[llvm-dev] Why does FPBinOp(X, undef) -> NaN?

[Cameron McInally via llvm-dev]

On Fri, Feb 7, 2020 at 1:48 PM Roman Lebedev <lebedev.ri at gmail.com> wrote:
> >     case ISD::ADD:
> >     case ISD::SUB:
>
> >     case ISD::UDIV:
> >     case ISD::SDIV:
> >     case ISD::UREM:
> >     case ISD::SREM:
> >       return getUNDEF(VT);       // fold op(arg1, undef) -> undef
> For these 4 - because divisor 'undef' can be defined to any value, including 0,
> but divisor can't be 0..

That makes sense. It's undefined behavior, so the "some value" as
output is fine.

The ADD and SUB I don't understand. Perhaps it's because there's no
single value for undef that will always give one defined result? Is
that a requirement?