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From: Richard Biener <richard.guenther@gmail.com>
Date: Mon, 5 Sep 2022 11:13:58 +0200
Message-ID: <CAFiYyc0zUGiQGCoSZBOQubrYWOrs-Lw6jciGy95jKnfpZ+6FDw@mail.gmail.com>
Subject: Re: [COMMITTED] Be even more conservative in intersection of NANs.
To: Jakub Jelinek <jakub@redhat.com>
Cc: Aldy Hernandez <aldyh@redhat.com>, GCC patches <gcc-patches@gcc.gnu.org>
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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.

Uh, that's confusing.  So [3, 5] U maybe NAN intersected with
][ NAN is ][ NAN.  [3, 5] !NAN intersected with ][ NAN is ][ !NAN.

In fact [3, 5] U maybe NAN is just [3, 5] U NAN, there's no "maybe" ranges,
if the value may be NAN then NAN is in the value-range.  So it's either
[3, 5] U NAN or [3, 5] (without U NAN).

Richard.

>
>         Jakub
>