Re: [PATCH] powerpc: slightly improve cache helpers

From: Segher Boessenkool
Date: Tue May 07 2019 - 11:11:50 EST

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").