Re: [PATCH] v2: Use gcc __builtin_stdc_* builtins in stdbit.h if possible

[Joseph Myers <josmyers@redhat.com>]

On Mon, 22 Jan 2024, Jakub Jelinek wrote:

> When the type-specific macros need to use inline functions, it means
> they won't be usable in constant expressions (sure, the standard doesn't
> require that).

Indeed - usability in constant expressions might wait for a future version 
of C to have some form of constexpr functions, rather than being expected 
now.

> Anyway, so do you want instead to just use the new GCC builtins only for the
> type-generic macros (as done in the following patch, tested like the earlier
> one on x86_64), or should the type-specific macros use different
> type-specific inlines then which would use the new builtins?  I guess if
> one can't use them in constant expressions, then the current implementations
> of those type-specific macros is just fine.

I think just use the new builtins for the type-generic macros.  I don't 
think there's any need to have different implementations of the inline 
functions.  (If any of them don't get compiled to essentially the same 
code as versions that do use the new builtins, that's best addressed as an 
optimization improvement in GCC.)

> diff --git a/manual/stdbit.texi b/manual/stdbit.texi
> index fe41c671d8..0878015cc7 100644
> --- a/manual/stdbit.texi
> +++ b/manual/stdbit.texi
> @@ -32,7 +32,9 @@ and @code{unsigned long long int}.  In addition, there is a
>  corresponding type-generic macro (not listed below), named the same as
>  the functions but without any suffix such as @samp{_uc}.  The
>  type-generic macro can only be used with an argument of an unsigned
> -integer type with a width of 8, 16, 32 or 64 bits.
> +integer type with a width of 8, 16, 32 or 64 bits, or when using
> +a compiler with support for stdc builtins such as GCC 14.1 or later
> +any unsigned integer type those builtins support.

I don't think a reference to "stdc builtins" is very useful for glibc 
users (but mentioning unsigned __int128 and unsigned _BitInt types as 
examples supported with GCC 14 would be).

-- 
Joseph S. Myers
josmyers@redhat.com