From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
"Gustavo A. R. Silva" <gustavoars@kernel.org>,
Nathan Chancellor <nathan@kernel.org>,
Nick Desaulniers <ndesaulniers@google.com>
Cc: Kees Cook <keescook@chromium.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [GIT PULL] fallthrough fixes for Clang for 5.14-rc2
Date: Thu, 15 Jul 2021 20:16:37 -0500 [thread overview]
Message-ID: <5d170cc8-501d-0cec-bf03-2f53108a8486@embeddedor.com> (raw)
In-Reply-To: <CAHk-=wjQeeUiv+P_4cZfCy-hY13yGqCGS-scKGhuJ-SAzz2doA@mail.gmail.com>
On 7/15/21 20:04, Linus Torvalds wrote:
> On Wed, Jul 14, 2021 at 1:03 PM Gustavo A. R. Silva
> <gustavoars@kernel.org> wrote:
>>
>> git://git.kernel.org/pub/scm/linux/kernel/git/gustavoars/linux.git tags/Wimplicit-fallthrough-clang-5.14-rc2
>
> Grr.
>
> I merged this, but when I actually tested it on my clang build, it
> turns out that the clang "-Wimplicit-fallthrough" flag is unbelievable
> garbage.
>
> I get
>
> warning: fallthrough annotation in unreachable code [-Wimplicit-fallthrough]
Kees just opened a bug report for this:
https://bugs.llvm.org/show_bug.cgi?id=51094
--
Gustavo
>
> and the stupid warning doesn't even say WHERE THE PROBLEM HAPPENS.
>
> No file name, no line numbers. Just this pointless garbage warning.
>
> Honestly, how does a compiler even do something that broken? Am I
> supposed to use my sixth sense to guide me in finding the warning?
>
> I like the concept of the fallthrough warning, but it looks like the
> clang implementation of it is so unbelievably broken that it's getting
> disabled again.
>
> Yeah, I can
>
> (a) build the kernel without any parallelism
>
> (b) use ">&" to get both output and errors into the same file
>
> (c) see that it says
>
> CC kernel/sched/core.o
> warning: fallthrough annotation in unreachable code [-Wimplicit-fallthrough]
> 1 warning generated.
>
> and now I see at least which _file_ it is that causes that warning.
>
> I can then use my incredible powers of deduction (it's almost like a
> sixth sense, but helped by the fact that there's only one single
> "fallthrough" statement in that file) to figure out that it's
> triggered by this code:
>
> case cpuset:
> if (IS_ENABLED(CONFIG_CPUSETS)) {
> cpuset_cpus_allowed_fallback(p);
> state = possible;
> break;
> }
> fallthrough;
> case possible:
>
> and it all makes it clear that the clang warning is just incredibly
> broken garbage not only in that lack of filename and line number, but
> just in general.
>
> Yeah, I'm a bit pissed off at this. This clang warning really is
> WRONG. It's so wrong in so many ways that I don't know what to say.
>
> Except "yeah, that broken option is getting reverted again, because
> the clang people messed up completely".
>
> It's sad to see that people wasted time and effort on trying to make
> clang happy, when it turns out that clang just gets this so _totally_
> wrong.
>
> Linus
>
next prev parent reply other threads:[~2021-07-16 1:35 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-14 20:05 [GIT PULL] fallthrough fixes for Clang for 5.14-rc2 Gustavo A. R. Silva
2021-07-15 21:15 ` pr-tracker-bot
2021-07-16 1:04 ` Linus Torvalds
2021-07-16 1:16 ` Gustavo A. R. Silva [this message]
2021-07-16 1:22 ` Linus Torvalds
2021-07-16 1:29 ` Gustavo A. R. Silva
2021-07-16 18:47 ` Nathan Chancellor
2021-07-16 18:57 ` Gustavo A. R. Silva
2021-07-16 19:18 ` Nathan Chancellor
2021-07-16 19:26 ` Linus Torvalds
2021-07-16 19:22 ` Linus Torvalds
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