diff --git a/lib/Target/Mips/MipsISelDAGToDAG.cpp b/lib/Target/Mips/MipsISelDAGToDAG.cpp
index 6e5bad7..027c171 100644
--- a/lib/Target/Mips/MipsISelDAGToDAG.cpp
+++ b/lib/Target/Mips/MipsISelDAGToDAG.cpp
@@ -213,7 +213,8 @@
     MachineInstr *MI = MO.getParent();
 
     // Do not replace if it is a phi's operand or is tied to def operand.
-    if (MI->isPHI() || MI->isRegTiedToDefOperand(U.getOperandNo()))
+    if (MI->isPHI() || MI->isRegTiedToDefOperand(U.getOperandNo()) ||
+        MI->isPseudo())
       continue;
 
     MO.setReg(ZeroReg);
diff --git a/test/CodeGen/Mips/atomic.ll b/test/CodeGen/Mips/atomic.ll
index a4763b1..e181610 100644
--- a/test/CodeGen/Mips/atomic.ll
+++ b/test/CodeGen/Mips/atomic.ll
@@ -242,3 +242,19 @@
 ; CHECK:   sync 0
 }
 
+; make sure that this assertion in
+; TwoAddressInstructionPass::TryInstructionTransform does not fail:
+;
+; line 1203: assert(TargetRegisterInfo::isVirtualRegister(regB) &&
+;
+; it failed when MipsDAGToDAGISel::ReplaceUsesWithZeroReg replaced an
+; operand of an atomic instruction with register $zero. 
+@a = external global i32
+
+define i32 @zeroreg() nounwind {
+entry:
+  %0 = cmpxchg i32* @a, i32 1, i32 0 seq_cst
+  %1 = icmp eq i32 %0, 1
+  %conv = zext i1 %1 to i32
+  ret i32 %conv
+}