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Message-ID: <ccc7fa72d0f83ddd62067092b105bd801479004b.camel@perches.com>
Subject: Re: [RFC PATCH] compiler_attributes.h: Add 'fallthrough' pseudo
 keyword for switch/case use
From:   Joe Perches <joe@perches.com>
To:     Pavel Machek <pavel@ucw.cz>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "Gustavo A . R . Silva" <gustavo@embeddedor.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Kan Liang <kan.liang@linux.intel.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Jiri Olsa <jolsa@redhat.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Shawn Landden <shawn@git.icu>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Date:   Wed, 31 Jul 2019 10:51:37 -0700
In-Reply-To: <20190731171429.GA24222@amd>
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         <20190731171429.GA24222@amd>
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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@ucw.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