Code Review Request: 6850846: G1: extend G1 marking verification (S)

[Tony Printezis]

Small change to extend marking verification in G1:

http://cr.openjdk.java.net/~tonyp/6850846/webrev.0/

Here's the blurb from the CR:

G1 uses two different versions of the marking information:

- "prev" is the last one completed (which is guaranteed to be consistent 
and complete, i.e., no live object should be pointing to a dead object, 
but maybe no up-to-date) and
- "next" is the marking information that is being computed next (and 
which might be incomplete, as we might be half-way through completing it)

Heap verification in G1 also checks the consistency of the "prev" 
marking information (given that it's the only marking information that 
we can trust to be complete). However, we would also like to check the 
consistency of the "next" marking information at the only place where we 
can, which is the end of the remark phase.

This CR proposes to change the G1 verification code to either use the 
"prev" or "next" marking info, depending on what the caller needs.

Additionally, it adds verification of the "next" marking info at the end 
of the remark (conditional on the VerifyDuringGC flag) and removes the 
#ifndef PRODUCT from the verification at the end of cleanup (it's good 
to be able to do heap / marking verification in a product build).

-- 
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