From: Jonathan Wakely <jwakely@redhat.com>
To: Ken Matsui <kmatsui@cs.washington.edu>
Cc: libstdc++@gcc.gnu.org
Subject: Re: Question about _GLIBCXX_HAVE_BUILTIN_IS_SAME
Date: Thu, 8 Feb 2024 00:13:39 +0000 [thread overview]
Message-ID: <CACb0b4=t69v6HaFH_mqRAvOOcSkv5ZHRz_dz+Ajq8DkpBoM_8Q@mail.gmail.com> (raw)
In-Reply-To: <sirw5skiqemdvyehahyuhudj7fy4e3hjidzgnfw6ofhzgy6giw@ojchughmbk5m>
[-- Attachment #1: Type: text/plain, Size: 2343 bytes --]
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.
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.
next prev parent reply other threads:[~2024-02-08 0:13 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-02-07 21:58 Ken Matsui
2024-02-08 0:13 ` Jonathan Wakely [this message]
2024-02-09 19:35 ` Ken Matsui
2024-02-13 23:34 ` Ken Matsui
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