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

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Message-ID: <765E740C-4259-4835-A58D-432006628BAC@zytor.com>
Date:   Wed, 31 Jul 2019 11:24:36 -0700
From:   hpa@...or.com
To:     Joe Perches <joe@...ches.com>, 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>,
        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 July 31, 2019 10:51:37 AM PDT, Joe Perches <joe@...ches.com> wrote:
>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

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.
-- 
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