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

Le 29/07/2019 à 22:32, Nathan Chancellor a écrit :

On Mon, Jul 29, 2019 at 01:25:41PM -0700, Nick Desaulniers wrote:

quoted

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/ (local)
Debugged-by: Nathan Chancellor [off-list ref]
Reported-by: Nathan Chancellor <redacted>
Reported-by: kbuild test robot <redacted>
Suggested-by: Nathan Chancellor <redacted>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.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.

Your example functions are too simple to show anything. The functions takes only one parameter so of course GCC won't use two registers allthough given the opportunity.

Christophe

Either way:

Reviewed-by: Nathan Chancellor <redacted>