diff --git a/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp b/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
index 919b45d3c7b1..add5432aa276 100644
--- a/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
@@ -86,7 +86,7 @@ static void ConnectProlog(Loop *L, Value *BECount, unsigned Count,
       if (L->contains(PN)) {
         NewPN->addIncoming(PN->getIncomingValueForBlock(NewPH), OrigPH);
       } else {
-        NewPN->addIncoming(Constant::getNullValue(PN->getType()), OrigPH);
+        NewPN->addIncoming(UndefValue::get(PN->getType()), OrigPH);
       }
 
       Value *V = PN->getIncomingValueForBlock(Latch);
diff --git a/llvm/test/Transforms/LoopUnroll/X86/mmx.ll b/llvm/test/Transforms/LoopUnroll/X86/mmx.ll
new file mode 100644
index 000000000000..2c4aa086e83c
--- /dev/null
+++ b/llvm/test/Transforms/LoopUnroll/X86/mmx.ll
@@ -0,0 +1,24 @@
+; RUN: opt < %s -S -loop-unroll | FileCheck %s
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define x86_mmx @f() #0 {
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %phi = phi i32 [ 1, %entry ], [ %add, %for.body ]
+  %add = add i32 %phi, 1
+  %cmp = icmp eq i32 %phi, 0
+  br i1 %cmp, label %exit, label %for.body
+
+exit:                                             ; preds = %for.body
+  %ret = phi x86_mmx [ undef, %for.body ]
+  ; CHECK: %[[ret_unr:.*]] = phi x86_mmx [ undef,
+  ; CHECK: %[[ret_ph:.*]]  = phi x86_mmx [ undef,
+  ; CHECK: %[[ret:.*]] = phi x86_mmx [ %[[ret_unr]], {{.*}} ], [ %[[ret_ph]]
+  ; CHECK: ret x86_mmx %[[ret]]
+  ret x86_mmx %ret
+}
+
+attributes #0 = { "target-cpu"="x86-64" }