Re: Question about _GLIBCXX_HAVE_BUILTIN_IS_SAME

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

On Fri, Feb 9, 2024 at 11:35 AM Ken Matsui <kmatsui@cs.washington.edu> wrote:
>
> On Wed, 07 Feb 2024 at 16:13, Jonathan Wakely <jwakely@redhat.com> wrote:
> >
> >
> > On Wed, 7 Feb 2024 at 21:59, Ken Matsui <kmatsui@cs.washington.edu> wrote:
> >
> >     Hi,
> >
> >     I found we are using _GLIBCXX_HAVE_BUILTIN_IS_SAME in type_traits, but I
> >     think we can use _GLIBCXX_USE_BUILTIN_TRAIT(__is_same) instead.  I feel
> >     this is a bit more readable and consistent with other traits.  With this
> >
> >
> > Agreed.
> >
> >
> >     change, AFAIK, _GLIBCXX_HAVE_BUILTIN_IS_SAME is not used anywhere, but
> >     can we completely remove it from gcc/libstdc++-v3/include/bits/c++config
> >     or is there any reason to keep it?
> >
> >
> > No, it can go.
> >
> > The reason we have that macro is historical. Originally, gcc did not define
> > __is_same, only __is_same_as. Clang defined __is_same. So we needed to use a
> > different built-in depending which compiler we were using.
> >
> > Since https://gcc.gnu.org/g:73ae6eb57251 we just use __is_same for both GCC and
> > Clang (and Intel). We can simplify it as you suggest.
> >
> >
> >
> >
> >        /// is_same
> >     +#if _GLIBCXX_USE_BUILTIN_TRAIT(__is_same)
> >        template<typename _Tp, typename _Up>
> >          struct is_same
> >     -#ifdef _GLIBCXX_HAVE_BUILTIN_IS_SAME
> >          : public __bool_constant<__is_same(_Tp, _Up)>
> >     +    { };
> >      #else
> >     +  template<typename _Tp, typename _Up>
> >     +    struct is_same
> >          : public false_type
> >     -#endif
> >          { };
> >
> >     -#ifndef _GLIBCXX_HAVE_BUILTIN_IS_SAME
> >        template<typename _Tp>
> >          struct is_same<_Tp, _Tp>
> >          : public true_type
> >         { };
> >     #endif
> >
> >     I am also wondering if we could replace other _GLIBCXX_HAVE_BUILTIN_*
> >     macros with _GLIBCXX_USE_BUILTIN_TRAIT(*):
> >
> >     * _GLIBCXX_HAVE_BUILTIN_HAS_UNIQ_OBJ_REP
> >     * _GLIBCXX_HAVE_BUILTIN_IS_AGGREGATE
> >     * _GLIBCXX_HAVE_BUILTIN_LAUNDER
> >
> >
> > Yes, let's normalize all of these to use the new form.
> >
>
> It seems that these macros are used for feature test macros.  I think
> it does not make sense to use _GLIBCXX_USE_BUILTIN_TRAIT for them IIUC.
> Or do you think we can replace them with _GLIBCXX_USE_BUILTIN_TRAIT?
>
> #if !defined(__cpp_lib_launder)
> # if (__cplusplus >= 201703L) && (defined(_GLIBCXX_HAVE_BUILTIN_LAUNDER))
> #  define __glibcxx_launder 201606L
> #  if defined(__glibcxx_want_all) || defined(__glibcxx_want_launder)
> #   define __cpp_lib_launder 201606L
> #  endif
> # endif
> #endif /* !defined(__cpp_lib_launder) && defined(__glibcxx_want_launder) */
> #undef __glibcxx_want_launder
>

I talked with Patrick about this.  Since we don't have a fallback
implementation for the following traits, it would be better to keep
the current implementation instead of using
_GLIBCXX_USE_BUILTIN_TRAIT.  For _GLIBCXX_HAS_BUILTIN, if it works
with Clang 14, I will submit a patch.  Thank you!

> >     * _GLIBCXX_HAVE_BUILTIN_HAS_UNIQ_OBJ_REP
> >     * _GLIBCXX_HAVE_BUILTIN_IS_AGGREGATE
> >     * _GLIBCXX_HAVE_BUILTIN_LAUNDER

> > At some point we should also simplify this:
> >
> > #ifdef __has_builtin
> > # ifdef __is_identifier
> > // Intel and older Clang require !__is_identifier for some built-ins:
> > #  define _GLIBCXX_HAS_BUILTIN(B) __has_builtin(B) || ! __is_identifier(B)
> > # else
> > #  define _GLIBCXX_HAS_BUILTIN(B) __has_builtin(B)
> > # endif
> > #endif
> >
> > I think we can stop supporting old versions of Clang where __is_builtin
> > (__is_same) is false, but we should verify which version of Clang stopped
> > requiring !__is_identifier for those built-ins. If it's older than Clang 14 we
> > can remove _GLIBCXX_HAS_BUILTIN and just use __has_builtin directly in
> > _GLIBCXX_USE_BUILTIN_TRAIT. That should wait for GCC 15 though.
> >
> >
> >
> >
> >
>
> --
> Ken Matsui