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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 11 May 2020 10:22:50 -0700
Message-ID: <CAKwvOdn6YUZvatghWSScNcJMd2Y3_dfU5ZtY2in=DDYdFweVOQ@mail.gmail.com>
Subject: Re: [PATCH v5] x86: bitops: fix build regression
To:     Andy Shevchenko <andy.shevchenko@gmail.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>,
        Brian Gerst <brgerst@gmail.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Ilie Halip <ilie.halip@gmail.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <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 Sat, May 9, 2020 at 5:20 AM Andy Shevchenko
<andy.shevchenko@gmail.com> wrote:
>
> On Fri, May 8, 2020 at 9:35 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.
> >
>
> Looks very good!
> One question though, does it work with minimum supported version of gcc?

Yes; the oldest version of GCC in godbolt.org is GCC 4.1.2 which
supports the syntax and generates the expected lower-8-bit register
operands.
-- 
Thanks,
~Nick Desaulniers