diff --git a/lib/CodeGen/MachineInstr.cpp b/lib/CodeGen/MachineInstr.cpp
index 97fe6f4..11ecc84 100644
--- a/lib/CodeGen/MachineInstr.cpp
+++ b/lib/CodeGen/MachineInstr.cpp
@@ -901,7 +901,8 @@
       Ops->push_back(i);
     if (MO.isUse())
       Use |= !MO.isUndef();
-    else if (MO.getSubReg())
+    else if (MO.getSubReg() && !MO.isUndef())
+      // A partial <def,undef> doesn't count as reading the register.
       PartDef = true;
     else
       FullDef = true;