Re: [PATCH] powerpc: slightly improve cache helpers

[Christophe Leroy]

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 
(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 âmâ or âesâ 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