Re: [PATCH] [PR68097] frange::set_nonnegative should not contain -NAN.

[Aldy Hernandez <aldyh@redhat.com>]

On Tue, Sep 20, 2022 at 2:51 PM Michael Matz <matz@suse.de> wrote:
>
> Hello,
>
> On Tue, 20 Sep 2022, Aldy Hernandez wrote:
>
> > > FWIW, in IEEE, 'abs' (like 'copy, 'copysign' and 'negate') are not
> > > arithmetic, they are quiet-computational.  Hence they don't rise
> > > anything, not even for sNaNs; they copy the input bits and appropriately
> > > modify the bit pattern according to the specification (i.e. fiddle the
> > > sign bit).
> > >
> > > That also means that a predicate like negative_p(x) that would be
> > > implemented ala
> > >
> > >   copysign(1.0, x) < 0.0
> >
> > I suppose this means -0.0 is not considered negative,
>
> It would be considered negative if the predicate is implemented like
> above:
>    copysign(1.0, -0.0) == -1.0
>
> But really, that depends on what _our_ definition of negative_p is
> supposed to be.  I think the most reasonable definition is indeed similar
> to above, which in turn is equivalent to simply looking at the sign bit
> (which is what copysign() does), i.e. ...
>
> > though it has
> > the signbit set?  FWIW, on real_value's real_isneg() returns true for
> > -0.0 because it only looks at the sign.
>
> ... this seems the sensible thing.  I just wanted to argue the case that
> set_negative (or the like) which "sets" the sign bit does not make the
> nan-ness go away.  They are orthogonal.
>
> > > deal with NaNs just fine and is required to correctly capture the sign of
> > > 'x'.  If frange::set_nonnegative is supposed to be used in such contexts
> > > (and I think it's a good idea if that were the case), then set_nonnegative
> > > does _not_ imply no-NaN.
> > >
> > > In particular I would assume that, given an VAYRING frange FR, that
> > > FR.set_nonnegative() would result in an frange {[+0.0,+inf],+nan} .
> >
> > That was my understanding as well, and what my original patch did.
> > But again, I'm just the messenger.
>
> Ah, I obviously haven't followed the thread carefully then.  If that's
> what it was doing then IMO it was the right thing.

This brings me back to my original patch :).

Richard, do you agree nonnegative should be [0.0, +INF] U +NAN.

Thanks.
Aldy