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

Gustavo A. R. Silva
2021-07-16T01:35:43Z

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
>