diff --git a/lib/CodeGen/MachineInstr.cpp b/lib/CodeGen/MachineInstr.cpp
index e553a04..46ea003 100644
--- a/lib/CodeGen/MachineInstr.cpp
+++ b/lib/CodeGen/MachineInstr.cpp
@@ -255,6 +255,10 @@
           OS << "imp-";
         OS << "def";
         NeedComma = true;
+        // <def,read-undef> only makes sense when getSubReg() is set.
+        // Don't clutter the output otherwise.
+        if (isUndef() && getSubReg())
+          OS << ",read-undef";
       } else if (isImplicit()) {
           OS << "imp-use";
           NeedComma = true;
@@ -271,7 +275,7 @@
           OS << "dead";
           NeedComma = true;
         }
-        if (isUndef()) {
+        if (isUndef() && isUse()) {
           if (NeedComma) OS << ',';
           OS << "undef";
           NeedComma = true;