Re: [PATCH v3 2/2] libstdc++: Use _GLIBCXX_HAS_BUILTIN_TRAIT

[Patrick Palka <ppalka@redhat.com>]

On Thu, 27 Jul 2023, Ken Matsui via Gcc-patches wrote:

> This patch uses _GLIBCXX_HAS_BUILTIN_TRAIT macro instead of
> __has_builtin in the type_traits header. This macro supports 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/std/type_traits (__has_builtin): Replace with ...
> 	(_GLIBCXX_HAS_BUILTIN): ... this.
> 
> Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
> ---
>  libstdc++-v3/include/std/type_traits | 26 +++++++++++++-------------
>  1 file changed, 13 insertions(+), 13 deletions(-)
> 
> diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
> index 9f086992ebc..12423361b6e 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -1411,7 +1411,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      : public __bool_constant<__is_base_of(_Base, _Derived)>
>      { };
>  
> -#if __has_builtin(__is_convertible)
> +#if _GLIBCXX_HAS_BUILTIN_TRAIT(__is_convertible)
>    template<typename _From, typename _To>
>      struct is_convertible
>      : public __bool_constant<__is_convertible(_From, _To)>
> @@ -1462,7 +1462,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  #if __cplusplus >= 202002L
>  #define __cpp_lib_is_nothrow_convertible 201806L
>  
> -#if __has_builtin(__is_nothrow_convertible)
> +#if _GLIBCXX_HAS_BUILTIN_TRAIT(__is_nothrow_convertible)
>    /// is_nothrow_convertible_v
>    template<typename _From, typename _To>
>      inline constexpr bool is_nothrow_convertible_v
> @@ -1537,7 +1537,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      { using type = _Tp; };
>  
>    /// remove_cv
> -#if __has_builtin(__remove_cv)
> +#if _GLIBCXX_HAS_BUILTIN_TRAIT(__remove_cv)
>    template<typename _Tp>
>      struct remove_cv
>      { using type = __remove_cv(_Tp); };
> @@ -1606,7 +1606,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>    // Reference transformations.
>  
>    /// remove_reference
> -#if __has_builtin(__remove_reference)
> +#if _GLIBCXX_HAS_BUILTIN_TRAIT(__remove_reference)
>    template<typename _Tp>
>      struct remove_reference
>      { using type = __remove_reference(_Tp); };
> @@ -2963,7 +2963,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        template<typename _Tp,
>  	       bool _Nothrow = noexcept(_S_conv<_Tp>(_S_get())),
>  	       typename = decltype(_S_conv<_Tp>(_S_get())),
> -#if __has_builtin(__reference_converts_from_temporary)
> +#if _GLIBCXX_HAS_BUILTIN_TRAIT(__reference_converts_from_temporary)
>  	       bool _Dangle = __reference_converts_from_temporary(_Tp, _Res_t)
>  #else
>  	       bool _Dangle = false
> @@ -3420,7 +3420,7 @@ template<typename _Ret, typename _Fn, typename... _Args>
>     */
>  #define __cpp_lib_remove_cvref 201711L
>  
> -#if __has_builtin(__remove_cvref)
> +#if _GLIBCXX_HAS_BUILTIN_TRAIT(__remove_cvref)
>    template<typename _Tp>
>      struct remove_cvref
>      { using type = __remove_cvref(_Tp); };
> @@ -3515,7 +3515,7 @@ template<typename _Ret, typename _Fn, typename... _Args>
>      : public bool_constant<is_unbounded_array_v<_Tp>>
>      { };
>  
> -#if __has_builtin(__is_layout_compatible)
> +#if _GLIBCXX_HAS_BUILTIN_TRAIT(__is_layout_compatible)

Hmm, I was thinking we'd use this macro only for traits that have a
fallback non-built-in implementation so that we could easily use/test
their fallback implementation.  For traits that don't have such a
fallback, using this macro would mean that trait would no longer get
defined at all, which doesn't seem as useful.  Perhaps let's initially
adjust only the traits that have a fallback implementation?

We could then verify that using the fallback implementation for all such
traits works as expected by running the testsuite with:

  make check RUNTESTFLAGS="conformance.exp --target_board=unix/-D_GLIBCXX_NO_BUILTIN_TRAITS"

>  
>    /// @since C++20
>    template<typename _Tp, typename _Up>
> @@ -3529,7 +3529,7 @@ template<typename _Ret, typename _Fn, typename... _Args>
>      constexpr bool is_layout_compatible_v
>        = __is_layout_compatible(_Tp, _Up);
>  
> -#if __has_builtin(__builtin_is_corresponding_member)
> +#if _GLIBCXX_HAS_BUILTIN_TRAIT(__builtin_is_corresponding_member)
>  #define __cpp_lib_is_layout_compatible 201907L
>  
>    /// @since C++20
> @@ -3540,7 +3540,7 @@ template<typename _Ret, typename _Fn, typename... _Args>
>  #endif
>  #endif
>  
> -#if __has_builtin(__is_pointer_interconvertible_base_of)
> +#if _GLIBCXX_HAS_BUILTIN_TRAIT(__is_pointer_interconvertible_base_of)
>    /// True if `_Derived` is standard-layout and has a base class of type `_Base`
>    /// @since C++20
>    template<typename _Base, typename _Derived>
> @@ -3554,7 +3554,7 @@ template<typename _Ret, typename _Fn, typename... _Args>
>      constexpr bool is_pointer_interconvertible_base_of_v
>        = __is_pointer_interconvertible_base_of(_Base, _Derived);
>  
> -#if __has_builtin(__builtin_is_pointer_interconvertible_with_class)
> +#if _GLIBCXX_HAS_BUILTIN_TRAIT(__builtin_is_pointer_interconvertible_with_class)
>  #define __cpp_lib_is_pointer_interconvertible 201907L
>  
>    /// True if `__mp` points to the first member of a standard-layout type
> @@ -3590,8 +3590,8 @@ template<typename _Ret, typename _Fn, typename... _Args>
>    template<typename _Tp>
>      inline constexpr bool is_scoped_enum_v = is_scoped_enum<_Tp>::value;
>  
> -#if __has_builtin(__reference_constructs_from_temporary) \
> -  && __has_builtin(__reference_converts_from_temporary)
> +#if _GLIBCXX_HAS_BUILTIN_TRAIT(__reference_constructs_from_temporary) \
> +  && _GLIBCXX_HAS_BUILTIN_TRAIT(__reference_converts_from_temporary)
>  
>  #define __cpp_lib_reference_from_temporary 202202L
>  
> @@ -3632,7 +3632,7 @@ template<typename _Ret, typename _Fn, typename... _Args>
>    template<typename _Tp, typename _Up>
>      inline constexpr bool reference_converts_from_temporary_v
>        = reference_converts_from_temporary<_Tp, _Up>::value;
> -#endif // __has_builtin for reference_from_temporary
> +#endif // _GLIBCXX_HAS_BUILTIN_TRAIT for reference_from_temporary
>  #endif // C++23
>  
>  #if _GLIBCXX_HAVE_IS_CONSTANT_EVALUATED
> -- 
> 2.41.0
> 
>