linux-kernel - Re: [PATCH] x86: bitops: fix build regression

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Message-ID: <8f53b69e-86cc-7ff9-671e-5e0a67ff75a2@zytor.com>
Date:   Fri, 8 May 2020 10:31:36 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Brian Gerst <brgerst@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Sedat Dilek <sedat.dilek@...il.com>,
        stable <stable@...r.kernel.org>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        "kernelci . org bot" <bot@...nelci.org>,
        Andy Shevchenko <andriy.shevchenko@...el.com>,
        Ilie Halip <ilie.halip@...il.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Marco Elver <elver@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Daniel Axtens <dja@...ens.net>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] x86: bitops: fix build regression

On 2020-05-08 10:21, Nick Desaulniers wrote:
>>
>> One last suggestion.  Add the "b" modifier to the mask operand: "orb
>> %b1, %0".  That forces the compiler to use the 8-bit register name
>> instead of trying to deduce the width from the input.
> 
> Ah right: https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#x86Operandmodifiers
> 
> Looks like that works for both compilers.  In that case, we can likely
> drop the `& 0xff`, too.  Let me play with that, then I'll hopefully
> send a v3 today.
> 

Good idea. I requested a while ago that they document these modifiers; they
chose not to document them all which in some ways is good; it shows what they
are willing to commit to indefinitely.

	-hpa