RFR: 8186670: Implement _onSpinWait() intrinsic for AArch64 [v4]

[Andrew Haley]

On Thu, 23 Sep 2021 11:04:32 GMT, Evgeny Astigeevich <github.com+42899633+eastig at openjdk.org> wrote:

>> src/hotspot/cpu/aarch64/globals_aarch64.hpp line 116:
>> 
>>> 114:   product(ccstr, OnSpinWaitImpl, "none",                                \
>>> 115:           "Use instructions to implement java.lang.Thread.onSpinWait()."\
>>> 116:           "Options: none, Nnop, Nisb, Nyield, where optional N is 2..9.")
>> 
>> Could this N be changed to 2..99 instead of 2..9? I tested this with SpinWaitBench.pong:totalSpins (ops/us), 9nop or 9yield is unable to provide a similar pause time as 1isb, maybe ~20 can do. A larger range can be more convenient for future experiments/tunings. Thanks.
>
> As Andrew suggested to have separated options for the name and the count, this won't be an issue.

It was more of a question than a suggestion. I'm not sure, and I'd like you to have a look at existing options to see if there's a precedent.

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PR: https://git.openjdk.java.net/jdk/pull/5562