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Date:   Wed, 31 Jul 2019 20:48:32 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     hpa@zytor.com
Cc:     Joe Perches <joe@perches.com>, Pavel Machek <pavel@ucw.cz>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Shawn Landden <shawn@git.icu>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH] compiler_attributes.h: Add 'fallthrough' pseudo
 keyword for switch/case use
Message-ID: <20190731184832.GZ31381@hirez.programming.kicks-ass.net>
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On Wed, Jul 31, 2019 at 11:24:36AM -0700, hpa@zytor.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.