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

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Date:   Fri, 9 Aug 2019 17:00:41 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Arnd Bergmann <arnd@...db.de>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Paul Mackerras <paulus@...ba.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        kbuild test robot <lkp@...el.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH] powerpc: fix inline asm constraints for dcbz

On Fri, Aug 09, 2019 at 10:03:01PM +0200, Christophe Leroy wrote:
> Arnd Bergmann <arnd@...db.de> a écrit :
> 
> >On Fri, Aug 9, 2019 at 8:21 PM 'Nick Desaulniers' via Clang Built
> >Linux <clang-built-linux@...glegroups.com> 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

And what about the other uses of "Z"?


Also, if you use C routines (instead of assembler code) for the basic
"clear a block" and the like routines, as there have been patches for
recently, the benefit is not zero.


Segher