[PATCH v2 3/3] libstdc++: Optimize is_fundamental performance by __is_arithmetic built-in

[Jonathan Wakely]

On Tue, 18 Jul 2023 at 07:28, Ken Matsui via Libstdc++ <
libstdc++@gcc.gnu.org> wrote:

> I will eventually work on disjunction to somehow optimize, but in the
> meantime, this might be a better implementation. Of course, my
> benchmark could be wrong.
>

You should use __or_ internally in libstdc++ code, not std::disjunction.

Patrick already optimized both of those, and __or_ is slightly faster
(because it doesn't have to conform to the full requirements of
std::disjunction).

A compiler built-in for __or_ / __disjunction might perform better. But
eventually if we're going to have built-ins for all of __is_arithmetic,
__is_void, and __is_null_pointer, then we would want simply:

__is_arithmetic(T) || __is_void(T) || __is_null_pointer(T)

and so we wouldn't need to avoid instantiating any class templates at all.



>
> On Mon, Jul 17, 2023 at 11:24 PM Ken Matsui <kmatsui@cs.washington.edu>
> wrote:
> >
> > Hi,
> >
> > I took a benchmark for this.
> >
> >
> https://github.com/ken-matsui/gcc-benches/blob/main/is_fundamental-disjunction.md#mon-jul-17-105937-pm-pdt-2023
> >
> > template<typename _Tp>
> > struct is_fundamental
> > : public std::bool_constant<__is_arithmetic(_Tp)
> >                             || std::is_void<_Tp>::value
> >                             || std::is_null_pointer<_Tp>::value>
> > { };
> >
> > is faster than:
> >
> > template<typename _Tp>
> > struct is_fundamental
> > : public std::bool_constant<__is_arithmetic(_Tp)
> >                             || std::disjunction<std::is_void<_Tp>,
> >                                                 std::is_null_pointer<_Tp>
> >                                                 >::value>
> > { };
> >
> > Time: -32.2871%
> > Peak Memory: -18.5071%
> > Total Memory: -20.1991%
> >
> > Sincerely,
> > Ken Matsui
> >
> > On Sun, Jul 16, 2023 at 9:49 PM Ken Matsui <kmatsui@cs.washington.edu>
> wrote:
> > >
> > > On Sun, Jul 16, 2023 at 5:41 AM François Dumont <frs.dumont@gmail.com>
> wrote:
> > > >
> > > >
> > > > On 15/07/2023 06:55, Ken Matsui via Libstdc++ wrote:
> > > > > This patch optimizes the performance of the is_fundamental trait by
> > > > > dispatching to the new __is_arithmetic built-in trait.
> > > > >
> > > > > libstdc++-v3/ChangeLog:
> > > > >
> > > > >       * include/std/type_traits (is_fundamental_v): Use
> __is_arithmetic
> > > > >       built-in trait.
> > > > >       (is_fundamental): Likewise. Optimize the original
> implementation.
> > > > >
> > > > > Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
> > > > > ---
> > > > >   libstdc++-v3/include/std/type_traits | 21 +++++++++++++++++----
> > > > >   1 file changed, 17 insertions(+), 4 deletions(-)
> > > > >
> > > > > diff --git a/libstdc++-v3/include/std/type_traits
> b/libstdc++-v3/include/std/type_traits
> > > > > index 7ebbe04c77b..cf24de2fcac 100644
> > > > > --- a/libstdc++-v3/include/std/type_traits
> > > > > +++ b/libstdc++-v3/include/std/type_traits
> > > > > @@ -668,11 +668,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > > > >   #endif
> > > > >
> > > > >     /// is_fundamental
> > > > > +#if __has_builtin(__is_arithmetic)
> > > > > +  template<typename _Tp>
> > > > > +    struct is_fundamental
> > > > > +    : public __bool_constant<__is_arithmetic(_Tp)
> > > > > +                             || is_void<_Tp>::value
> > > > > +                             || is_null_pointer<_Tp>::value>
> > > > > +    { };
> > > >
> > > > What about doing this ?
> > > >
> > > >    template<typename _Tp>
> > > >      struct is_fundamental
> > > >      : public __bool_constant<__is_arithmetic(_Tp)
> > > >                               || __or_<is_void<_Tp>,
> > > >                                       is_null_pointer<_Tp>>::value>
> > > >      { };
> > > >
> > > > Based on your benches it seems that builtin __is_arithmetic is much
> better that std::is_arithmetic. But __or_ could still avoid instantiation
> of is_null_pointer.
> > > >
> > > Let me take a benchmark for this later.
>
>