linux-kernel - Re: [PATCH 2/3] powerpc: fix build errors

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Date:   Fri, 25 Feb 2022 20:51:59 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Anders Roxell <anders.roxell@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        "# 3.4.x" <stable@...r.kernel.org>
Subject: Re: [PATCH 2/3] powerpc: fix build errors

Excerpts from Arnd Bergmann's message of February 25, 2022 6:33 pm:
> On Fri, Feb 25, 2022 at 1:32 AM Nicholas Piggin <npiggin@...il.com> wrote:
>> Excerpts from Segher Boessenkool's message of February 25, 2022 3:12 am:
>> >> +#ifdef CONFIG_CC_IS_GCC
>> >> +#if (GCC_VERSION >= 100000)
>> >> +#if (CONFIG_AS_VERSION == 23800)
>> >> +asm(".machine any");
>> >> +#endif
>> >> +#endif
>> >> +#endif
>> >> +#endif /* __ASSEMBLY__ */
>> >
>> > Abusing toplevel asm like this is broken and you *will* end up with
>> > unhappiness all around.
>>
>> It actually unbreaks things and reduces my unhappiness. It's only done
>> for broken compiler versions and only where as does not have the
>> workaround for the breakage.
> 
> It doesn't work with clang, which always passes explicit .machine
> statements around each inline asm, and it's also fundamentally
> incompatible with LTO builds. Generally speaking, you can't expect
> a top-level asm statement to have any effect inside of another
> function.

You have misunderstood my patch. It is not supposed to "work" with
clang and it explicitly is complied out of clang. It's not intended
to have any implementation independent meaning. It's working around
a very specific issue with specific versions of gcc, and that's what
it does.

It's also not intended to be the final solution, it's a workaround
hack. We will move away from -many of course. I will post it as a
series since which hopefully will make it less confusing to people.

Thanks,
Nick