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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/113770] _Float64x support on g++ 13.2.0
Date: Mon, 05 Feb 2024 15:16:51 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113770

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Masahide Kashiwagi from comment #5)
> Thank you very much.
>=20
> I understand that g++ 13 no longer supports _Float64x in a very limited w=
ay.

You understand it wrong.  _Float64x wasn't supported at all in GCC 12 and
earlier.
The reason you don't get an error is because of glibc typedefs.
Now it is supported in a limited way.

> Does this mean that if I want to use 80-bit extended floating point numbe=
rs
> in g++ 13 or later, is it best to use long double?
>=20
> Since long double may not be 80-bit on non-x86 architectures, I wanted to
> use _Float64x or __float80, which are clearly 80-bit, if possible.

_Float64x certainly doesn't mean 80-bit.  In C, it doesn't have to be suppo=
rted
at all (such as when IEEE double is the largest type supported by hw and/or
emulation), or can be IEEE quad, 80-bit extended or whatever else.
So, if you need only 80-bit and want an error otherwise, use __float80.=