, i64, i32, %struct.s3*, %struct.s3*, [49 x i64] }
 %struct.s3 = type { %struct.s3*, %struct.s3*, i32, i32, i32 }
 
-define fastcc i8* @t(i64 %size) nounwind {
+define fastcc i8* @t(i32 %base) nounwind {
 entry:
 ; CHECK: t:
 ; CHECK: leaq (%rax,%rax,4)
-  %0 = zext i32 undef to i64
+  %0 = zext i32 %base to i64
   %1 = getelementptr inbounds %struct.s2* null, i64 %0
   br i1 undef, label %bb1, label %bb2