Index: lib/CodeGen/UnreachableBlockElim.cpp
===================================================================
--- lib/CodeGen/UnreachableBlockElim.cpp
+++ 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: test/CodeGen/X86/unreachable-mbb-undef-phi.ll
===================================================================
--- /dev/null
+++ test/CodeGen/X86/unreachable-mbb-undef-phi.ll
@@ -0,0 +1,33 @@
+; RUN: llc -optimize-regalloc -O0 %s -o - -stop-after=unreachable-mbb-elimination | FileCheck %s
+
+define void @f() {
+bb0:
+  br label %bb1
+
+bb1:                                           ; preds = %bb2, %bb0
+  %inc1 = phi i32 [ %inc, %bb2 ], [ undef, %bb0 ]
+  %inc = add nsw i32 %inc1, 1
+  %tobool = icmp eq i32 0, 0
+  br i1 %tobool, label %bb3, label %bb2
+
+bb2:                                           ; preds = %bb1
+  br label %bb1
+
+bb3:                                           ; preds = %bb1
+  ret void
+}
+
+; bb2 is dead and should be removed and the add input should be marked with
+; undef since the bb0 value in the PHI is undef. Shouldn't crash.
+
+; CHECK: bb.0.bb0:
+; CHECK:   successors: %bb.1.bb1
+; CHECK:   JMP_1 %bb.1.bb1
+
+; CHECK: bb.1.bb1:
+; CHECK:   successors: %bb.2.bb3
+; CHECK:   dead %eax = ADD32ri8 undef %eax, 1, implicit-def dead %eflags
+; CHECK:   JMP_1 %bb.2.bb3
+
+; CHECK: bb.2.bb3:
+; CHECK:   RETQ