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

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Date:   Thu, 7 May 2020 15:29:35 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Brian Gerst <brgerst@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        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 Thu, May 7, 2020 at 12:19 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Thu, May 7, 2020 at 7:00 AM Brian Gerst <brgerst@...il.com> wrote:
> >
> > This change will make sparse happy and allow these cleanups:
> > #define CONST_MASK(nr)                 ((u8)1 << ((nr) & 7))
>
> yep, this is more elegant, IMO.  Will send a v3 later with this
> change.  Looking at the uses of CONST_MASK, I noticed
> arch_change_bit() currently has the (u8) cast from commit
> 838e8bb71dc0c ("x86: Implement change_bit with immediate operand as
> "lock xorb""), so that instance can get cleaned up with the above
> suggestion.

Oh, we need the cast to be the final operation.  The binary AND and
XOR in 2 of the 3 uses of CONST_MASK implicitly promote the operands
of the binary operand to int, so the type of the evaluated
subexpression is int.
https://wiki.sei.cmu.edu/confluence/display/c/EXP14-C.+Beware+of+integer+promotion+when+performing+bitwise+operations+on+integer+types+smaller+than+int
So I think this version (v2) is most precise fix, and would be better
than defining more macros or (worse) using metaprogramming.
-- 
Thanks,
~Nick Desaulniers