Re: [PATCH v2 2/3] libstdc++: Optimize is_arithmetic performance by __is_arithmetic built-in

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

On 17/07/2023 06:48, Ken Matsui wrote:
> On Sun, Jul 16, 2023 at 5:32 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_arithmetic trait by
>>> dispatching to the new __is_arithmetic built-in trait.
>>>
>>> libstdc++-v3/ChangeLog:
>>>
>>>        * include/std/type_traits (is_arithmetic): Use __is_arithmetic
>>>        built-in trait.
>>>        (is_arithmetic_v): Likewise.
>>>
>>> Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
>>> ---
>>>    libstdc++-v3/include/std/type_traits | 14 ++++++++++++++
>>>    1 file changed, 14 insertions(+)
>>>
>>> diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
>>> index 0e7a9c9c7f3..7ebbe04c77b 100644
>>> --- a/libstdc++-v3/include/std/type_traits
>>> +++ b/libstdc++-v3/include/std/type_traits
>>> @@ -655,10 +655,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>>>        { };
>>>
>>>      /// is_arithmetic
>>> +#if __has_builtin(__is_arithmetic)
>>> +  template<typename _Tp>
>>> +    struct is_arithmetic
>>> +    : public __bool_constant<__is_arithmetic(_Tp)>
>>> +    { };
>>> +#else
>>>      template<typename _Tp>
>>>        struct is_arithmetic
>>>        : public __or_<is_integral<_Tp>, is_floating_point<_Tp>>::type
>>>        { };
>>> +#endif
>>>
>>>      /// is_fundamental
>>>      template<typename _Tp>
>>> @@ -3198,8 +3205,15 @@ template <typename _Tp>
>>>      inline constexpr bool is_reference_v<_Tp&> = true;
>>>    template <typename _Tp>
>>>      inline constexpr bool is_reference_v<_Tp&&> = true;
>>> +
>>> +#if __has_builtin(__is_arithmetic)
>>> +template <typename _Tp>
>>> +  inline constexpr bool is_arithmetic_v = __is_arithmetic(_Tp);
>>> +#else
>>>    template <typename _Tp>
>>>      inline constexpr bool is_arithmetic_v = is_arithmetic<_Tp>::value;
>>> +#endif
>>> +
>>>    template <typename _Tp>
>>>      inline constexpr bool is_fundamental_v = is_fundamental<_Tp>::value;
>>>    template <typename _Tp>
>> Same remark as the one I did for __is_pointer in cpp_type_traits.h. You
>> could implement it as:
>>
>>     template<typename _Tp>
>>       struct __is_arithmetic_t
>>       : public __truth_type<__is_arithmetic(_Tp)>
>>       { };
>>
>> François
>>
> Thank you for your review! This is from the type_traits header, so the
> name should be as-is.

Here I meant that current libstdc++ implementation of 
std::__is_arithmetic in cpp_type_traits.h should also make use of the 
builtin __is_arithmetic that you are introducing. That is to say replace 
this:

   template<typename _Tp>
     struct __is_arithmetic
     : public __traitor<__is_integer<_Tp>, __is_floating<_Tp> >
     { };

by:

#if __has_builtin(__is_arithmetic)

   template<typename _Tp>
     struct __is_arithmetic_t
     : public __truth_type<__is_arithmetic<_Tp>>
     { };

#else

   template<typename _Tp>
     struct __is_arithmetic_t
     : public __traitor<__is_integer<_Tp>, __is_floating<_Tp> >
     { };

#endif

if you replace '__is_arithmetic' by '__is_arithmetic_t' for the 
libstdc++, just adapt to the name you eventually adopt.