Index: llvm/trunk/lib/CodeGen/UnreachableBlockElim.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/UnreachableBlockElim.cpp
+++ llvm/trunk/lib/CodeGen/UnreachableBlockElim.cpp
@@ -207,11 +207,12 @@
           MachineRegisterInfo &MRI = F.getRegInfo();
           unsigned InputSub = Input.getSubReg();
           if (InputSub == 0 &&
-              MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg))) {
+              MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg)) &&
+              !Input.isUndef()) {
             MRI.replaceRegWith(OutputReg, InputReg);
           } else {
             // The input register to the PHI has a subregister or it can't be
-            // constrained to the proper register class:
+            // constrained to the proper register class or it is undef:
             // insert a COPY instead of simply replacing the output
             // with the input.
             const TargetInstrInfo *TII = F.getSubtarget().getInstrInfo();
Index: llvm/trunk/test/CodeGen/MIR/X86/unreachable-mbb-undef-phi.mir
===================================================================
--- llvm/trunk/test/CodeGen/MIR/X86/unreachable-mbb-undef-phi.mir
+++ llvm/trunk/test/CodeGen/MIR/X86/unreachable-mbb-undef-phi.mir
@@ -0,0 +1,38 @@
+# RUN: llc %s -o - -run-pass=processimpdefs -run-pass=unreachable-mbb-elimination | FileCheck %s
+---
+name:            f
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: gr32, preferred-register: '' }
+  - { id: 1, class: gr32, preferred-register: '' }
+  - { id: 2, class: gr32, preferred-register: '' }
+body:             |
+  bb.0:
+    %0 = IMPLICIT_DEF
+    JMP_1 %bb.1
+
+  bb.1:
+    %1 = PHI %0, %bb.0, %2, %bb.2
+    %2 = ADD32ri8 killed %1, 1, implicit-def %eflags
+    JMP_1 %bb.3
+
+  bb.2:
+    JMP_1 %bb.1
+
+  bb.3:
+...
+
+# bb2 above is dead and should be removed and the PHI should be replaced with a
+# COPY from an undef value since the bb0 value in the PHI is undef.
+
+# CHECK:  bb.0:
+# CHECK:    successors: %bb.1
+# CHECK:    JMP_1 %bb.1
+
+# CHECK:  bb.1:
+# CHECK:    successors: %bb.2
+# CHECK:    [[TMP1:%[0-9]+]] = COPY undef %{{[0-9]+}}
+# CHECK:    %{{[0-9]+}} = ADD32ri8 killed [[TMP1]], 1
+# CHECK:    JMP_1 %bb.2
+
+# CHECK:  bb.2: