Re: [PATCH] powerpc: fix inline asm constraints for dcbz

[Christophe Leroy]

Arnd Bergmann <arnd@xxxxxxxx> a ÃcritÂ:

> On Fri, Aug 9, 2019 at 8:21 PM 'Nick Desaulniers' via Clang Built
> Linux <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
>
> >  static inline void dcbz(void *addr)
> >  {
> > -       __asm__ __volatile__ ("dcbz %y0" : : "Z"(*(u8 *)addr) : "memory");
> > +       __asm__ __volatile__ ("dcbz %y0" : "=Z"(*(u8 *)addr) :: "memory");
> >  }
> >
> >  static inline void dcbi(void *addr)
> >  {
> > -       __asm__ __volatile__ ("dcbi %y0" : : "Z"(*(u8 *)addr) : "memory");
> > +       __asm__ __volatile__ ("dcbi %y0" : "=Z"(*(u8 *)addr) :: "memory");
> >  }
>
> I think the result of the discussion was that an output argument only kind-of
> makes sense for dcbz, but for the others it's really an input, and clang is
> wrong in the way it handles the "Z" constraint by making a copy, which it
> doesn't do for "m".
>
> I'm not sure whether it's correct to use "m" instead of "Z" here, which
> would be a better workaround if that works. More importantly though,
> clang really needs to be fixed to handle "Z" correctly.

As the benefit is null, I think the best is probably to reverse my  
original commit until at least CLang is fixed, as initialy suggested  
by mpe

Christophe