RFR: Add generations to freeset [v20]

[Y. Srinivas Ramakrishna]

On Sat, 29 Apr 2023 00:36:15 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

>> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Fix white space
>
> src/hotspot/share/gc/shenandoah/shenandoahFreeSet.hpp line 41:
> 
>> 39: 
>> 40: class ShenandoahSetsOfFree {
>> 41:   friend class ShenandoahFreeSet;
> 
> Is the friendliness necessary, or is this a case of leakage of abstraction. Could the requisite stuff just be made public? May be as I look more closely at uses of non-public methods by the friend, I'll understand why the friendliness seems necessary. It'd be good to document that, if nothing else. That process of documentation itself might make one see a better approach.

Hmm, wait, the only non-public method is `clear_internal` which is anyway exposed via `clear_all`. 

In that case, is the friend accessing a private data member field? 

Or perhaps the friend annotation is now obsolete from an earlier round of development and must now be removed.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/250#discussion_r1180899032