RFR: Reduce waste in humongous allocations

[Roland Westrelin]

> It's not painful in terms of performance, but painful in terms of
> implemention. We cannot easily acquire the Heap_lock on allocations
> because the allocation might come out of a write barrier, and that Java
> thread is not-in-VM (they call into the VM via a cheap leaf-call). We
> could change that (and have been there already) to use regular calls
> like, e.g. allocations do, but this opens up a whole new class of other
> problems. For example, we need oopmaps at write-barriers which, iirc,
> presented us some serious optimization problems in C2 land. With
> Roland's work, those might have gone away though (seems like we can
> well live with control inputs to write barriers now..)

If we have a blocking runtime call at a write barrier then
deoptimization at a a write barrier is possible and we need debug info
at the write barrier. Having debug info and allowing the write barrier
to move around would be quite complicated.

Roland.