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

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Date:   Wed, 31 Jul 2019 20:48:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     hpa@...or.com
Cc:     Joe Perches <joe@...ches.com>, Pavel Machek <pavel@....cz>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "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>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] compiler_attributes.h: Add 'fallthrough' pseudo
 keyword for switch/case use

On Wed, Jul 31, 2019 at 11:24:36AM -0700, hpa@...or.com wrote:
> >> > +/*
> >> > + * 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
> >> > + */
> >> > +#if __has_attribute(__fallthrough__)
> >> > +# define fallthrough                   __attribute__((__fallthrough__))
> >> > +#else
> >> > +# define fallthrough                    do {} while (0)  /* fallthrough */
> >> > +#endif
> >> > +

> If the comments are stripped, how would the compiler see them to be
> able to issue a warning? I would guess that it is retained or replaced
> with some other magic token.

Everything that has the warning (GCC-7+/CLANG-9) has that attribute.