On Fri, Oct 15, 2021 at 08:05:38PM +0200, Thomas Koenig wrote:
> > with -mabi=ibmlongdouble, I see 31 and 291, while with -mabi=ieeelongdouble
> > 33 and 4931. The 0.0_8 precision/range values are 15 and 307, so neither
> > precision of C long double if it is double-double nor range matches
> > anything.
> > If we do implement double-double support, I think KIND=15 would be better
> > than KIND=17, it is true that double-double has for certain numbers much
> > higher precision than IEEE quad, but the precision depends on the numbers
> > and most of the time is smaller, the range is always smaller. And
> > the PRECISION/RANGE intrinsic numbers are also both smaller.
>
> There is one potential problem: selected_real_kind.
>
> The standard says about that...
>
> # If more than one kind type parameter value meets the criteria, the
> # value returned is the one with the smallest decimal precision, unless
> # there are several such values, in which case the smallest of these
> # kind values is returned
>
> So, selected_real_kind(25) would yield double double, and we would
> have to violate the standard there if we wanted people to have
> IEEE QP in that case.
That would be true if some kind exist for double double, whether
it is kind == 15 or kind == 17, no?
Because precision (0.0_double_double_kind) < precision (0.0_ieee_quad_kind)
so the "smallest of the kind values" doesn't trigger.
Jakub
