RFR: 8156548: gc/gctests/StringInternSyncWithGC2 fails with Test level exit status: 151

[Kim Barrett]

Please review this fix for a serious performance bug in non-product
verification code in CMS precleaning.

When precleaning an object, the mark bits for the object are verified,
including scanning for invalid set mark bits interior to the object.
If there are multiple dirty ranges applicable to the object,
processing each range leads to mark bit verification.  If the object
is large and there are many applicable dirty ranges, a lot of time may
be spent on this verification.

For example, in a test that creates many large objArrays (O(64) of
O(16M) elements) and randomly scribbles on them, we've seen preclean
times of 20+ minutes because of this.
 
This verification isn't precise, since it is performed when the object
is visited during precleaning.  Not all live objects will require
precleaning, and this won't catch post-visit mark corruption.  This
verification is also redundant with that done later, during the sweep
phase, where it *is* precise.  So the only benefit of the problematic
verification is that it might catch a problem earlier.

Since the problematic verification is redundant with later checks, we
simply remove it.

CR:
https://bugs.openjdk.java.net/browse/JDK-8156548

Webrev:
http://cr.openjdk.java.net/~kbarrett/8156548/webrev.00/

Testing:
Locally tested before and after change, using GC logging to see
precleaning time drop from 15+ minutes to 20 seconds.