Re: [PATCH 2/2] powerpc/64: Use -mtune=pwr10/9/8 for clang

From: Nathan Chancellor <nathan@kernel.org>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: linuxppc-dev@lists.ozlabs.org, linux-kbuild@vger.kernel.org,
	llvm@lists.linux.dev, Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: [PATCH 2/2] powerpc/64: Use -mtune=pwr10/9/8 for clang
Date: Fri, 3 Mar 2023 08:14:00 -0700	[thread overview]
Message-ID: <20230303151400.GA2665@dev-arch.thelio-3990X> (raw)
In-Reply-To: <878rgelnhd.fsf@mpe.ellerman.id.au>

On Fri, Mar 03, 2023 at 10:53:02AM +1100, Michael Ellerman wrote:
> Nathan Chancellor <nathan@kernel.org> writes:
> > Hi Michael,
> >
> > Thanks for the workaround and sorry this has come to bite us :/
> >
> > On Fri, Mar 03, 2023 at 12:16:56AM +1100, Michael Ellerman wrote:
> >> For the -mtune option clang doesn't accept power10/9/8, instead it
> >> accepts pwr10/9/8. That will be fixed in future versions of clang, but
> >> the kernel must support the clang versions in the wild.
> >> 
> >> So add support for the "pwr" spelling if clang is in use.
> >> 
> >> Reported-by: Nathan Chancellor <nathan@kernel.org>
> >
> > I think that should actually be
> >
> > Reported-by: Nick Desaulniers <ndesaulniers@google.com>
> 
> I guess yeah.
> 
> >> BugLink: https://github.com/ClangBuiltLinux/linux/issues/1799
> >> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> >
> > Reviewed-by: Nathan Chancellor <nathan@kernel.org>
> 
> Thanks.
> 
> >> ---
> >>  arch/powerpc/platforms/Kconfig.cputype | 4 ++++
> >>  1 file changed, 4 insertions(+)
> >> 
> >> Need to confirm the clang <= 16 statement is correct.
> >
> > Currently, this is indeed the case. It is possible that Nemanja's patch
> > will get applied to release/16.x before 16.0.0 final but it might not.
> >
> > We can always update it later. I think we do want to push to get that
> > patch applied because I forgot that it is only in 16.0.0 that '-mtune'
> > starts to do something on PowerPC:
> >
> > https://github.com/llvm/llvm-project/commit/1dc26b80b872a94c581549a21943756a8c3448a3
> >
> > Prior to that change, '-mtune' was accepted but did nothing. It is only
> > once it was hooked up to the backend that we got the spew of warnings. I
> > think that warrants us trying to get Nemanja's patch into 16.0.0, which
> > may allow us to drop this workaround altogether...
> 
> Aha OK, I missed that the warning was new in 16.
> 
> I'll sit on this for now then until we know if that change will make it
> into clang 16.

It was merged into release/16.x a few hours ago:

https://github.com/llvm/llvm-project/issues/61128

https://github.com/llvm/llvm-project/commit/9b2e09e9fb1aa3bbe3668d7cc585188a3014d1b9

So I think this particular workaround is no longer needed :)

Cheers,
Nathan