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

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Date:   Wed, 8 May 2019 09:40:58 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc: slightly improve cache helpers

On Tue, May 07, 2019 at 06:53:30PM +0200, Christophe Leroy wrote:
> Le 07/05/2019 à 17:10, Segher Boessenkool a écrit :
> >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':

It says it *usually* is better to use "m".  What it really should say is
it is better to use "m" _when that is valid_.  It is not valid for the
cache block instructions.

I'll fix up the comment...  "es" is ancient, too, nowadays it is
equivalent to just "m" (and you need "m<>" to allow pre-modify addressing).

> 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.

Yeah, the manual text could be clearer.


Segher