Index: llvm/trunk/lib/Analysis/InstructionSimplify.cpp
===================================================================
--- llvm/trunk/lib/Analysis/InstructionSimplify.cpp
+++ llvm/trunk/lib/Analysis/InstructionSimplify.cpp
@@ -3045,9 +3045,16 @@
     std::swap(LHS, RHS);
     Pred = CmpInst::getSwappedPredicate(Pred);
   }
+  assert(!isa<UndefValue>(LHS) && "Unexpected icmp undef,%X");
 
   Type *ITy = GetCompareTy(LHS); // The return type.
 
+  // For EQ and NE, we can always pick a value for the undef to make the
+  // predicate pass or fail, so we can return undef.
+  // Matches behavior in llvm::ConstantFoldCompareInstruction.
+  if (isa<UndefValue>(RHS) && ICmpInst::isEquality(Pred))
+    return UndefValue::get(ITy);
+
   // icmp X, X -> true/false
   // icmp X, undef -> true/false because undef could be X.
   if (LHS == RHS || isa<UndefValue>(RHS))
Index: llvm/trunk/test/Transforms/InstCombine/icmp.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/icmp.ll
+++ llvm/trunk/test/Transforms/InstCombine/icmp.ll
@@ -69,14 +69,14 @@
 ; PR4837
 define <2 x i1> @test5_eq(<2 x i64> %x) {
 ; CHECK-LABEL: @test5_eq(
-; CHECK-NEXT:    ret <2 x i1> <i1 true, i1 true>
+; CHECK-NEXT:    ret <2 x i1> undef
 ;
   %V = icmp eq <2 x i64> %x, undef
   ret <2 x i1> %V
 }
 define <2 x i1> @test5_ne(<2 x i64> %x) {
 ; CHECK-LABEL: @test5_ne(
-; CHECK-NEXT:    ret <2 x i1> zeroinitializer
+; CHECK-NEXT:    ret <2 x i1> undef
 ;
   %V = icmp ne <2 x i64> %x, undef
   ret <2 x i1> %V
Index: llvm/trunk/test/Transforms/NewGVN/pr31613.ll
===================================================================
--- llvm/trunk/test/Transforms/NewGVN/pr31613.ll
+++ llvm/trunk/test/Transforms/NewGVN/pr31613.ll
@@ -71,32 +71,35 @@
 
 declare void @c.d.p(i64, i8*)
 
-define void @e() {
+define void @e(i32 %a0, i32 %a1, %struct.a** %p2) {
 ; CHECK-LABEL: @e(
 ; CHECK-NEXT:    [[F:%.*]] = alloca i32
-; CHECK-NEXT:    store i32 undef, i32* [[F]], !g !0
+; CHECK-NEXT:    store i32 [[A0:%.*]], i32* [[F]], !g !0
 ; CHECK-NEXT:    br label [[H:%.*]]
 ; CHECK:       h:
 ; CHECK-NEXT:    call void @c.d.p(i64 8, i8* undef)
+; CHECK-NEXT:    [[I:%.*]] = load i32, i32* [[F]]
 ; CHECK-NEXT:    [[J:%.*]] = load i32, i32* null
-; CHECK-NEXT:    br i1 true, label [[L:%.*]], label [[Q:%.*]]
+; CHECK-NEXT:    [[K:%.*]] = icmp eq i32 [[I]], [[J]]
+; CHECK-NEXT:    br i1 [[K]], label [[L:%.*]], label [[Q:%.*]]
 ; CHECK:       l:
 ; CHECK-NEXT:    br label [[R:%.*]]
 ; CHECK:       q:
-; CHECK-NEXT:    store i8 undef, i8* null
+; CHECK-NEXT:    [[M:%.*]] = load %struct.a*, %struct.a** null
 ; CHECK-NEXT:    br label [[R]]
 ; CHECK:       r:
 ; CHECK-NEXT:    switch i32 undef, label [[N:%.*]] [
 ; CHECK-NEXT:    i32 0, label [[S:%.*]]
 ; CHECK-NEXT:    ]
 ; CHECK:       s:
+; CHECK-NEXT:    store i32 [[A1:%.*]], i32* [[F]], !g !0
 ; CHECK-NEXT:    br label [[H]]
 ; CHECK:       n:
-; CHECK-NEXT:    [[O:%.*]] = load %struct.a*, %struct.a** null
+; CHECK-NEXT:    [[O:%.*]] = load %struct.a*, %struct.a** [[P2:%.*]]
 ; CHECK-NEXT:    ret void
 ;
   %f = alloca i32
-  store i32 undef, i32* %f, !g !0
+  store i32 %a0, i32* %f, !g !0
   br label %h
 
 h:                                                ; preds = %s, %0
@@ -120,11 +123,11 @@
   ]
 
 s:                                                ; preds = %r
-  store i32 undef, i32* %f, !g !0
+  store i32 %a1, i32* %f, !g !0
   br label %h
 
 n:                                                ; preds = %r
-  %o = load %struct.a*, %struct.a** null
+  %o = load %struct.a*, %struct.a** %p2
   %2 = bitcast %struct.a* %o to %struct.b*
   ret void
 }