On Tue, Sep 6, 2022 at 9:59 AM Jakub Jelinek <jakub@redhat.com> wrote:
>
> On Tue, Sep 06, 2022 at 09:49:55AM +0200, Aldy Hernandez wrote:
> > On Tue, Sep 6, 2022 at 9:44 AM Jakub Jelinek <jakub@redhat.com> wrote:
> > >
> > > On Tue, Sep 06, 2022 at 09:40:59AM +0200, Aldy Hernandez wrote:
> > > > if (x <= Inf)
> > >
> > > This will be [-Inf, Inf] !NAN on the true side and
> > > NAN (either sign) on the false side indeed.
> > >
> > > > if (x < -Inf)
> > >
> > > will be NAN (either sign) on the true side and
> > > [-Inf, Inf] !NAN on the false side.
> >
> > Sweet, that's exactly what I thought, thus the patch.
> >
> > Furthermore, for !HONOR_NANS I would expect the NAN sides above to be
> > UNDEFINED/unreachable.  That is, the false side of x <= Inf when
> > !HONOR_NANS is unreachable.
>
> In practice, there is no real format that has NaNs and doesn't have Infs
> or vice versa and similarly we have just one switch to cover both Infinities
> and NaNs, so either both are supported, or neither of them, or both
> are supported but neither of them should appear in a valid program
> (-ffinite-math-only on most floating point formats).
> So the answer in that case is a little bit fuzzy because one shouldn't
> compare against infinity in that case (or for !MODE_HAS_INFINITIES even
> can't).  But sure, if NaNs aren't possible or can't appear and you compare
> x <= Largest_possible_float, then it is always true and so UNDEFINED on the
> false edge.

OK, let's leave it as undefined to be consistent.

Come to think of it, perhaps we could represent the endpoints
(varying, [x, +INF], etc) as the min/max representable values for the
type (for !HONOR_NANS).  I don't think it would make a big difference,
but we might get better results for some corner cases.

Question...for !HONOR_NANS or !HONOR_INFINITIES or whatever, say the
range for the domain is [-MIN, +MAX] for the min and max representable
numbers.  What happens for MAX+1?  Is that undefined?  I wonder what
real.cc does for that.

Attached is the final version of the patch I'm pushing.  Tested (+mpfr
tests) on x86-64 Linux.

Aldy