diff --git a/lib/CodeGen/MachineCopyPropagation.cpp b/lib/CodeGen/MachineCopyPropagation.cpp
index 0e9d3a4..9ed7b73 100644
--- a/lib/CodeGen/MachineCopyPropagation.cpp
+++ b/lib/CodeGen/MachineCopyPropagation.cpp
@@ -99,6 +99,31 @@
   return true;
 }
 
+/// isNopCopy - Return true if the specified copy is really a nop. That is
+/// if the source of the copy is the same of the definition of the copy that
+/// supplied the source. If the source of the copy is a sub-register than it
+/// must check the sub-indices match. e.g.
+/// ecx = mov eax
+/// al  = mov cl
+/// But not
+/// ecx = mov eax
+/// al  = mov ch
+static bool isNopCopy(MachineInstr *CopyMI, unsigned Def, unsigned Src,
+                      const TargetRegisterInfo *TRI) {
+  unsigned SrcSrc = CopyMI->getOperand(1).getReg();
+  if (Def == SrcSrc)
+    return true;
+  if (TRI->isSubRegister(SrcSrc, Def)) {
+    unsigned SrcDef = CopyMI->getOperand(0).getReg();
+    unsigned SubIdx = TRI->getSubRegIndex(SrcSrc, Def);
+    if (!SubIdx)
+      return false;
+    return SubIdx == TRI->getSubRegIndex(SrcDef, Src);
+  }
+
+  return false;
+}
+
 bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
   SmallSetVector<MachineInstr*, 8> MaybeDeadCopies; // Candidates for deletion
   DenseMap<unsigned, MachineInstr*> AvailCopyMap;   // Def -> available copies map
@@ -122,10 +147,9 @@
       DenseMap<unsigned, MachineInstr*>::iterator CI = AvailCopyMap.find(Src);
       if (CI != AvailCopyMap.end()) {
         MachineInstr *CopyMI = CI->second;
-        unsigned SrcSrc = CopyMI->getOperand(1).getReg();
         if (!ReservedRegs.test(Def) &&
             (!ReservedRegs.test(Src) || NoInterveningSideEffect(CopyMI, MI)) &&
-            (SrcSrc == Def || TRI->isSubRegister(SrcSrc, Def))) {
+            isNopCopy(CopyMI, Def, Src, TRI)) {
           // The two copies cancel out and the source of the first copy
           // hasn't been overridden, eliminate the second one. e.g.
           //  %ECX<def> = COPY %EAX<kill>