Index: llvm/trunk/lib/Transforms/Vectorize/SLPVectorizer.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ llvm/trunk/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -785,9 +785,10 @@
           break;
         case ReorderingMode::Opcode:
           // We accept both Instructions and Undefs, but with different scores.
-          if ((isa<Instruction>(Op) &&
+          if ((isa<Instruction>(Op) && isa<Instruction>(OpLastLane) &&
                cast<Instruction>(Op)->getOpcode() ==
                    cast<Instruction>(OpLastLane)->getOpcode()) ||
+              (isa<UndefValue>(OpLastLane) && isa<Instruction>(Op)) ||
               isa<UndefValue>(Op)) {
             // An instruction has a higher score than an undef.
             unsigned Score = (isa<UndefValue>(Op)) ? GoodScore : BestScore;
Index: llvm/trunk/test/Transforms/SLPVectorizer/X86/crash_reordering_undefs.ll
===================================================================
--- llvm/trunk/test/Transforms/SLPVectorizer/X86/crash_reordering_undefs.ll
+++ llvm/trunk/test/Transforms/SLPVectorizer/X86/crash_reordering_undefs.ll
@@ -0,0 +1,47 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -slp-vectorizer -S < %s -mtriple=x86_64-unknown-linux -mcpu=corei7-avx | FileCheck %s
+
+define i32 @crash_reordering_undefs() {
+; CHECK-LABEL: @crash_reordering_undefs(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[OR0:%.*]] = or i64 undef, undef
+; CHECK-NEXT:    [[CMP0:%.*]] = icmp eq i64 undef, [[OR0]]
+; CHECK-NEXT:    [[ADD0:%.*]] = select i1 [[CMP0]], i32 65536, i32 65537
+; CHECK-NEXT:    [[ADD1:%.*]] = add i32 undef, [[ADD0]]
+; CHECK-NEXT:    [[CMP1:%.*]] = icmp eq i64 undef, undef
+; CHECK-NEXT:    [[ADD2:%.*]] = select i1 [[CMP1]], i32 65536, i32 65537
+; CHECK-NEXT:    [[ADD3:%.*]] = add i32 [[ADD1]], [[ADD2]]
+; CHECK-NEXT:    [[CMP2:%.*]] = icmp eq i64 undef, undef
+; CHECK-NEXT:    [[ADD4:%.*]] = select i1 [[CMP2]], i32 65536, i32 65537
+; CHECK-NEXT:    [[ADD5:%.*]] = add i32 [[ADD3]], [[ADD4]]
+; CHECK-NEXT:    [[ADD6:%.*]] = add i32 [[ADD5]], undef
+; CHECK-NEXT:    [[ADD7:%.*]] = add i32 [[ADD6]], undef
+; CHECK-NEXT:    [[ADD8:%.*]] = add i32 [[ADD7]], undef
+; CHECK-NEXT:    [[OR1:%.*]] = or i64 undef, undef
+; CHECK-NEXT:    [[CMP3:%.*]] = icmp eq i64 undef, [[OR1]]
+; CHECK-NEXT:    [[ADD9:%.*]] = select i1 [[CMP3]], i32 65536, i32 65537
+; CHECK-NEXT:    [[ADD10:%.*]] = add i32 [[ADD8]], [[ADD9]]
+; CHECK-NEXT:    [[ADD11:%.*]] = add i32 [[ADD10]], undef
+; CHECK-NEXT:    ret i32 [[ADD11]]
+;
+entry:
+  %or0 = or i64 undef, undef
+  %cmp0 = icmp eq i64 undef, %or0
+  %add0 = select i1 %cmp0, i32 65536, i32 65537
+  %add1 = add i32 undef, %add0
+  %cmp1 = icmp eq i64 undef, undef
+  %add2 = select i1 %cmp1, i32 65536, i32 65537
+  %add3 = add i32 %add1, %add2
+  %cmp2 = icmp eq i64 undef, undef
+  %add4 = select i1 %cmp2, i32 65536, i32 65537
+  %add5 = add i32 %add3, %add4
+  %add6 = add i32 %add5, undef
+  %add7 = add i32 %add6, undef
+  %add8 = add i32 %add7, undef
+  %or1 = or i64 undef, undef
+  %cmp3 = icmp eq i64 undef, %or1
+  %add9 = select i1 %cmp3, i32 65536, i32 65537
+  %add10 = add i32 %add8, %add9
+  %add11 = add i32 %add10, undef
+  ret i32 %add11
+}