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

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Date:   Thu, 01 Aug 2019 13:09:24 -0700
From:   hpa@...or.com
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Joe Perches <joe@...ches.com>, Pavel Machek <pavel@....cz>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.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>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] compiler_attributes.h: Add 'fallthrough' pseudo keyword for switch/case use

On August 1, 2019 5:24:29 AM PDT, Peter Zijlstra <peterz@...radead.org> wrote:
>On Wed, Jul 31, 2019 at 11:10:36PM -0700, hpa@...or.com wrote:
>> On July 31, 2019 4:55:47 PM PDT, Miguel Ojeda
><miguel.ojeda.sandonis@...il.com> wrote:
>> >On Wed, Jul 31, 2019 at 11:01 PM <hpa@...or.com> wrote:
>> >>
>> >> The standard is moving toward adding this as an attribute with the
>> >[[fallthrough]] syntax; it is in C++17, not sure when it will be in
>C
>> >be if it isn't already.
>> >
>> >Not yet, but it seems to be coming:
>> >
>> >  http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2268.pdf
>> >
>> >However, even if C2x gets it, it will be quite a while until the GCC
>> >minimum version gets bumped up to that, so...
>> >
>> >Cheers,
>> >Miguel
>> 
>> The point was that we should plan ahead in whatever we end up doing.
>
>By reserving 'fallthrough' as a keyword we do exactly that. We can then
>define it to whatever the compiler/tool at hand requires.
>
>Once GCC gains support for that [[attribute]] nonsense, we can detector
>that and use that over the __attribute__(())
>
>[ Also the Cxx attribute syntax is an abomination -- just a lesser one
>than reading actual comments :-) ]

I'm not disagreeing... I think using a macro makes sense.

Not sure if I agree about the syntax... I think it's rather friendly compared to gcc's ;)
-- 
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