On Wed, May 06, 2020 at 11:36:00AM +1000, Michael Ellerman wrote:
> >> As far as I understood that's mandatory on recent gcc to get the 
> >> pre-update form of the instruction. With older versions "m" was doing 
> >> the same, but not anymore.
> >
> > Yes.  How much that matters depends on the asm.  On older CPUs (6xx/7xx,
> > say) the update form was just as fast as the non-update form.  On newer
> > or bigger CPUs it is usually executed just the same as an add followed
> > by the memory access, so it just saves a bit of code size.
> The update-forms are stdux, sthux etc. right?

And stdu, sthu, etc.  "x" is "indexed form" (reg+reg addressing).

> I don't see any change in the number of those with or without the
> constraint. That's using GCC 9.3.0.

It's most useful in loops (and happens more often there).  You probably
do not have many loops that allow update form insns.

> >> Should we ifdef the "m<>" or "m" based on GCC 
> >> version ?
> >
> > That will be a lot of churn.  Just make 4.8 minimum?
> As I said in my other mail that's not really up to us. We could mandate
> a higher minimum for powerpc, but I'd rather not.

Yeah, I quite understand that.

> I think for now I'm inclined to just drop the "<>", and we can revisit
> in a release or two when hopefully GCC 4.8 has become the minimum.

An unhappy resolution, but it leaves a light on the horizon :-)

In that case, leave the "%Un", if you will but the "<>" back soonish?
Not much point removing it and putting it back later (it is harmless,
there are more instances of it in the kernel as well, since many years).



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