RFR (S): 8223693: Memory wastage in size of per-region type buffers in GC

[Thomas Schatzl]

Hi all,

  can I have reviews for this small change in the constants for
G1HeapRegionAttr::_type field, eliminating some confusion and minor
memory wastage?

Perf testing showed no measurable impact of the change.

CR:
https://bugs.openjdk.java.net/browse/JDK-8223693
Webrev:
http://cr.openjdk.java.net/~tschatzl/8223693/webrev/
Testing:
hs-tier1-5 in conjunction with other changes; perf testing showed no
measurable impact

Thanks,
  Thomas