[9] RFR(S): 8034839: jvm hangs with gc/gctests/LoadUnloadGC test

[Albert]

Hi,

could I get reviews for this small patch?

Bug: https://bugs.openjdk.java.net/browse/JDK-8034839

Problem: The problem is that the patch (7194669) - which was supposed to 
speed-up dependency checking
                causes a performance regression. The reason for the 
performance regression is that most dependencies
                are unique, so we have the overhead of determining if 
the dependency is already checked plus the
                overhead of dependency checking. The overhead of 
searching is significant, since we perform
                a linear search on 6000+ items each time.

Solution: Use a hashtable instead of linear search to lookup already 
checked dependencies. The new hashtable
                is very rudimentary. It provides only the required 
functionality to solve this bug. However, the functionality
                can be easily extended as needed.

Testing: jprt, failing test case, nashorn. The failing test case 
completes in approx. the same time as before 7194669.
              For nashorn + Octane, this patch yields the following 
times spent for dependency checking:

               with this patch:  844s
                        7194669: 1080s
            before 7194669: 5223s

webrev: http://cr.openjdk.java.net/~anoll/8034939/webrev.00/

Thanks,
Albert

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