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

[Jonathan Wakely]

On Tue, 18 Jul 2023 at 07:25, Ken Matsui via Libstdc++ <
libstdc++@gcc.gnu.org> 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%
>

But what about the fallback implementation of is_fundamental where we don't
have the __is_arithmetic built-in?

-    : public __or_<is_arithmetic<_Tp>, is_void<_Tp>,
-                  is_null_pointer<_Tp>>::type
+    : public __bool_constant<is_arithmetic<_Tp>::value
+                             || is_void<_Tp>::value
+                             || is_null_pointer<_Tp>::value>

Here the use of __or_ means that for is_fundamental<int> we don't
instantiate is_void<int> and is_null_pointer<int>. Isn't that still
worthwhile?





>
> 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.
>
>