Track transitive pinning in the pinning profiler

[d-netto]

We are only tracking objects pinned by the runtime and during conservative register/stack scanning.

We should track transitively pinned objects too.

[d-netto]

@qinsoon, @udesou: any chance one of you could take this task, since it may involve some changes in MMTk Core?

[d-netto]

Please take a look at #247 to see what changes I implemented in the binding to track pinning from conservative scanning.

[qinsoon]

@qinsoon, @udesou: any chance one of you could take this task, since it may involve some changes in MMTk Core.

I think this is low priority for now. Currently the only place that uses transitive pinning is GC.@preserve. We have an implementation that counts how many objects are transitively pinned in mmtk-core, and it is generally a very small number.

We only count the number of objects, rather than from which location the objects get transitively pinned. The latter is not that easy to implement, as it means we need to track the root from which an object is reachable -- MMTk does not keep that information.