linux-kernel - Re: [GIT PULL] fallthrough fixes for Clang for 5.14-rc2

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Date:   Thu, 15 Jul 2021 20:16:37 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [GIT PULL] fallthrough fixes for Clang for 5.14-rc2



On 7/15/21 20:04, Linus Torvalds wrote:
> On Wed, Jul 14, 2021 at 1:03 PM Gustavo A. R. Silva
> <gustavoars@...nel.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
>