RFC: (How to?) Replace use of mark-word in leak-profiler (Lilliput)

[Roman Kennke]

Hi there,

I'm currently thinking about an issue that came up when I tested 
JFR/Leakprofiler with Lilliput.

First come context: In Lilliput I am storing a (compressed) Klass* in 
the object header, and want to phase out all uses of the dedicated 
Klass*-word and then remove that. This means that we need to be a little 
more careful when accessing the object header and/or the object's klass.

In the leak-profiler, we (temporarily) store an Edge* into the object 
header, and preserve the actual mark in a table. However, that means 
that the heap traversal would not work anymore because the object 
doesn't have a valid Klass* anymore.

Therefore I'm thinking about alternative ways to associate objects with 
an Edge*:

1. Use a (hash-)table for it (oop->Edge*), in EdgeStore.
2. Compress the Edge* to 32bits and store that only in the lower 32bits 
of the header (the Klass* is in the upper 32bits, currently). Not quite 
sure how to do this, though, it means we have to control allocation of 
the Edge instances in a contiguous space.
3. Store the object Klass* somewhere as long as we have it (before 
overriding it in EdgeStore::associate_leak_content_with_candidate()), 
and use that for iterating the object (in BFSClosure::iterate()). But 
where? Maybe in the Edge to the object? I.o.w. each Edge would also 
store the Klass* of its pointee?

I wanted to check with you if any of those approaches make sense to you, 
or maybe you have even better ideas?

Thanks for helping,
Roman