RFR: 8316493: Make immutable maps @ValueBased [v6]

[Stuart Marks]

On Wed, 18 Oct 2023 15:24:14 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 incrementally with one additional commit since the last revision:
> 
>   Add 'fresh' to implSpec

In TreeMap, should the cached entrySet and navigableKeySet fields also be removed?

I'm not sure about removal of the clone() method. Certainly it will need to be adjusted to remove mention of the now-removed fields, but I haven't thought through all the compatibility implications of removing the method entirely.

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PR Comment: https://git.openjdk.org/jdk/pull/15614#issuecomment-1769358474