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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/113099] locale without RTTI uses dynamic_cast before
 gcc 13.2 or has ODR violation since gcc 13.2
Date: Tue, 26 Dec 2023 12:31:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113099

--- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to andysem from comment #11)
> > I'm not sure what you mean by "the compiler is free to generate code th=
at takes it into account." Takes what into account? What problem does that =
freedom cause?
>=20
> I mean the compiler could move (some part of) dynamic_cast to precede the
> check for the standard facet. I.e. of something like this:
>=20
>   template< typename _Facet >
>   const _Facet* __try_use_facet(locale const& loc)
>   {
>     const size_t __i =3D _Facet::id._M_id();
>     const locale::facet** __facets =3D __loc._M_impl->_M_facets;
>     const _Facet* __dyn_facet =3D __dynamic_cast< const _Facet*
> >(__facets[__i]);
>=20
>     // checks for every standard facet type
>     if (__is_same(_Facet, ...))
>       return static_cast<const _Facet*>(__facets[__i]);
>=20
>     return __dyn_facet;
>   }

But why? Maybe I'm being slow but I still don't understand what problem is
being solved here.

Also this would defeat the optimization that avoids the unnecessary overhea=
d of
dynamic_cast for standard facets.

> AFAIK, the standard or libstdc++ docs do not require RTTI for std::locale=
 to
> function.

The standard requires RTTI, period. Using -fno-rtti is completely non-stand=
ard
and so the standard has nothing to say about it.=