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

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Message-ID: <20190709133540.GJ30355@gate.crashing.org>
Date:   Tue, 9 Jul 2019 08:35:40 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        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 ?

I would think so, but cannot tell from the given information.

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

That's what we need to start debugging this, yup.

> Segher, any idea ?

There is nothing I recognise, no.


Segher