linux-kernel - Re: [PATCH v6 1/2] x86: fix bitops.h warning with a moved cast

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Date:   Mon, 4 May 2020 18:14:43 -0700
From:   Jesse Brandeburg <jesse.brandeburg@...el.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     <clang-built-linux@...glegroups.com>,
        <andriy.shevchenko@...el.com>, <bp@...en8.de>,
        <dan.j.williams@...el.com>, <linux-kernel@...r.kernel.org>,
        <linux@...musvillemoes.dk>, <mingo@...hat.com>,
        <peterz@...radead.org>, <tglx@...utronix.de>, <x86@...nel.org>,
        <ilie.halip@...il.com>, <natechancellor@...il.com>
Subject: Re: [PATCH v6 1/2] x86: fix bitops.h warning with a moved cast

On Mon, 4 May 2020 12:51:12 -0700
Nick Desaulniers <ndesaulniers@...gle.com> wrote:

> Sorry for the very late report.  It turns out that if your config
> tickles __builtin_constant_p just right, this now produces invalid
> assembly:
> 
> $ cat foo.c
> long a(long b, long c) {
>   asm("orb\t%1, %0" : "+q"(c): "r"(b));
>   return c;
> }
> $ gcc foo.c
> foo.c: Assembler messages:
> foo.c:2: Error: `%rax' not allowed with `orb'
> 
> The "q" constraint only has meanting on -m32 otherwise is treated as
> "r".
> 
> Since we have the mask (& 0xff), can we drop the `b` suffix from the
> instruction? Or is a revert more appropriate? Or maybe another way to
> skin this cat?

Figures that such a small change can create problems :-( Sorry for the
thrash!

The patches in the link below basically add back the cast, but I'm
interested to see if any others can come up with a better fix that
a) passes the above code generation test
b) still keeps sparse happy
c) passes the test module and the code inspection

If need be I'm OK with a revert of the original patch to fix the issue
in the short term, but it seems to me there must be a way to satisfy
both tools.  We went through several iterations on the way to the final
patch that we might be able to pluck something useful from.

> Link: https://github.com/ClangBuiltLinux/linux/issues/961
> This is blowing up our KernelCI reports.

ASIDE: 
Bummer, how come none of those KernelCI reports are part of
zero-day testing at https://01.org/lkp/documentation/0-day-test-service
I'm interested in your answer but don't want to pollute this thread,
feel free to contact me directly for this one or start a new thread?