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

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Date:   Mon, 29 Jul 2019 13:32:46 -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: workaround clang codegen bug in dcbz

On Mon, Jul 29, 2019 at 01:25:41PM -0700, Nick Desaulniers wrote:
> Commit 6c5875843b87 ("powerpc: slightly improve cache helpers") exposed
> what looks like a codegen bug in Clang's handling of `%y` output
> template with `Z` constraint. This is resulting in panics during boot
> for 32b powerpc builds w/ Clang, as reported by our CI.
> 
> Add back the original code that worked behind a preprocessor check for
> __clang__ until we can fix LLVM.
> 
> Further, it seems that clang allnoconfig builds are unhappy with `Z`, as
> reported by 0day bot. This is likely because Clang warns about inline
> asm constraints when the constraint requires inlining to be semantically
> valid.
> 
> Link: https://bugs.llvm.org/show_bug.cgi?id=42762
> Link: https://github.com/ClangBuiltLinux/linux/issues/593
> Link: https://lore.kernel.org/lkml/20190721075846.GA97701@archlinux-threadripper/
> Debugged-by: Nathan Chancellor <natechancellor@...il.com>
> Reported-by: Nathan Chancellor <natechancellor@...il.com>
> Reported-by: kbuild test robot <lkp@...el.com>
> Suggested-by: Nathan Chancellor <natechancellor@...il.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
> Alternatively, we could just revert 6c5875843b87. It seems that GCC
> generates the same code for these functions for out of line versions.
> But I'm not sure how the inlined code generated would be affected.

For the record:

https://godbolt.org/z/z57VU7

This seems consistent with what Michael found so I don't think a revert
is entirely unreasonable.

Either way:

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>