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

[Paul Hohensee]

On Tue, 16 Nov 2021 18:14:15 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

I'd explicitly set OnSpinWaitInstCount because one has to go find the default value in another file to understand what's going to happen. So I'd add:

FLAG_SET_DEFAULT(OnSpinWaitInstCount, 1);

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Changes requested by phh (Reviewer).

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