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

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Message-ID: <20190625071846.GN3436@hirez.programming.kicks-ass.net>
Date:   Tue, 25 Jun 2019 09:18:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
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>
Subject: Re: [PATCH] perf/x86/intel: Mark expected switch fall-throughs

On Tue, Jun 25, 2019 at 12:28:23AM +0200, Miguel Ojeda wrote:
> On Mon, Jun 24, 2019 at 10:53 PM Gustavo A. R. Silva
> <gustavo@...eddedor.com> wrote:
> >
> > Once the C++17 `__attribute__((fallthrough))` is more widely handled by C compilers,
> > static analyzers, and IDEs, we can switch to using that instead. Also, we are a few
> > warnings away (less than five) from being able to enable -Wimplicit-fallthrough. After
> > this option has been finally enabled (in v5.3) we can easily go and replace the comments
> > to whatever we agree upon.
> 
> Indeed -- the decision last year was to wait for a while since not
> everyone had support for it. My branch is waiting here:
> 
>     https://github.com/ojeda/linux/tree/compiler-attributes-fallthrough
> 
> The good news is that there is some progress. For instance, LLVM is
> working on supporting the GNU spelling:
> 
>     https://reviews.llvm.org/D63260
>     https://bugs.llvm.org/show_bug.cgi?id=37135
>     https://github.com/ClangBuiltLinux/linux/issues/235

Can it build a kernel without patches yet? That is, why should I care
what LLVM does?

> 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?