RFR: JDK-8277520: Implement JDK-8 default methods for IdentityHashMap [v2]

[Roger Riggs]

On Thu, 16 Dec 2021 20:48:39 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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:
> 
>  - Merge branch 'master' of https://git.openjdk.java.net/jdk into identityhashmap-default
>  - update dates
>  - Also test cme for identity hash map
>  - Fix putIfAbsent
>  - JDK-8277520: Implement JDK-8 default methods for IdentityHashMap

The PR description doesn't do the change request justice. At least the first paragraph from the bug report would be useful.

The rationale seems ok but not compelling: 
"may save a few hash table lookups and moderately boost the performance of IdentityHashMap".

Please quantify the improvement with JMH test results. 
There may be an existing test in `test/micro/org/openjdk/bench/java/util` to build on.

Thanks

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