RFR: Traversal: Don't traverse new objects

[Roman Kennke]

With Traversal GC I'm currently observing final-pauses of several dozen 
(e.g. ~70) ms. Turns out that almost all of it is just traversing 
just-allocated objects. It occurs to me that this is pointless work: 
we're not going to evacuate any new object during this cycle, and 
liveness information may not be that useful either into next cycle 
because it usually degenerates quickly (for new stuff). Let's not 
traverse new-alloc-regions, and don't count liveness there, which means 
it's going to end up in next cycle's cset (just like the alloc-regions 
between cycles). This cuts pauses down to 3-4ms. Yay!

Notice that this is safe because new objects don't point to anything, 
and any ref stored into new objects during the pause get picked up by 
the enqueing barrier for I-U.

Notice that we *do* need to traverse and account evac-regions, because 
they have references pointing to interesting stuff. Hence the 
disctinction for is_alloc_region() (better ideas welcome!).

Also notice that this opens up an interesting opportunity for optimizing 
the enqueue-barrier: we could now elide ENQBs on objects known to be 
new. (In addition to known-NULL, which we already check, and 
already-enqueued, which we don't do yet. We could also do pretty much 
what we do with WBs and RBs and hoist them out of loops, etc. Lots of 
work to do there, and opportunities for improvements).

http://cr.openjdk.java.net/~rkennke/traversal-no-traverse-new/webrev.00/

Ok?

Roman