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Date:   Fri, 9 Aug 2019 17:00:41 -0500
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Christophe Leroy <christophe.leroy@c-s.fr>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Paul Mackerras <paulus@samba.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        kbuild test robot <lkp@intel.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: [PATCH] powerpc: fix inline asm constraints for dcbz
Message-ID: <20190809220041.GO31406@gate.crashing.org>
References: <87h873zs88.fsf@concordia.ellerman.id.au> <20190809182106.62130-1-ndesaulniers@google.com> <CAK8P3a3LynWTbpV8=VPm2TqgNM2MnoEyCPJd0PL2D+tcZqJgHg@mail.gmail.com> <20190809220301.Horde.AR6y4Bx4WGIq58V9K0En9g4@messagerie.si.c-s.fr>
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On Fri, Aug 09, 2019 at 10:03:01PM +0200, Christophe Leroy wrote:
> Arnd Bergmann <arnd@arndb.de> a ?crit?:
> 
> >On Fri, Aug 9, 2019 at 8:21 PM 'Nick Desaulniers' via Clang Built
> >Linux <clang-built-linux@googlegroups.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