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    SubjectRe: [PATCH] x86: bitops: fix build regression
    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

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