Re: [PATCH v2 1/2] c++, libstdc++: implement __is_pointer built-in trait

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

On Wed, Jul 12, 2023 at 3:01 AM Jonathan Wakely <jwakely@redhat.com> wrote:
>
> On Mon, 10 Jul 2023 at 06:51, Ken Matsui via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
> >
> > Hi,
> >
> > Here is the benchmark result for is_pointer:
> >
> > https://github.com/ken-matsui/gcc-benches/blob/main/is_pointer.md#sun-jul--9-103948-pm-pdt-2023
> >
> > Time: -62.1344%
> > Peak Memory Usage: -52.4281%
> > Total Memory Usage: -53.5889%
>
> Wow!
>
> Although maybe we could have improved our std::is_pointer_v anyway, like so:
>
> template <typename _Tp>
>   inline constexpr bool is_pointer_v = false;
> template <typename _Tp>
>   inline constexpr bool is_pointer_v<_Tp*> = true;
> template <typename _Tp>
>   inline constexpr bool is_pointer_v<_Tp* const> = true;
> template <typename _Tp>
>   inline constexpr bool is_pointer_v<_Tp* volatile> = true;
> template <typename _Tp>
>   inline constexpr bool is_pointer_v<_Tp* const volatile> = true;
>
> I'm not sure why I didn't already do that.
>
> Could you please benchmark that? And if it is better than the current
> impl using is_pointer<_Tp>::value then we should do this in the
> library:
>
> #if __has_builtin(__is_pointer)
> template <typename _Tp>
>   inline constexpr bool is_pointer_v = __is_pointer(_Tp);
> #else
> template <typename _Tp>
>   inline constexpr bool is_pointer_v = false;
> template <typename _Tp>
>   inline constexpr bool is_pointer_v<_Tp*> = true;
> template <typename _Tp>
>   inline constexpr bool is_pointer_v<_Tp* const> = true;
> template <typename _Tp>
>   inline constexpr bool is_pointer_v<_Tp* volatile> = true;
> template <typename _Tp>
>   inline constexpr bool is_pointer_v<_Tp* const volatile> = true;
> #endif

Hi François and Jonathan,

Thank you for your reviews! I will rename the four underscores to the
appropriate name and take a benchmark once I get home.

If I apply your change on is_pointer_v, is it better to add the
`Co-authored-by:` line in the commit?