Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH] powerpc: slightly improve cache helpers
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
References: <0b460a85319fb89dab2c5d1200ac69a3e1b7c1ef.1557235807.git.christophe.leroy@c-s.fr>
 <20190507151030.GF8599@gate.crashing.org>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <720e7c77-3f5c-83f3-6013-36b265c1ba73@c-s.fr>
Date:   Tue, 7 May 2019 18:53:30 +0200
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:60.0) Gecko/20100101
 Thunderbird/60.6.1
MIME-Version: 1.0
In-Reply-To: <20190507151030.GF8599@gate.crashing.org>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: fr
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


Le 07/05/2019 à 17:10, Segher Boessenkool a écrit :
> Hi Christophe,
> 
> On Tue, May 07, 2019 at 01:31:39PM +0000, Christophe Leroy wrote:
>> Cache instructions (dcbz, dcbi, dcbf and dcbst) take two registers
>> that are summed to obtain the target address. Using '%y0' argument
>> gives GCC the opportunity to use both registers instead of only one
>> with the second being forced to 0.
> 
> That's not quite right.  Sorry if I didn't explain it properly.
> 
> "m" allows all memory.  But this instruction only allows reg,reg and
> 0,reg addressing.  For that you need to use constraint "Z".

But gcc help 
(https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html#Machine-Constraints) 
says it is better to use 'm':

Z

     Memory operand that is an indexed or indirect from a register (it 
is usually better to use ‘m’ or ‘es’ in asm statements)

That's the reason why I used 'm', I thought it was equivalent.

Christophe

> 
> The output modifier "%y0" just makes [reg] (i.e. simple indirect addressing)
> print as "0,reg" instead of "0(reg)" as it would by default (for just "%0").
> 
> 
> Segher
>