Re: [PATCH] powerpc: slightly improve cache helpers

From: Christophe Leroy
Date: Tue May 07 2019 - 12:54:26 EST

Le 07/05/2019 Ã 17:10, Segher Boessenkool a ÃcritÂ:
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 ( says it is better to use 'm':


Memory operand that is an indexed or indirect from a register (it is usually better to use âmâ or âesâ in asm statements)

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


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").