RFR: 8322535: Change default AArch64 SpinPause instruction

[Stuart Monteith]

On Mon, 15 Jan 2024 16:25:08 GMT, Fredrik Bredberg <fbredberg at openjdk.org> wrote:

> The Java options OnSpinWaitInst lets you choose which AArch64 instruction should be used in `SpinPause()`. Valid values are "none", "nop", "isb" and "yield". Today the default value for OnSpinWaitInst is unfortunately "none".
> 
> However some CPUs changes the default SpinPause instruction to something better if the user hasn't used the OnSpinWaitInst option. For instance if you run a Neoverse N1, N2, V1 or V2, the default SpinPause instruction will be changed to "isb". After doing some measurements on Apple's M1-M3 CPUs it also seems like "isb" is the best yielding instruction on on those CPUs.
> 
> This PR changes the default SpinPause instruction to "yield" on all AArch64 platforms except on Apple's M1, M2 and M3 CPUs on which the default value will be "isb".
> 
> Tested tier1-tier7 successfully on linux-aarch64 and macosx-aarch64.

For the most part, "YIELD" is probably going to be equivalent to a "NOP". Unless there is a a demonstrable reason for this change, I would leave it as it is.
With regards to the change, do you have a suite of benchmark data that demonstrates this is a benefit on Apple Silicon?
Otherwise, as @theRealAph  says, microbenchmarks can demonstrate an benefit from ISBs, but applications overall won't necessarily show any benefit.

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PR Comment: https://git.openjdk.org/jdk/pull/17430#issuecomment-1893545918