RFR: 8277137: Set OnSpinWaitInst default value to "isb" for Arm Neoverse N1 [v2]

[Nick Gasson]

On Tue, 16 Nov 2021 19:15:11 GMT, Evgeny Astigeevich <duke at openjdk.java.net> wrote:

>> One `ISB` implementation of `Thread.OnSpinWait` shows performance improvements on Graviton2 (Arm Neoverse N1 implementation), e.g. https://github.com/openjdk/jdk/pull/5562#issuecomment-966153163. 
>> 
>> Testing:
>> - `make test TEST=gtest`: Passed
>> - `make run-test TEST=tier1`: Passed
>> - `make run-test TEST=tier2`: Passed
>> - `make run-test TEST=hotspot/jtreg/compiler/onSpinWait`: Passed
>
> Evgeny Astigeevich has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Explicitly set OnSpinWaitInstCount to 1

src/hotspot/cpu/aarch64/vm_version_aarch64.cpp line 206:

> 204:     }
> 205: 
> 206:     if (FLAG_IS_DEFAULT(OnSpinWaitInst) && FLAG_IS_DEFAULT(OnSpinWaitInstCount)) {

Should these two be set independently? If I pass `-XX:OnSpinWaitInstCount=2` then `OnSpinWaitInst` will default to "none".

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PR: https://git.openjdk.java.net/jdk/pull/6415