request for review (small): 6749695

[Y Srinivas Ramakrishna]

6749695 CMS: SIGEGV thrown on CMSCleanOnEnter optimization

   http://bugs.sun.com/view_bug.do?bug_id=6749695
   (modulo a bugs.sun.com bug that causes "object" to appear as "customer")

   webrev: http://webrev.invokedynamic.info/ysr/6749695/

The problem, as described in the bug report, was that we were
clearing too many cards, namely those prior to the "finger",
which would not be scanned after the clearing of the MUT card,
thus compromising correctness.
The fix is to clear only the portion after the start of the
object. Note that this optimization triggers only when the
marking phase finds objects that straddle (or start at)
page boundaries, limiting its efficacy somewhat.

Testing: jprt with -XX:-UseParNewGC; no test case provided

Thanks for your reviews.
-- ramki