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

From: Christophe Leroy
Date: Fri Aug 09 2019 - 16:02:42 EST

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