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

["François Dumont" <frs.dumont@gmail.com>]

It seems rather logical cause std::disjunction is supposed to avoid 
instantiations but in case of:

std::disjunction<std::is_void<_Tp>, std::is_null_pointer<_Tp>>

you'll avoid std::is_null_pointer instantiation only for 'void' type and 
at the price of instantiating std::disjunction so 2 instantiations at 
best but most of the time 3, clearly useless here.

On 18/07/2023 08:24, Ken Matsui 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.