RFR: 8038925: Java 7 with G1 crashes in dump_instance_fields using jmap or jcmd without fullgc

[Andreas Eriksson]

Hi,

Could I have a review for this G1 jdk7 only fix please?

Description:
When using G1 and doing a heap dump while no fullgc has been done, 
hotspot crashes.
This is because G1CollectedHeap::safe_object_iterate uses 
ContiguousSpace::object_iterate_from, which does not avoid dead objects 
that can still be on the heap when using G1.

The fix adds a G1 specific safe_object_iterate code path that avoids 
dead objects.

http://cr.openjdk.java.net/~aeriksso/8038925/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8038925

Thanks to Mikael Gerdin for his help with this.

Regards,
Andreas