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 10:51:37 -0700
From:   Joe Perches <joe@...ches.com>
To:     Pavel Machek <pavel@....cz>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        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, 2019-07-31 at 19:14 +0200, Pavel Machek wrote:
> On Tue 2019-07-30 22:35:18, Joe Perches 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.
> 
> Acked-by: Pavel Machek <pavel@....cz>
> 
> > +/*
> > + * 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
> > +
> 
> Will various checkers (and gcc) recognize, that comment in a macro,
> and disable the warning accordingly?

Current non-gcc tools:  I doubt it.

And that's unlikely as the comments are supposed to be stripped
before the macro expansion phase.

gcc 7+, which by now probably most developers actually use, will
though
and likely that's sufficient.

> Explanation that the comment is "magic" might not be a bad idea.

The comment was more for the reader.

cheers, Joe