diff --git a/lib/CodeGen/DeadMachineInstructionElim.cpp b/lib/CodeGen/DeadMachineInstructionElim.cpp
index 305ec0e7988..0d043f795a2 100644
--- a/lib/CodeGen/DeadMachineInstructionElim.cpp
+++ b/lib/CodeGen/DeadMachineInstructionElim.cpp
@@ -126,7 +126,10 @@ bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) {
           unsigned Reg = MO.getReg();
           if (Reg != 0 && TargetRegisterInfo::isPhysicalRegister(Reg)) {
             LivePhysRegs.reset(Reg);
-            for (const unsigned *AliasSet = TRI->getAliasSet(Reg);
+            // Check the subreg set, not the alias set, because a def
+            // of a super-register may still be partially live after
+            // this def.
+            for (const unsigned *AliasSet = TRI->getSubRegisters(Reg);
                  *AliasSet; ++AliasSet)
               LivePhysRegs.reset(*AliasSet);
           }