Re: Improving spin-lock implementation on ARM.

Alexander Korotkov <aekorotkov(at)gmail(dot)com> writes:
> I tend to think that LSE is enabled by default in Apple's clang based
> on your previous message[1]. In order to dispel the doubts could you
> please provide assembly of SpinLockAcquire for following clang
> options.
> "-O2"
> "-O2 -march=armv8-a+lse"
> "-O2 -march=armv8-a"

Huh. Those options make exactly zero difference to the code generated
for SpinLockAcquire/SpinLockRelease; it's the same as I showed upthread,
for either the HEAD definition of TAS() or the CAS patch's version.

So now I'm at a loss as to the reason for the performance difference
I got. -march=armv8-a+lse does make a difference to code generation
someplace, because the overall size of the postgres executable changes
by 16kB or so. One might argue that the performance difference is due
to better code elsewhere than the spinlocks ... but the test I'm running
is basically just

while (count-- > 0)
{
XLogGetLastRemovedSegno();

CHECK_FOR_INTERRUPTS();
}

so it's hard to see where a non-spinlock-related code change would come
in. That loop itself definitely generates the same code either way.

I did find this interesting output from "clang -v":

-target-cpu vortex -target-feature +v8.3a -target-feature +fp-armv8 -target-feature +neon -target-feature +crc -target-feature +crypto -target-feature +fullfp16 -target-feature +ras -target-feature +lse -target-feature +rdm -target-feature +rcpc -target-feature +zcm -target-feature +zcz -target-feature +sha2 -target-feature +aes

whereas adding -march=armv8-a+lse changes that to just

-target-cpu vortex -target-feature +neon -target-feature +lse -target-feature +zcm -target-feature +zcz

On the whole, that would make one think that -march=armv8-a+lse
should produce worse code than the default settings.

regards, tom lane