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

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Date:   Tue, 7 May 2019 10:10:30 -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

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".

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