Re: git: cebb8646c4ee - main - Support byte-sized enums

On 5 Jul 2023, at 16:38, Mateusz Guzik <mjg@FreeBSD.org> wrote:
> 
> The branch main has been updated by mjg:
> 
> URL: https://cgit.FreeBSD.org/src/commit/?id=cebb8646c4ee559aedcbc1b27bf30faa70a1f716
> 
> commit cebb8646c4ee559aedcbc1b27bf30faa70a1f716
> Author:     Mateusz Guzik <mjg@FreeBSD.org>
> AuthorDate: 2023-03-12 19:29:20 +0000
> Commit:     Mateusz Guzik <mjg@FreeBSD.org>
> CommitDate: 2023-07-05 14:46:30 +0000
> 
>    Support byte-sized enums
> 
>    To that end add __enum_uint8_decl and __enum_uint8.
> 
>    By default enums take 4 bytes, but vast majority of them have values
>    which would fit in a byte.
> 
>    One can try to workaround the problem by using bitfields, like so:
>    enum some_small_enum foo:8;
> 
>    but that's ugly and runs into trouble when using atomic_load (you can't
>    take an address of a bitfield, even if it is sized to a multiply of a
>    byte).
> 
>    Both gcc 13 and clang support specifying the size, and for older
>    variants one can fallback to the "packed" attribute.
> 
>    Names are mangled in order to avoid mix use with plain enum.
> 
>    Reviewed by:
>    Differential Revision:  https://reviews.freebsd.org/D39031
> ---
> sys/sys/types.h | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
> 
> diff --git a/sys/sys/types.h b/sys/sys/types.h
> index 0846f2a57670..c6f7abe1f524 100644
> --- a/sys/sys/types.h
> +++ b/sys/sys/types.h
> @@ -347,6 +347,18 @@ __makedev(int _Major, int _Minor)
>    ((dev_t)(_Minor & 0xff00) << 24) | (_Minor & 0xffff00ff));
> }
> 
> +#if (defined(__clang__) || (defined(__GNUC__) && __GNUC__ >= 13))
> +#define __enum_uint8_decl(name) enum enum_ ## name ## _uint8 : uint8_t
> +#define __enum_uint8(name) enum enum_ ## name ## _uint8

What happened to zlei@’s feedback about making it so __enum_uint8 isn’t
needed everywhere?

Jess

> +#else
> +/*
> + * Note: there is no real size checking here, but the code below can be
> + * removed once we require GCC 13.
> + */
> +#define __enum_uint8_decl(name) enum __attribute__((packed)) enum_ ## name ## _uint8
> +#define __enum_uint8(name) enum __attribute__((packed)) enum_ ## name ## _uint8
> +#endif
> +
> /*
>  * These declarations belong elsewhere, but are repeated here and in
>  * <stdio.h> to give broken programs a better chance of working with