On Tue, 18 Jul 2023 at 08:44, Ken Matsui via Libstdc++ <
libstd...@gcc.gnu.org> wrote:
> This patch optimizes the performance of the is_compound trait by
> dispatching to the new __is_arithmetic built-in trait.
>
> libstdc++-v3/ChangeLog:
>
> * include/std/type_traits (is_compound): Use __is_arithmetic
> built-in trait.
> (is_compound_v): Use is_fundamental_v instead.
>
> Signed-off-by: Ken Matsui <kmat...@gcc.gnu.org>
> ---
> libstdc++-v3/include/std/type_traits | 11 ++++++++++-
> 1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/include/std/type_traits
> b/libstdc++-v3/include/std/type_traits
> index cf24de2fcac..73d9a2b16fc 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -702,9 +702,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> { };
>
> /// is_compound
> +#if __has_builtin(__is_arithmetic)
> + template<typename _Tp>
> + struct is_compound
> + : public __bool_constant<!(__is_arithmetic(_Tp)
> + || is_void<_Tp>::value
> + || is_null_pointer<_Tp>::value)>
> + { };
> +#else
> template<typename _Tp>
> struct is_compound
> : public __not_<is_fundamental<_Tp>>::type { };
> +#endif
>
I think it would be simpler to just do this unconditionally (i.e. just a
single definition without using __has_builtin):
template<typename _Tp>
struct is_compound
: __bool_constant<!is_fundamental<_Tp>::value>
{ };
This still avoids instantiating __not_. If is_fundamental is much more
efficient now, then I think it's OK to instantiate it here. Otherwise we're
duplicating the logic for is_fundamental, and just giving ourselves more
code to maintain.
Nobody ever uses is_compound anyway!
> /// @cond undocumented
> template<typename _Tp>
> @@ -3234,7 +3243,7 @@ template <typename _Tp>
> template <typename _Tp>
> inline constexpr bool is_scalar_v = is_scalar<_Tp>::value;
> template <typename _Tp>
> - inline constexpr bool is_compound_v = is_compound<_Tp>::value;
> + inline constexpr bool is_compound_v = !is_fundamental_v<_Tp>;
> template <typename _Tp>
> inline constexpr bool is_member_pointer_v =
> is_member_pointer<_Tp>::value;
> template <typename _Tp>
> --
> 2.41.0
>
>
