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

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Message-ID: <CAKwvOdkBaxyOiO8+_cdBenkOj5cqcn9OhThOLmbt1uMyTW1P6g@mail.gmail.com>
Date:   Thu, 7 May 2020 12:22:30 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Brian Gerst <brgerst@...il.com>
Cc:     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>,
        "H. Peter Anvin" <hpa@...or.com>, Marco Elver <elver@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.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 Wed, May 6, 2020 at 11:18 PM Brian Gerst <brgerst@...il.com> wrote:
>
> I think a better fix would be to make CONST_MASK() return a u8 value
> rather than have to cast on every use.
>
> Also I question the need for the "q" constraint.  It was added in
> commit 437a0a54 as a workaround for GCC 3.4.4.  Now that the minimum
> GCC version is 4.6, is this still necessary?

TL;DR yes

ah, thanks for the git archeology, it's useful.  I don't think this is
a compiler bug however, just the compiler being more strict that the
`b` suffix on `orb` requires a 8b register operand.  For 32b x86, `q`
asm constraint is required, because not all GPR's had lower 8b
register aliases, as Arnd found and HPA noted as well.

I like your suggested change to CONST_MASK, and will send that in a bit.
-- 
Thanks,
~Nick Desaulniers