RFR: 8353225: Add a way to iterate Klass inside the loaded CDS archive

[Thomas Stuefe]

On Thu, 3 Apr 2025 12:51:40 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> There's only a small number of classes that have allocated instances in the CDS archive. I think it's better to build a table of these classes and create KLUT information for them.
>> 
>> BTW, none of the classes in the `LambdaProxyClassDictionary` have allocated instances.
>> 
>> 
>> $ LOG=-Xlog:cds+map=trace,cds+map+oops=trace:file=cds.oops.txt:none:filesize=0
>> $ java -Xshare:dump $LOG
>> $ grep '@@ Object' cds.oops.txt | cut -b 44- | sed -e 's/ .*//g' | sort | uniq | wc
>>      79      79    2456
>> $ java -Xshare:dump $LOG -XX:+AOTClassLinking 
>> $ grep '@@ Object' cds.oops.txt | cut -b 44- | sed -e 's/ .*//g' | sort | uniq | wc
>>     172     172    6068
>
> I wonder if you could generate the KLUT information when objects are allocated rather than eagerly when classes are loaded.

@coleenp Thank you. I think would even be worse; you'd pay at every allocation instead of at oop iteration time; the latter only pays when visiting liveset objects, ideally a tiny subset of those that had been allocated.

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PR Review Comment: https://git.openjdk.org/jdk/pull/24311#discussion_r2027235957