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

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Message-ID: <CAMzpN2gu4stkRKTsMTVxyzckO3SMhfA+dmCnSu6-aMg5QAA_JQ@mail.gmail.com>
Date:   Thu, 7 May 2020 21:57:07 -0400
From:   Brian Gerst <brgerst@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.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 6:29 PM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> 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.

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.

--
Brian Gerst