Question about _GLIBCXX_HAVE_BUILTIN_IS_SAME

[Jonathan Wakely]

On Wed, 7 Feb 2024 at 21:59, Ken Matsui <kmatsui@cs.washington.edu> wrote:

> Hi,
>
> I found we are using _GLIBCXX_HAVE_BUILTIN_IS_SAME in type_traits, but I
> think we can use _GLIBCXX_USE_BUILTIN_TRAIT(__is_same) instead.  I feel
> this is a bit more readable and consistent with other traits.  With this
>

Agreed.


> change, AFAIK, _GLIBCXX_HAVE_BUILTIN_IS_SAME is not used anywhere, but
> can we completely remove it from gcc/libstdc++-v3/include/bits/c++config
> or is there any reason to keep it?
>

No, it can go.

The reason we have that macro is historical. Originally, gcc did not define
__is_same, only __is_same_as. Clang defined __is_same. So we needed to use
a different built-in depending which compiler we were using.

Since https://gcc.gnu.org/g:73ae6eb57251 we just use __is_same for both GCC
and Clang (and Intel). We can simplify it as you suggest.



>
>    /// is_same
> +#if _GLIBCXX_USE_BUILTIN_TRAIT(__is_same)
>    template<typename _Tp, typename _Up>
>      struct is_same
> -#ifdef _GLIBCXX_HAVE_BUILTIN_IS_SAME
>      : public __bool_constant<__is_same(_Tp, _Up)>
> +    { };
>  #else
> +  template<typename _Tp, typename _Up>
> +    struct is_same
>      : public false_type
> -#endif
>      { };
>
> -#ifndef _GLIBCXX_HAVE_BUILTIN_IS_SAME
>    template<typename _Tp>
>      struct is_same<_Tp, _Tp>
>      : public true_type
>     { };
> #endif
>
> I am also wondering if we could replace other _GLIBCXX_HAVE_BUILTIN_*
> macros with _GLIBCXX_USE_BUILTIN_TRAIT(*):
>
> * _GLIBCXX_HAVE_BUILTIN_HAS_UNIQ_OBJ_REP
> * _GLIBCXX_HAVE_BUILTIN_IS_AGGREGATE
> * _GLIBCXX_HAVE_BUILTIN_LAUNDER
>

Yes, let's normalize all of these to use the new form.

At some point we should also simplify this:

#ifdef __has_builtin
# ifdef __is_identifier
// Intel and older Clang require !__is_identifier for some built-ins:
#  define _GLIBCXX_HAS_BUILTIN(B) __has_builtin(B) || ! __is_identifier(B)
# else
#  define _GLIBCXX_HAS_BUILTIN(B) __has_builtin(B)
# endif
#endif

I think we can stop supporting old versions of Clang where
__is_builtin(__is_same) is false, but we should verify which version of
Clang stopped requiring !__is_identifier for those built-ins. If it's older
than Clang 14 we can remove _GLIBCXX_HAS_BUILTIN and just use __has_builtin
directly in _GLIBCXX_USE_BUILTIN_TRAIT. That should wait for GCC 15 though.
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