RFR: 8322535: Change default AArch64 SpinPause instruction

[Andrew Haley]

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.

ISB isn't really the right thing for this. Sure, it causes a delay, but the extent of the delay depends on what else the processor is doing. In some cases an ISB can work well, in other cases not. Some micro benchmarks show a great improvement with ISB. 
It doesn't depend only on the target hardware, but on the application. Sure, in some cases an ISB is going to be exactly right, but on others it might be too much.

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PR Review: https://git.openjdk.org/jdk/pull/17430#pullrequestreview-1822989370