linux-kernel - Re: [PATCH] perf/x86/intel: Mark expected switch fall-throughs

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Message-ID: <CANiq72=zzZ+Cx8uM+5UW7HeB9XtbXRhXmC2y2tz5EzPX77gHMw@mail.gmail.com>
Date:   Tue, 25 Jun 2019 14:47:29 +0200
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Joe Perches <joe@...ches.com>, Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Shawn Landden <shawn@....icu>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>
Subject: Re: [PATCH] perf/x86/intel: Mark expected switch fall-throughs

On Tue, Jun 25, 2019 at 9:19 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> Can it build a kernel without patches yet? That is, why should I care
> what LLVM does?

Having more than a single compiler is always a good idea. You benefit
from more warnings, more tooling, a second implementation for
reference/comparison, etc.

As for what is the current state, I think they are close, specially
for aarch64, but I let Nick, Nathan et. al. answer that! :-) (Cc'd).
They had a talk in FOSDEM 2019 about it, too.

Also CC'ing Luc since he changed sparse to stop ignoring the attribute
so that __has_attribute() would work, but I am not sure if there has
been further work on supporting it properly.

> > Also note that C2x may get [[fallthrough]]. See N2267 and N2335. At
> > that point, surely tools/IDEs/analyzers will support it :-) The
> > question is whether we want to wait that long to replace the comments.
>
> #define __fallthrough [[fallthrough]]
>
> right?

Yes and no. The exact spelling we use does not matter much. My point
with that paragraph was that since C2x will (maybe) add fallthrough,
as C++17 did, every compiler/analyzer/IDE/etc. that is still missing
support for it will have to eventually add it even if they ignore GNU
attributes. At that point, I would guess most will likely add all
spellings too.

Cheers,
Miguel