Re: Question about _GLIBCXX_HAVE_BUILTIN_IS_SAME

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

On Wed, 07 Feb 2024 at 16:13, Jonathan Wakely <jwakely@redhat.com> wrote:
> 
> 
> 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.
> 

It seems that these macros are used for feature test macros.  I think
it does not make sense to use _GLIBCXX_USE_BUILTIN_TRAIT for them IIUC.
Or do you think we can replace them with _GLIBCXX_USE_BUILTIN_TRAIT?

#if !defined(__cpp_lib_launder)
# if (__cplusplus >= 201703L) && (defined(_GLIBCXX_HAVE_BUILTIN_LAUNDER))
#  define __glibcxx_launder 201606L
#  if defined(__glibcxx_want_all) || defined(__glibcxx_want_launder)
#   define __cpp_lib_launder 201606L
#  endif
# endif
#endif /* !defined(__cpp_lib_launder) && defined(__glibcxx_want_launder) */
#undef __glibcxx_want_launder

> 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.
> 
> 
> 
> 
>  

-- 
Ken Matsui