RFR (M): 8067336: Allow that PLAB allocations at the end of regions are flexible

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that greatly reduces the amount of
PLAB waste caused by blocks of memory that cannot be used at the end of
regions?

The idea is simple: instead of allocating a PLAB that does not fit the
current region in a new region, look if it is possible to reuse the
remainder of the current region for the current PLAB.

E.g. if the current allocation region during GC has 19k words left, but
PLAB size is 20k, and the next allocation is just a few words, return a
19k PLAB to the caller instead of wasting the 19k.

There were no performance issues noticed here, the number of GCs tends
to decrease significantly though.

The main implementation idea is that in many allocation calls the single
"word_size" parameter is replaced by minimum, desired and actual word
size parameters which represent the minimum and desired (input)
allocation sizes, and the actual word size an output parameter that
indicates the actual number of words allocated.

CR:
https://bugs.openjdk.java.net/browse/JDK-8067336
Webrev:
http://cr.openjdk.java.net/~tschatzl/8067336/webrev/
Testing:
jprt, vm.gc test list

Thanks,
  Thomas