RFR: 8322535: Change default AArch64 SpinPause instruction

[Coleen Phillimore]

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.

It seems to me that we should just change the default to 'yield' and let other platforms determine the best tuning, since this might be the cause of aarch64 regression in JDK-8324221.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/17430#issuecomment-1915206782