linux-kernel - Re: [RFC PATCH v1] perf build: Fix build for clang's -Wunreachable-code

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Message-ID: <aDdU1npHL2Vczhsa@google.com>
Date: Wed, 28 May 2025 11:24:22 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Ian Rogers <irogers@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Kan Liang <kan.liang@...ux.intel.com>, linux-kernel@...r.kernel.org,
	linux-perf-users@...r.kernel.org
Subject: Re: [RFC PATCH v1] perf build: Fix build for clang's
 -Wunreachable-code

On Tue, May 27, 2025 at 01:53:37PM -0700, Ian Rogers wrote:
> On Fri, Apr 11, 2025 at 3:14 PM Ian Rogers <irogers@...gle.com> wrote:
> >
> > On Fri, Apr 11, 2025 at 2:34 PM Namhyung Kim <namhyung@...nel.org> wrote:
> > >
> > > Hi Ian,
> > >
> > > On Thu, Apr 10, 2025 at 01:26:47PM -0700, Ian Rogers wrote:
> > > > Clang's unreachable code warning is able to catch bugs like the famous
> > > > "goto fail" which gcc's unreachable code warning fails to warn about
> > > > (it will complain about misleading indent). The changes here are
> > > > sufficient to get perf building with clang with -Wunreachable code,
> > > > but they don't really fix any bugs. Posting as an RFC to see if anyone
> > > > things this is worth pursuing.
> > >
> > > I'm not sure if it's useful and don't see what kind of bugs it can
> > > address.  The proposed changes don't look like an improvement.
> >
> > The goto fail case was in OpenSSL the code from a bad merge:
> > ```
> > if (...)
> >   goto fail;
> >   goto fail;
> > ```
> > Meaning the fail path was always taken and checking on the non-fail
> > code never executed. Newer GCCs will warn of this because of the
> > "misleading indent" but  clang won't. It is easy to imagine similar
> > mistakes creeping in, so using compiler warnings to avoid the bug
> > could be useful.

It doesn't look very convincing to me but it might be valuable in some
rare cases.  But the proposed changes - basically replace exit() to
__builtin_unreachable() - seem weird.  Why is calling it a problem?  I
guess it already has some kind of annotation like "noreturn"?

Thanks,
Namhyung