Re: Improving spin-lock implementation on ARM.

On Fri, Nov 27, 2020 at 2:20 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Alexander Korotkov <aekorotkov(at)gmail(dot)com> writes:
> > On Thu, Nov 26, 2020 at 1:32 PM Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
> >> Is there some official ARM documentation, like a programmer's reference
> >> manual or something like that, that would show a reference
> >> implementation of a spinlock on ARM? It would be good to refer to an
> >> authoritative source on this.
>
> > I've compared assembly output of gcc implementations of CAS and TAS.
>
> FWIW, I see quite different assembly using Apple's clang on their M1
> processor. What I get for SpinLockAcquire on HEAD is (lock pointer
> initially in x0):

Yep, arm v8.1 implements single-instruction atomic operations swpal
and casa, which much more look like x86 atomic instructions rather
than loops of ldxr/stlxr.

So, all the reasoning upthread shouldn't work here, but the advantage
is much more huge.

------
Regards,
Alexander Korotkov