RFR (M): 8047328: Improve memory usage for cards in SparsePRTEntry

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that cuts SparsePRTEntry usage by
half (asymptotically at least :) )?

This change is based on a change from A. Sjöberg (which will be
credited by me), where instead of having special -1 entries in the
SparsePRTEntry array of cards, use a dedicated next-pointer for storing
where to put the next card.

This allows us to internally use uint16_t's instead of CardIdx_t's
(which are 32 bit) for storing.

This also limits the max region size to 32M: there is an assert that
checks that - unfortunately it is not possible to have a STATIC_ASSERT
here, as HeapRegionBounds::MAX_REGION_SIZE is private.
If you think that it is worth to have a static assert, at the cost of
exposing that member (or has another idea on how to do that without
adding too many dependencies somewhere), I would be happy to change the
code accordingly.

This reduces memory usage of sparse prt entries, and also somewhat
speeds up searching and iterating entries.

There are no significant throughput gains for that change though, but
the memory savings (particularly on large heaps) are nice to have.

(Even in the worst case, i.e. very small default number of cards in a
SparsePRTEntry we are not worse than before in that regard).

CR:
https://bugs.openjdk.java.net/browse/JDK-8047328
Webrev:
http://cr.openjdk.java.net/~tschatzl/8047328/webrev/
Testing:
jprt, vm.gc, perf benchmarks

Thanks,
  Thomas