RFR: 8316493: Make immutable maps @ValueBased [v3]

[Per Minborg]

On Thu, 21 Sep 2023 16:40:59 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> This PR outlines a solution for making immutable maps `@ValueBased` by removing cacheing of certain values in `AbstractMap`.
>> 
>> By removing these caching fields in `AbstractMap`, we can make the immutable maps `@ValueBased` and at the same time, performance is likely improved because the JVM is probably able to optimize away object creation anyway via escape analysis. Also, all maps will occupy less space as we get rid of a number of objects and references stored for each map.
>> 
>> We need to benchmark this solution to better understand its implications.
>
> Per Minborg 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' into vb-map2
>  - Remove redundant impl spec parts
>  - Merge pull request #4 from cl4es/HashMapViews
>    
>    Add simple HashMapViews microbenchmark
>  - Add simple HashMapViews microbenchmark
>  - Remove caching in AbstractMap and make immutable maps @ValueBased

Looking at the potential compatibility issues (See CSR), I have produced an alternate draft PR that solves the problem by simply not overriding `AbstractMap`: https://github.com/openjdk/jdk/pull/15887

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PR Comment: https://git.openjdk.org/jdk/pull/15614#issuecomment-1731232410