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

[Ken Matsui <kmatsui@cs.washington.edu>]

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?
>
>>
>>
>> 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
>>