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SubjectRe: [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

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