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

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

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
>