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

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Date:   Fri, 9 Aug 2019 13:36:45 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     mpe@...erman.id.au, christophe.leroy@....fr,
        segher@...nel.crashing.org, arnd@...db.de,
        kbuild test robot <lkp@...el.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] powerpc: fix inline asm constraints for dcbz

On Fri, Aug 09, 2019 at 11:21:05AM -0700, Nick Desaulniers wrote:
> The input parameter is modified, so it should be an output parameter
> with "=" to make it so that a copy of the input is not made by Clang.
> 
> Link: https://bugs.llvm.org/show_bug.cgi?id=42762
> Link: https://gcc.gnu.org/onlinedocs/gcc/Modifiers.html#Modifiers
> Link: https://github.com/ClangBuiltLinux/linux/issues/593
> Link: https://godbolt.org/z/QwhZXi
> Link: https://lore.kernel.org/lkml/20190721075846.GA97701@archlinux-threadripper/
> Fixes: 6c5875843b87 ("powerpc: slightly improve cache helpers")
> Debugged-by: Nathan Chancellor <natechancellor@...il.com>
> Reported-by: Nathan Chancellor <natechancellor@...il.com>
> Reported-by: kbuild test robot <lkp@...el.com>
> Suggested-by: Arnd Bergmann <arnd@...db.de>
> Suggested-by: Nathan Chancellor <natechancellor@...il.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>

I applied this patch as well as a revert of the original patch and both
clang and GCC appear to generate the same code; I think a straight
revert would be better.

Crude testing script and the generated files attached.

Cheers,
Nathan

Download attachment "tmp.bRmcRT0jd0.sh" of type "application/x-sh" (2653 bytes)

Download attachment "testing-output.tar.gz" of type "application/gzip" (16412 bytes)