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

[Nick Desaulniers]

On Thu, May 7, 2020 at 12:19 PM Nick Desaulniers
<ndesaulniers@xxxxxxxxxx> wrote:
>
> On Thu, May 7, 2020 at 7:00 AM Brian Gerst <brgerst@xxxxxxxxx> 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