Re: [COMMITTED] Be even more conservative in intersection of NANs.

[Richard Biener <richard.guenther@gmail.com>]

On Mon, Sep 5, 2022 at 11:12 AM Aldy Hernandez <aldyh@redhat.com> wrote:
>
> On Mon, Sep 5, 2022 at 11:06 AM Jakub Jelinek <jakub@redhat.com> wrote:
> >
> > On Mon, Sep 05, 2022 at 11:00:54AM +0200, Richard Biener wrote:
> > > On Mon, Sep 5, 2022 at 8:24 AM Aldy Hernandez via Gcc-patches
> > > <gcc-patches@gcc.gnu.org> wrote:
> > > >
> > > > Intersecting two ranges where one is a NAN is keeping the sign bit of
> > > > the NAN range.  This is not correct as the sign bits may not match.
> > > >
> > > > I think the only time we're absolutely sure about the intersection of
> > > > a NAN and something else, is when both are a NAN with exactly the same
> > > > properties (sign bit).  If we're intersecting two NANs of differing
> > > > sign, we can decide later whether that's undefined or just a NAN with
> > > > no known sign.  For now I've done the latter.
> > > >
> > > > I'm still mentally working on intersections involving NANs, especially
> > > > if we want to keep track of signbits.  For now, let's be extra careful
> > > > and only do things we're absolutely sure about.
> > > >
> > > > Later we may want to fold the intersect of [NAN,NAN] and say [3,5]
> > > > with the posibility of NAN, to a NAN, but I'm not 100% sure.
> > >
> > > The intersection of [NAN, NAN] and [3, 5] is empty.  The intersection
> > > of [NAN, NAN] and VARYING is [NAN, NAN].
> >
> > I think [3.0, 5.0] printed that way currently means U maybe NAN,
> > it would be [3.0, 5.0] !NAN if it was known not to be NAN.
>
> Right.  I don't print any of the "maybe" properties, just if they're
> definitely set or definitely clear.  I'm open to suggestions as to how
> to display them.  Perhaps NAN, !NAN, ?NAN.

There's no NAN tristate.  Your "definitely NAN" would be simply
][ NAN, that is, the value range only contains NAN.  Your !NAN
is <whatever range> and non NAN.  Likewise for the sign, the
range either includes -NAN and NAN or one or none of those.
For signed zeros you either have [-0, upper-bound] or [0, upper-bound]
where it either includes both -0 and 0 or just one of them

> I'm mostly worried about removing a NAN from the IL that was going to
> signal, or some such.  While I agree with you Richard, I just want to
> make real sure, because getting something wrong in the frange or
> range-ops bowels means the problem becomes pervasive to all of ranger
> ...and threader...and loop ch...and vrp, etc etc.  I just want to take
> more time to test things.  I promise it won't stay varying too long.

There's sNANs and qNANs, but I think for value-ranges we should
concern ourselves only with qNANs for now and leave sNANs VARYING.
All operations only ever produce qNANs (loads can produce sNANs).

Richard.

> Aldy
>