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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Fri, 8 May 2020 10:21:48 -0700
Message-ID: <CAKwvOd=hVKrFU+imSGeX+MEKMpW97gE7bzn1C637qdns9KSnUA@mail.gmail.com>
Subject: Re: [PATCH] x86: bitops: fix build regression
To:     Brian Gerst <brgerst@gmail.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Sedat Dilek <sedat.dilek@gmail.com>,
        stable <stable@vger.kernel.org>,
        Jesse Brandeburg <jesse.brandeburg@intel.com>,
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        Andy Shevchenko <andriy.shevchenko@intel.com>,
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        Marco Elver <elver@google.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
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        Masahiro Yamada <yamada.masahiro@socionext.com>,
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On Thu, May 7, 2020 at 6:57 PM Brian Gerst <brgerst@gmail.com> wrote:
>
> On Thu, May 7, 2020 at 6:29 PM Nick Desaulniers <ndesaulniers@google.com> wrote:
> >
> > On Thu, May 7, 2020 at 12:19 PM Nick Desaulniers
> > <ndesaulniers@google.com> wrote:
> > >
> > > On Thu, May 7, 2020 at 7:00 AM Brian Gerst <brgerst@gmail.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.

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.
-- 
Thanks,
~Nick Desaulniers