Subject: Re: [PATCH] powerpc: fix inline asm constraints for dcbz

From: Nathan Chancellor <[email-masked]>
Date: Fri, 9 Aug 2019 13:36:45 -0700

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 <[email-masked]>
> Reported-by: Nathan Chancellor <[email-masked]>
> Reported-by: kbuild test robot <[email-masked]>
> Suggested-by: Arnd Bergmann <[email-masked]>
> Suggested-by: Nathan Chancellor <[email-masked]>
> Signed-off-by: Nick Desaulniers <[email-masked]>

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