linux-kernel - Re: [PATCH v2] powerpc: slightly improve cache helpers

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Date:   Mon, 8 Jul 2019 23:49:52 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        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 v2] powerpc: slightly improve cache helpers

On Tue, Jul 09, 2019 at 07:04:43AM +0200, Christophe Leroy wrote:
> 
> 
> Le 08/07/2019 à 21:14, Nathan Chancellor a écrit :
> > On Mon, Jul 08, 2019 at 11:19:30AM +1000, Michael Ellerman wrote:
> > > On Fri, 2019-05-10 at 09:24:48 UTC, Christophe Leroy wrote:
> > > > Cache instructions (dcbz, dcbi, dcbf and dcbst) take two registers
> > > > that are summed to obtain the target address. Using 'Z' constraint
> > > > and '%y0' argument gives GCC the opportunity to use both registers
> > > > instead of only one with the second being forced to 0.
> > > > 
> > > > Suggested-by: Segher Boessenkool <segher@...nel.crashing.org>
> > > > Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> > > 
> > > Applied to powerpc next, thanks.
> > > 
> > > https://git.kernel.org/powerpc/c/6c5875843b87c3adea2beade9d1b8b3d4523900a
> > > 
> > > cheers
> > 
> > This patch causes a regression with clang:
> 
> Is that a Clang bug ?

No idea, it happens with clang-8 and clang-9 though (pretty sure there
were fixes for PowerPC in clang-8 so something before it probably won't
work but I haven't tried).

> 
> Do you have a disassembly of the code both with and without this patch in
> order to compare ?

I can give you whatever disassembly you want (or I can upload the raw
files if that is easier).

Cheers,
Nathan

> 
> Segher, any idea ?
> 
> Christophe
> 
> > 
> > https://travis-ci.com/ClangBuiltLinux/continuous-integration/jobs/213944668
> > 
> > I've attached my local bisect/build log.
> > 
> > Cheers,
> > Nathan
> >