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

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

On Tue, Aug 8, 2023 at 1:19 PM Jonathan Wakely <jwakely@redhat.com> wrote:
>
>
>
> 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?

That fallback implementation would be this:
https://github.com/ken-matsui/gsoc23/blob/967e20770599f2a8925c9794669111faef11beb7/is_fundamental.cc#L11-L15.

The is_fundamental-disjunction.cc benchmark used the USE_BUILTIN
macro, but in this benchmark, I used it to just switch two different
implementations that use 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?
>
Let me take a benchmark with __or_ later! We may see a difference.

>
>
>
>>
>>
>> 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.
>>