Re: [PATCH v4 4/4] libstdc++: Optimize is_compound trait performance

[Jonathan Wakely <jwakely@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 2302 bytes --]

On Tue, 18 Jul 2023 at 08:44, Ken Matsui via Libstdc++ <
libstdc++@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 <kmatsui@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
>
>