Re: [PATCH v5 6/6] libstdc++: make std::is_object dispatch to new built-in traits

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

On Tue, Jun 13, 2023 at 10:10 PM François Dumont <frs.dumont@gmail.com> wrote:
>
>
> On 13/06/2023 00:22, Ken Matsui via Libstdc++ wrote:
> > This patch gets std::is_object to dispatch to new built-in traits,
> > __is_function, __is_reference, and __is_void.
> >
> > libstdc++-v3/ChangeLog:
> >       * include/std/type_traits (is_object): Use new built-in traits,
> >       __is_function, __is_reference, and __is_void.
> >       (__is_object): Define this built-in-like macro.
> >       (is_object_v): Use built-in traits through the build-in-like macro.
> >
> > Signed-off-by: Ken Matsui <kmatsui@cs.washington.edu>
> > ---
> >   libstdc++-v3/include/std/type_traits | 19 +++++++++++++++++++
> >   1 file changed, 19 insertions(+)
> >
> > diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
> > index 780fcc00135..93335f94385 100644
> > --- a/libstdc++-v3/include/std/type_traits
> > +++ b/libstdc++-v3/include/std/type_traits
> > @@ -682,11 +682,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >       { };
> >
> >     /// is_object
> > +#if __has_builtin(__is_function) && __has_builtin(__is_reference) \
> > +    && __has_builtin(__is_void)
> > +
> > +#define __is_object(_Tp) \
> > +  (!(__is_function(_Tp) || __is_reference(_Tp) || __is_void(_Tp)))
>
> Is this evaluation order random ? Are all those builtin functions
> performances equivalent ?
>
> I would have felt that __is_void is the simplest/fastest cause only for
> 'void' so would have put it first.

This particular order is derived from the original implementation:

```
   template<typename _Tp>
     struct is_object
     : public __not_<__or_<is_function<_Tp>, is_reference<_Tp>,
                           is_void<_Tp>>>::type
     { };
```

From what I can see, it appears there shouldn't be any disparities in
performance based on these implementations in /gcc/cp/semantics.cc:

```
+    case CPTK_IS_FUNCTION:
+      return type_code1 == FUNCTION_TYPE;

+    case CPTK_IS_REFERENCE:
+      return type_code1 == REFERENCE_TYPE;

+    case CPTK_IS_VOID:
+      return VOID_TYPE_P (type1);
```

VOID_TYPE_P: gcc/tree.h

```
/* Nonzero if this type is the (possibly qualified) void type.  */
#define VOID_TYPE_P(NODE) (TREE_CODE (NODE) == VOID_TYPE)
```

> > +
> > +  template<typename _Tp>
> > +    struct is_object
> > +    : public __bool_constant<__is_object(_Tp)>
> > +    { };
> > +#else
> >     template<typename _Tp>
> >       struct is_object
> >       : public __not_<__or_<is_function<_Tp>, is_reference<_Tp>,
> >                             is_void<_Tp>>>::type
> >       { };
> > +#endif
> >
> >     template<typename>
> >       struct is_member_pointer;
> > @@ -3235,8 +3247,15 @@ template <typename _Tp>
> >     inline constexpr bool is_arithmetic_v = is_arithmetic<_Tp>::value;
> >   template <typename _Tp>
> >     inline constexpr bool is_fundamental_v = is_fundamental<_Tp>::value;
> > +
> > +#ifdef __is_object
> > +template <typename _Tp>
> > +  inline constexpr bool is_object_v = __is_object(_Tp);
> > +#else
> >   template <typename _Tp>
> >     inline constexpr bool is_object_v = is_object<_Tp>::value;
> > +#endif
> > +
> >   template <typename _Tp>
> >     inline constexpr bool is_scalar_v = is_scalar<_Tp>::value;
> >   template <typename _Tp>