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

From: Arnd Bergmann
Date: Fri Aug 09 2019 - 14:28:37 EST

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.