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

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Message-ID: <CAK8P3a3LynWTbpV8=VPm2TqgNM2MnoEyCPJd0PL2D+tcZqJgHg@mail.gmail.com>
Date:   Fri, 9 Aug 2019 20:28:19 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        christophe leroy <christophe.leroy@....fr>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        kbuild test robot <lkp@...el.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] powerpc: fix inline asm constraints for dcbz

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.

        Arnd