RFR: JDK-8277520: Implement JDK-8 default methods for IdentityHashMap [v4]

[liach]

On Mon, 21 Feb 2022 23:36:19 GMT, liach <duke at openjdk.java.net> wrote:

>> Might need a CSR as now `computeIfAbsent` `computeIfPresent` `compute` `merge` would throw CME if the functions modified the map itself, and there are corresponding specification changes.
>
> liach has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - merge branch 'identityhashmap-bench' of https://github.com/liachmodded/jdk into identityhashmap-default
>  - fix whitespace

@stuart-marks Could you help me with this?

In my JMH benchmark runs, I often find that ambient differences (from the jdk processes, the random test data generated, etc.) hide the actual performance difference caused by the patches.

Hence, I need help in these two area:
1. I probably need to make a baseline benchmark that can discount the fundamental differences between jdk processes. What should it be?
2. How do I generate consistent data set for all test runs to avoid contributing to errors?

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PR: https://git.openjdk.java.net/jdk/pull/6532