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

[Evgeny Astigeevich]

On Thu, 23 Sep 2021 10:45:10 GMT, Patrick Zhang <qpzhang at openjdk.org> wrote:

>> Evgeny Astigeevich has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Move spin_wait in cpp file with removal of loop macro
>>   
>>   In addition, comments are added to a checking method of a test.
>
> 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.

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