RFR: 8144072: G1ParScanThreadState::update_rs does not need to call is_in_reserved

[Erik Helin]

Hi all,

based on the patch "8144067: Pass obj directly to
G1ParScanThreadState::update_rs", here is another small optimization
that can utilize that G1ParScanThreadState::update_rs now has access to
both the field and the obj. Instead of calling from->is_in_reserved(o),
which is defined as

  bool is_in_reserved(const void* p) {
    return _bottom <= p && p < _end;
  }

we can do the same things as the write barrier for G1:

  bool is_in_same_region(T* p, oop o) {
    return ((p ^ o) >> Log2HeapRegionSize) == 0;
  }

which requires no loads at all.

This patch adds the new static function HeapRegion::is_in_same_region
and patches G1ParScanThreadState::update_rs to make use of it.

Thanks to Thomas for pointing this optimization out.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8144072

Webrev:
http://cr.openjdk.java.net/~ehelin/8144072/webrev.00/

Please note this patch is based upon the patch for 8144067.

Testing:
- GC test suite with full verification.

Thanks,
Erik