linux-kernel - Re: [PATCH] powerpc: workaround clang codegen bug in dcbz

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Message-ID: <20190730170728.GQ31406@gate.crashing.org>
Date:   Tue, 30 Jul 2019 12:07:28 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     kbuild test robot <lkp@...el.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Paul Mackerras <paulus@...ba.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] powerpc: workaround clang codegen bug in dcbz

On Tue, Jul 30, 2019 at 11:16:37AM -0500, Segher Boessenkool wrote:
> in_le32 and friends?  Yeah, huh.  If LLVM copies that to the stack as
> well, its (not byte reversing) read will be atomic just fine, so things
> will still work correctly.
> 
> The things defined with DEF_MMIO_IN_D (instead of DEF_MMIO_IN_X) do not
> look like they will work correctly if an update form address is chosen,
> but that won't happen because the constraint is "m" instead of "m<>",
> making the %Un pretty useless (it will always be the empty string).

Btw, this is true since GCC 4.8; before 4.8, plain "m" *could* have an
automodify (autoinc, autodec, etc.) side effect.  What is the minimum
GCC version required, these days?

https://gcc.gnu.org/PR44492
https://gcc.gnu.org/r161328


Segher