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Date:   Wed, 31 Jul 2019 14:01:05 -0700
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Subject: Re: [RFC PATCH] compiler_attributes.h: Add 'fallthrough' pseudo keyword for switch/case use
To:     Peter Zijlstra <peterz@infradead.org>
CC:     Joe Perches <joe@perches.com>, Pavel Machek <pavel@ucw.cz>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Borislav Petkov <bp@alien8.de>,
        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
From:   hpa@zytor.com
Message-ID: <F1AB2846-CA91-41ED-B8E7-3799895DCF06@zytor.com>
Sender: linux-kernel-owner@vger.kernel.org
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On July 31, 2019 11:48:32 AM PDT, Peter Zijlstra <peterz@infradead=2Eorg> w=
rote:
>On Wed, Jul 31, 2019 at 11:24:36AM -0700, hpa@zytor=2Ecom 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=2Egnu=2Eorg/onlinedocs/gcc/Statement-Attributes=2Ehtml#State=
ment-Attributes
>> >> > + */
>> >> > +#if __has_attribute(__fallthrough__)
>> >> > +# define fallthrough                 =20
>__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=2E
>
>Everything that has the warning (GCC-7+/CLANG-9) has that attribute=2E

The standard is moving toward adding this as an attribute with the [[fallt=
hrough]] syntax; it is in C++17, not sure when it will be in C be if it isn=
't already=2E
--=20
Sent from my Android device with K-9 Mail=2E Please excuse my brevity=2E