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From:   Brian Gerst <brgerst@gmail.com>
Date:   Mon, 11 May 2020 14:52:16 -0400
Message-ID: <CAMzpN2hUQwGoYQnsKZJHFY=p-=-zXTwhRnCejhoztCvPX8e=CA@mail.gmail.com>
Subject: Re: [PATCH v5] x86: bitops: fix build regression
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Jesse Brandeburg <jesse.brandeburg@intel.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        "kernelci . org bot" <bot@kernelci.org>,
        Andy Shevchenko <andriy.shevchenko@intel.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Ilie Halip <ilie.halip@gmail.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Marco Elver <elver@google.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Daniel Axtens <dja@axtens.net>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
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On Fri, May 8, 2020 at 2:32 PM Nick Desaulniers <ndesaulniers@google.com> wrote:
>
> This is easily reproducible via CC=clang+CONFIG_STAGING=y+CONFIG_VT6656=m.
>
> It turns out that if your config tickles __builtin_constant_p via
> differences in choices to inline or not, these statements produce
> 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'
>
> Use the `%b` "x86 Operand Modifier" to instead force register allocation
> to select a lower-8-bit GPR operand.
>
> The "q" constraint only has meaning on -m32 otherwise is treated as
> "r". Not all GPRs have low-8-bit aliases for -m32.
>
> Cc: Jesse Brandeburg <jesse.brandeburg@intel.com>
> Link: https://github.com/ClangBuiltLinux/linux/issues/961
> Link: https://lore.kernel.org/lkml/20200504193524.GA221287@google.com/
> Link: https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#x86Operandmodifiers
> Fixes: 1651e700664b4 ("x86: Fix bitops.h warning with a moved cast")
> Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
> Reported-by: kernelci.org bot <bot@kernelci.org>
> Suggested-by: Andy Shevchenko <andriy.shevchenko@intel.com>
> Suggested-by: Brian Gerst <brgerst@gmail.com>
> Suggested-by: H. Peter Anvin <hpa@zytor.com>
> Suggested-by: Ilie Halip <ilie.halip@gmail.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

Reviewed-By: Brian Gerst <brgerst@gmail.com>