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

Gustavo A. R. Silva
2021-07-16T18:55:13Z

On 7/16/21 13:47, Nathan Chancellor wrote:
> On Thu, Jul 15, 2021 at 06:04:15PM -0700, 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]
>>
>> 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.
>
> I commented this on the LLVM bug tracker but I will copy and paste it
> here for posterity:
>
> "It is actually the fact that
>
> case 1:
> if (something || !IS_ENABLED(CONFIG_SOMETHING))
> return blah;
> fallthrough;
> case 2:
>
> looks like
>
> case 1:
> return blah;
> fallthrough;
> case 2:
>
> For example: https://godbolt.org/z/GdPeMbdo8
>
> int foo(int a) {
> switch (a) {
> case 0:
> if (0)
> return 0;
> __attribute__((__fallthrough__)); // no warning
> case 1:
> if (1)
> return 1;
> __attribute__((__fallthrough__)); // warning

I think that if the "1" in this case, depends on the initial
configuration, as it is the case with CONFIG_CPUSETS, then
Clang should not cause a warning either. That's how GCC seems
to be treating these scenarios.

--
Gustavo

> case 2:
> return 3;
> default:
> return 4;
> }
> }
>
> I am not really sure how to resolve that within checkFallThroughIntoBlock() or
> fillReachableBlocks() but given that this is something specific to the kernel,
> we could introduce -Wimplicit-fallthrough-unreachable then disable it within
> the kernel.
>
> The file location not showing up was fixed by commit 1b4800c26259
> ("[clang][parser] Set source ranges for GNU-style attributes"). The
> differential revision mentions this issue specifically."
>
> Hopefully that would be an adequate solution, otherwise someone with more clang
> internal will have to take a look.
>
> Cheers,
> Nathan
>