RFR: 8205683: Refactor heap allocation to separate concerns

[Erik Österlund]

Hi,

After a bunch of stuff was added to the CollectedHeap object allocation 
path, it is starting to look quite ugly. It has function pointers passed 
around with arguments to be sent to them, mixing up allocation, 
initialization, tracing, sampling and exception handling all over the place.

I propose to improve the situation by having an abstract MemAllocator 
stack object own the allocation path, with suitable subclasses, 
ObjAllocator, ClassAllocator and ObjArrayAllocator for allocating the 3 
types we support allocating on the heap. Variation points now simply use 
virtual calls, and most of the tracing/sampling code can be moved out 
from the allocation path. This should make it easier to understand what 
is going on here, and the different concerns are more separated.

A collector can override the virtual member functions for allocating 
objects, arrays, and classes, and inject their own variation points into 
this framework. For example for installing a forwarding pointer in the 
cell header required by Brooks pointers, if you are into that kind of stuff.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8205683

Webrev:
http://cr.openjdk.java.net/~eosterlund/8205683/webrev.02/

Thanks,
/Erik