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From: Jesse Brandeburg <jesse.brandeburg@intel.com>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: <clang-built-linux@googlegroups.com>,
	<andriy.shevchenko@intel.com>, <bp@alien8.de>,
	<dan.j.williams@intel.com>, <linux-kernel@vger.kernel.org>,
	<linux@rasmusvillemoes.dk>, <mingo@redhat.com>,
	<peterz@infradead.org>, <tglx@linutronix.de>, <x86@kernel.org>,
	<ilie.halip@gmail.com>, <natechancellor@gmail.com>
Subject: Re: [PATCH v6 1/2] x86: fix bitops.h warning with a moved cast
Date: Mon, 4 May 2020 18:14:43 -0700	[thread overview]
Message-ID: <20200504181443.00007a3d@intel.com> (raw)
In-Reply-To: <20200504193524.GA221287@google.com>

On Mon, 4 May 2020 12:51:12 -0700
Nick Desaulniers <ndesaulniers@google.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?


  reply	other threads:[~2020-05-05  1:14 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-10 22:17 Jesse Brandeburg
2020-03-10 22:17 ` [PATCH v6 2/2] lib: make a test module with set/clear bit Jesse Brandeburg
2020-04-15 14:46   ` Andy Shevchenko
2020-03-11  4:37 ` [PATCH v6 1/2] x86: fix bitops.h warning with a moved cast Luc Van Oostenryck
2020-03-17 19:25 ` Peter Zijlstra
2020-03-18 21:59 ` [tip: x86/asm] x86: Fix " tip-bot2 for Jesse Brandeburg
2020-05-04 19:37 ` [PATCH v6 1/2] x86: fix " Nick Desaulniers
2020-05-05  1:14   ` Jesse Brandeburg [this message]
2020-05-05 15:14     ` Andy Shevchenko
2020-05-05 17:29       ` Nick Desaulniers
2020-05-05 17:47         ` Nick Desaulniers

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