Re: [PATCH v2] libstdc++: Define _GLIBCXX_HAS_BUILTIN_TRAIT

[Jonathan Wakely <jwakely@redhat.com>]

On Thu, 31 Aug 2023 at 13:33, Ken Matsui <kmatsui@cs.washington.edu> wrote:
>
> On Tue, Aug 8, 2023 at 1:23 PM Jonathan Wakely <jwakely@redhat.com> wrote:
> >
> >
> >
> > On Wed, 19 Jul 2023 at 20:33, Ken Matsui via Libstdc++ <libstdc++@gcc.gnu.org> wrote:
> >>
> >> This patch defines _GLIBCXX_HAS_BUILTIN_TRAIT macro, which will be used
> >> as a flag to toggle the use of built-in traits in the type_traits header
> >> through _GLIBCXX_NO_BUILTIN_TRAITS macro, without needing to modify the
> >> source code.
> >>
> >> libstdc++-v3/ChangeLog:
> >>
> >>         * include/bits/c++config (_GLIBCXX_HAS_BUILTIN_TRAIT): Define.
> >>         (_GLIBCXX_HAS_BUILTIN): Keep defined.
> >
> >
> > I think this would be a little better as:
> >
> >         * include/bits/c++config (_GLIBCXX_HAS_BUILTIN): Do not undef.
> >         (_GLIBCXX_HAS_BUILTIN_TRAIT): Define.
> >
> > OK for trunk with that change, thanks.
> >
> Thank you for your review! Patrick and I were discussing the naming
> conventions for the macros _GLIBCXX_HAS_BUILTIN_TRAIT and
> _GLIBCXX_NO_BUILTIN_TRAITS. It was brought to our attention that these
> namings might be ambiguous, as there are implementations that have
> corresponding built-ins but do not have fallback. Therefore, we
> believe that using _GLIBCXX_USE_BUILTIN_TRAIT instead of
> _GLIBCXX_HAS_BUILTIN_TRAIT would be more appropriate. Similarly, we
> think that _GLIBCXX_AVOID_BUILTIN_TRAITS would be a better choice than
> _GLIBCXX_NO_BUILTIN_TRAITS, as the latter implies that there are no
> built-ins, when in fact it is meant to express that the use of
> built-ins should be avoided when defining this macro. Could you please
> let me know your thoughts on these updated namings?

Yes, I agree that makes sense. I think personally I'd go with
"DISABLE" instead of "AVOID", or even "DO_NOT_USE" (to mirror "USE" in
the macro for individual traits), but either is OK.

> >
> >>
> >>
> >> Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
> >> ---
> >>  libstdc++-v3/include/bits/c++config | 10 +++++++++-
> >>  1 file changed, 9 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/libstdc++-v3/include/bits/c++config b/libstdc++-v3/include/bits/c++config
> >> index dd47f274d5f..984985d6fff 100644
> >> --- a/libstdc++-v3/include/bits/c++config
> >> +++ b/libstdc++-v3/include/bits/c++config
> >> @@ -854,7 +854,15 @@ namespace __gnu_cxx
> >>  # define _GLIBCXX_HAVE_BUILTIN_LAUNDER 1
> >>  #endif
> >>
> >> -#undef _GLIBCXX_HAS_BUILTIN
> >> +// Returns 1 if _GLIBCXX_NO_BUILTIN_TRAITS is not defined and the compiler
> >> +// has a corresponding built-in type trait, 0 otherwise.
> >> +// _GLIBCXX_NO_BUILTIN_TRAITS can be defined to disable the use of built-in
> >> +// traits.
> >> +#ifndef _GLIBCXX_NO_BUILTIN_TRAITS
> >> +# define _GLIBCXX_HAS_BUILTIN_TRAIT(BT) _GLIBCXX_HAS_BUILTIN(BT)
> >> +#else
> >> +# define _GLIBCXX_HAS_BUILTIN_TRAIT(BT) 0
> >> +#endif
> >>
> >>  // Mark code that should be ignored by the compiler, but seen by Doxygen.
> >>  #define _GLIBCXX_DOXYGEN_ONLY(X)
> >> --
> >> 2.41.0
> >>
>