Re: [PATCH] [range-ops] Implement sqrt.

[Aldy Hernandez <aldyh@redhat.com>]

This may be DCE.

DOM uses ranger through simplify_using_ranges::fold_cond() to fold the
following conditional to false, because we know x_185 is a NAN:

 x_185 = __builtin_sqrtf (-1.0e+0);
if (x_185 ord x_185)

I believe we can do that, because there are no user observable
effects.  But DCE removes the sqrt which could trap:

Eliminating unnecessary statements:
Deleting : x_185 = __builtin_sqrtf (-1.0e+0);

Is DCE allowed to remove that sqrtf call?

Thanks.
Aldy

On Thu, Nov 17, 2022 at 5:48 PM Aldy Hernandez <aldyh@redhat.com> wrote:
>
>
>
> On 11/17/22 17:40, Aldy Hernandez wrote:
> > To go along with whatever magic we're gonna tack along to the
> > range-ops sqrt implementation, here is another revision addressing the
> > VARYING issue you pointed out.
> >
> > A few things...
> >
> > Instead of going through trees, I decided to call do_mpfr_arg1
> > directly.  Let's not go the wide int <-> tree rat hole in this one.
> >
> > The function do_mpfr_arg1 bails on +INF, so I had to handle it manually.
> >
> > There's a regression in gfortran.dg/ieee/ieee_6.f90, which I'm not
> > sure how to handle.  We are failing because we are calculating
> > sqrt(-1) and expecting certain IEEE flags set.  These flags aren't
> > set, presumably because we folded sqrt(-1) into a NAN directly:
> >
> >      // All negatives.
> >      if (real_compare (LT_EXPR, &lh_ub, &dconst0))
> >        {
> >      real_nan (&lb, "", 0, TYPE_MODE (type));
> >      ub = lb;
> >      maybe_nan = true;
> >      return;
> >        }
>
> FWIW, we could return [-0.0, +INF] +-NAN which would keep us from
> eliding the sqrt, but it'd be a pity to keep the sqrt unless it's
> mandated by some IEEE canon.
>
> Aldy