linux-kernel - Re: [PATCH 2/4] compiler_attributes.h: Add 'fallthrough' pseudo keyword for switch/case use

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Date:   Sat, 5 Oct 2019 19:17:36 +0200
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Joe Perches <joe@...ches.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Pavel Machek <pavel@....cz>,
        "Gustavo A . R . Silva" <gustavo@...eddedor.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Shawn Landden <shawn@....icu>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Miller <davem@...emloft.net>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH 2/4] compiler_attributes.h: Add 'fallthrough' pseudo
 keyword for switch/case use

Hi Joe,

On Sat, Oct 5, 2019 at 6:46 PM Joe Perches <joe@...ches.com> wrote:
>
> Reserve the pseudo keyword 'fallthrough' for the ability to convert the
> various case block /* fallthrough */ style comments to appear to be an
> actual reserved word with the same gcc case block missing fallthrough
> warning capability.
>
> All switch/case blocks now must end in one of:
>
>         break;
>         fallthrough;
>         goto <label>;
>         return [expression];
>         continue;
>
> fallthough is gcc's __attribute__((__fallthrough__)) which was introduced
> in gcc version 7..

Nits: double period, missing "r" in fallthough.

> fallthrough devolves to an empty "do {} while (0)" if the compiler
> version (any version less than gcc 7) does not support the attribute.

Perhaps add a short note why (empty statement warnings maybe? I don't
remember them but it was months ago so maybe it changed).

> Signed-off-by: Joe Perches <joe@...ches.com>
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Please add Dan's Suggested-by and copy the things I wrote in the
commit message when I proposed this:

  https://github.com/ojeda/linux/commit/668f011a2706ea555987e263f609a5deba9c7fc4

> ---
>  include/linux/compiler_attributes.h | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
>
> diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
> index 6b318efd8a74..cdf016596659 100644
> --- a/include/linux/compiler_attributes.h
> +++ b/include/linux/compiler_attributes.h
> @@ -40,6 +40,7 @@
>  # define __GCC4_has_attribute___noclone__             1
>  # define __GCC4_has_attribute___nonstring__           0
>  # define __GCC4_has_attribute___no_sanitize_address__ (__GNUC_MINOR__ >= 8)
> +# define __GCC4_has_attribute___fallthrough__         0

This goes after __externally_visible__.

>  #endif
>
>  /*
> @@ -185,6 +186,22 @@
>  # define __noclone
>  #endif
>
> +/*
> + * Add the pseudo keyword 'fallthrough' so case statement blocks
> + * must end with any of these keywords:
> + *   break;
> + *   fallthrough;
> + *   goto <label>;
> + *   return [expression];
> + *
> + *  gcc: https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#Statement-Attributes

This also goes after __externally_visible__.

Please add:

  * Optional: only supported since gcc >= 7.1
  * Optional: only supported since clang >= 10
  * Optional: not supported by icc

As well as:

  clang: https://clang.llvm.org/docs/AttributeReference.html#fallthrough

See how I did it in the link above:

  https://github.com/ojeda/linux/commit/668f011a2706ea555987e263f609a5deba9c7fc4

> + */
> +#if __has_attribute(__fallthrough__)
> +# define fallthrough                    __attribute__((__fallthrough__))
> +#else
> +# define fallthrough                    do {} while (0)  /* fallthrough */
> +#endif
> +
>  /*
>   * Note the missing underscores.
>   *
> --
> 2.15.0
>

Cheers,
Miguel