On Fri, Oct 14, 2022 at 4:33 PM Jakub Jelinek <jakub@redhat.com> wrote:
>
> On Fri, Oct 14, 2022 at 04:26:50PM +0200, Aldy Hernandez via Gcc-patches wrote:
> > Similar to what we do for NANs when !HONOR_NANS and Inf when
> > flag_finite_math_only, we can remove -0.0 from the range at creation
> > time.
> >
> > We were kinda sorta doing this because there is a bug in
> > real_isdenormal that is causing flush_denormals_to_zero to saturate
> > [x, -0.0] to [x, +0.0] when !HONOR_SIGNED_ZEROS.  Fixing this bug
> > (upcoming), causes us to leave -0.0 in places where we aren't
> > expecting it (the intersection code).
> >
> > gcc/ChangeLog:
> >
> >       * value-range.cc (frange::set): Drop -0.0 for !HONOR_SIGNED_ZEROS.
>
> This looks wrong to me.
> !HONOR_NANS is different from !HONOR_SIGNED_ZEROS.
> The former says that either NaNs aren't supported or if they appear,
> it will be UB.
> The latter says that either -0.0 doesn't exist, or user doesn't care
> if -0.0 or 0.0 is used.
>
> So, what you do is ok for !MODE_HAS_SIGNED_ZEROS (TYPE_MODE (m_type)),
> but otherwise we want to canonicalize [x, -0.0] to [x, 0.0] and
> [0.0, y] to [-0.0, y].

If the user doesn't care, I would expect  they'd be ok with it being
+0.0, but I must say, this is way beyond my paygrade.

How does this patch in testing look?

Thanks.
Aldy