Re: [PATCH v4 4/4] libstdc++: Optimize is_compound trait performance

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

On Tue, Aug 8, 2023 at 1:33 PM Jonathan Wakely <jwakely@redhat.com> wrote:
>
>
>
> On Tue, 18 Jul 2023 at 08:44, Ken Matsui via Libstdc++ <libstdc++@gcc.gnu.org> wrote:
>>
>> This patch optimizes the performance of the is_compound trait by
>> dispatching to the new __is_arithmetic built-in trait.
>>
>> libstdc++-v3/ChangeLog:
>>
>>         * include/std/type_traits (is_compound): Use __is_arithmetic
>>         built-in trait.
>>         (is_compound_v): Use is_fundamental_v instead.
>>
>> Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
>> ---
>>  libstdc++-v3/include/std/type_traits | 11 ++++++++++-
>>  1 file changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
>> index cf24de2fcac..73d9a2b16fc 100644
>> --- a/libstdc++-v3/include/std/type_traits
>> +++ b/libstdc++-v3/include/std/type_traits
>> @@ -702,9 +702,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>>      { };
>>
>>    /// is_compound
>> +#if __has_builtin(__is_arithmetic)
>> +  template<typename _Tp>
>> +    struct is_compound
>> +    : public __bool_constant<!(__is_arithmetic(_Tp)
>> +                             || is_void<_Tp>::value
>> +                             || is_null_pointer<_Tp>::value)>
>> +    { };
>> +#else
>>    template<typename _Tp>
>>      struct is_compound
>>      : public __not_<is_fundamental<_Tp>>::type { };
>> +#endif
>
>
> I think it would be simpler to just do this unconditionally (i.e. just a single definition without using __has_builtin):
>
> template<typename _Tp>
>   struct is_compound
>   : __bool_constant<!is_fundamental<_Tp>::value>
>   { };
>
> This still avoids instantiating __not_. If is_fundamental is much more efficient now, then I think it's OK to instantiate it here. Otherwise we're duplicating the logic for is_fundamental, and just giving ourselves more code to maintain.
>
> Nobody ever uses is_compound anyway!
>
Agreed! Will fix this patch. Thank you!

>
>
>>
>>    /// @cond undocumented
>>    template<typename _Tp>
>> @@ -3234,7 +3243,7 @@ template <typename _Tp>
>>  template <typename _Tp>
>>    inline constexpr bool is_scalar_v = is_scalar<_Tp>::value;
>>  template <typename _Tp>
>> -  inline constexpr bool is_compound_v = is_compound<_Tp>::value;
>> +  inline constexpr bool is_compound_v = !is_fundamental_v<_Tp>;
>>  template <typename _Tp>
>>    inline constexpr bool is_member_pointer_v = is_member_pointer<_Tp>::value;
>>  template <typename _Tp>
>> --
>> 2.41.0
>>