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

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Message-ID: <20190801122429.GY31398@hirez.programming.kicks-ass.net>
Date:   Thu, 1 Aug 2019 14:24:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     hpa@...or.com
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 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 :-) ]