Perf regression accessing fields in JDK21

[Radim Vansa]

Thank you for the reply, I've created 
https://bugs.openjdk.org/browse/JDK-8352075

> I recall looking at this when optimizing heap dumps, and I think we 
> would not be able to avoid O(n)
> on initial field resolution. But I would expect incremental 
> improvements to this code are possible.

You're right, even prior to the regression the code was O(n), but 
apparently each iteration was significantly cheaper. We could also think 
about different strategies for regular 'small' classes where the 
overhead is bearable, vs. big classes where this builds up and it might 
be worth sacrificing the memory for a more efficient lookup. If we can't 
have both at once.

Cheers,

Radim