Index: lib/Analysis/InstructionSimplify.cpp
===================================================================
--- lib/Analysis/InstructionSimplify.cpp
+++ lib/Analysis/InstructionSimplify.cpp
@@ -3040,7 +3040,8 @@
   if (Pred == FCmpInst::FCMP_TRUE)
     return ConstantInt::get(GetCompareTy(LHS), 1);
 
-  if (isa<UndefValue>(RHS))                  // fcmp pred X, undef -> undef
+  // fcmp pred undef, undef -> undef
+  if (isa<UndefValue>(RHS) && isa<UndefValue>(LHS))
     return UndefValue::get(GetCompareTy(LHS));
 
   // fcmp x,x -> true/false.  Not all compares are foldable.
Index: test/Transforms/InstCombine/fcmp.ll
===================================================================
--- test/Transforms/InstCombine/fcmp.ll
+++ test/Transforms/InstCombine/fcmp.ll
@@ -240,3 +240,29 @@
   %conv = zext i1 %cmp to i32
   ret i32 %conv
 }
+
+; Can't fold fcmp with undef on one side
+; To be able to fold
+;   fcmp pred x, undef -> undef
+; one need to prove that for all possible value of x, there exist
+; one value for undef that makes the comparison false, and another
+; value value for undef that makes the comparison true
+; The possibility of NaN prevents the folding
+define i32 @test18_undef(float %a) nounwind {
+; CHECK-LABEL: @test18_undef
+; CHECK-NOT: ret 0
+; CHECK-NOT: ret 1
+  %cmp = fcmp ueq float %a, undef
+  %conv = zext i1 %cmp to i32
+  ret i32 %conv
+}
+
+; Can fold fcmp with undef on both side
+;   fcmp pred undef, undef -> undef
+; even if one NaN is undef, you can alway chose
+define i1 @test19_undef() nounwind {
+; CHECK-LABEL: @test19_undef
+; CHECK: ret i1 undef
+  %cmp = fcmp ueq float undef, undef
+  ret i1 %cmp
+}