Re: [PATCH v6 1/2] x86: fix bitops.h warning with a moved cast

[Nick Desaulniers <ndesaulniers@google.com>]

On Tue, May 5, 2020 at 8:14 AM Andy Shevchenko
<andriy.shevchenko@intel.com> wrote:
>
> On Mon, May 04, 2020 at 06:14:43PM -0700, Jesse Brandeburg wrote:
> > On Mon, 4 May 2020 12:51:12 -0700
> > Nick Desaulniers <ndesaulniers@google.com> wrote:
> >
> > > Sorry for the very late report.  It turns out that if your config
> > > tickles __builtin_constant_p just right, this now produces invalid
> > > assembly:
> > >
> > > $ cat foo.c
> > > long a(long b, long c) {
> > >   asm("orb\t%1, %0" : "+q"(c): "r"(b));
> > >   return c;
> > > }
> > > $ gcc foo.c
> > > foo.c: Assembler messages:
> > > foo.c:2: Error: `%rax' not allowed with `orb'
> > >
> > > The "q" constraint only has meanting on -m32 otherwise is treated as
> > > "r".
> > >
> > > Since we have the mask (& 0xff), can we drop the `b` suffix from the
> > > instruction? Or is a revert more appropriate? Or maybe another way to
> > > skin this cat?
> >
> > Figures that such a small change can create problems :-( Sorry for the
> > thrash!
> >
> > The patches in the link below basically add back the cast, but I'm
> > interested to see if any others can come up with a better fix that
> > a) passes the above code generation test
> > b) still keeps sparse happy
> > c) passes the test module and the code inspection
> >
> > If need be I'm OK with a revert of the original patch to fix the issue
> > in the short term, but it seems to me there must be a way to satisfy
> > both tools.  We went through several iterations on the way to the final
> > patch that we might be able to pluck something useful from.
>
> For me the below seems to work:

Yep:
https://github.com/ClangBuiltLinux/linux/issues/961#issuecomment-623785987
https://github.com/ClangBuiltLinux/linux/issues/961#issuecomment-624162497
Sedat wrote the same patch 22 days ago; I didn't notice before
starting this thread.  I will sign off on his patch, add your
Suggested by tag, and send shortly.

>
>
> diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
> index b392571c1f1d1..139122e5b25b1 100644
> --- a/arch/x86/include/asm/bitops.h
> +++ b/arch/x86/include/asm/bitops.h
> @@ -54,7 +54,7 @@ arch_set_bit(long nr, volatile unsigned long *addr)
>         if (__builtin_constant_p(nr)) {
>                 asm volatile(LOCK_PREFIX "orb %1,%0"
>                         : CONST_MASK_ADDR(nr, addr)
> -                       : "iq" (CONST_MASK(nr) & 0xff)
> +                       : "iq" ((u8)(CONST_MASK(nr) & 0xff))
>                         : "memory");
>         } else {
>                 asm volatile(LOCK_PREFIX __ASM_SIZE(bts) " %1,%0"
> @@ -74,7 +74,7 @@ arch_clear_bit(long nr, volatile unsigned long *addr)
>         if (__builtin_constant_p(nr)) {
>                 asm volatile(LOCK_PREFIX "andb %1,%0"
>                         : CONST_MASK_ADDR(nr, addr)
> -                       : "iq" (CONST_MASK(nr) ^ 0xff));
> +                       : "iq" ((u8)(CONST_MASK(nr) ^ 0xff)));
>         } else {
>                 asm volatile(LOCK_PREFIX __ASM_SIZE(btr) " %1,%0"
>                         : : RLONG_ADDR(addr), "Ir" (nr) : "memory");
>
>
> --
> With Best Regards,
> Andy Shevchenko
>
>


-- 
Thanks,
~Nick Desaulniers