RFR: 8371672: G1: G1YoungGenSizer handling of NewRatio, NewSize and MaxNewSize [v2]

[Thomas Schatzl]

On Thu, 13 Nov 2025 06:22:38 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> G1YoungGenSizer ignores these values if they are not from the command line. Resulting in a different _sizer_kind than expected.
>> The NewSize and MaxNewSize are also not bounded by the Initial and Max heap size respectively ([JDK-8370494](https://bugs.openjdk.org/browse/JDK-8370494)).
>> 
>> I suggest we rewrite this logic using `!FLAG_IS_DEFAULT` to always get the correct behaviour. And only use `FLAG_IS_CMDLINE` for issuing warnings to the user when we ergonomically change or ignore these values to be compatible.
>> As well as adding the proper truncation for HeapSize incompatible values.
>> 
>> * Testing
>>   * GHA
>>   * Tier 1-5 on Oracle Supported Platforms
>
> Axel Boldt-Christmas has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Apply @walulyai suggestion

Marked as reviewed by tschatzl (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/28255#pullrequestreview-3459993443