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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/99117] [9/10/11 Regression] cannot accumulate
 std::valarray
Date: Tue, 23 Feb 2021 11:43:34 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99117

--- Comment #12 from rguenther at suse dot de <rguenther at suse dot de> ---
On Tue, 23 Feb 2021, redi at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99117
>=20
> --- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
> So is Jakub wrong in comment 5 when he suggests that this should "lose" t=
he
> __restrict__ qualification?
>=20
>       _Tp* __p (__a._M_data);

Yes.  __p is clearly based on __a._M_data and thus it can be used
to access data pointed to by the restrict qualified __a._M_data
which in turn means we can, if we prove value equivalence, place
the same restrictions on accesses via __p as we can on accesses
via __a._M_data.  It would be way more awkward implementation-wise
if we couldn't do that.

> Is there a better way to drop that qual so that we can assign something t=
o the
> elements of __a._M_data that aliases itself?
>=20
> Presumably this would be guaranteed to work?
>=20
>       _Tp* __p (__a._M_data);
>       for (size_t __i =3D 0; __i < __n; ++__i, ++__p)
>       {
>         _Tp __tmp =3D __e[__i];
>         *__p =3D __tmp;
>       }

That's no different.  There's no way to lose restrict qualification
but to obfuscate the code in a way to make the compiler not see
the "based-on" relation of __a._M_data and the pointer used to=20
access the data.  Passing __a by reference should also run into
limitations of points-to analysis but I guess it would be an ABI
change.=