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

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Date:   Tue, 9 Jul 2019 07:04:43 +0200
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Nathan Chancellor <natechancellor@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Segher Boessenkool <segher@...nel.crashing.org>
Cc:     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



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 ?

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

Segher, any idea ?

Christophe

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