RFR (S): 8200233: Simple G1 evacuation path performance enhancements

[Thomas Schatzl]

Hi all,

  can I have reviews for these very tiny (and ultimately note really
measureable) improvements to the G1 evacuation path?

They are:

- some stores can be made OOP_NOT_NULL 
- the path for compressed/uncompressed oops when popping elements from
the task queue during evacuation can be specialized: compressed oops
are never array oops so saving the useless test
- some load of the "from" region in the evacuation path can be delayed
and in many cases actually avoided

CR:
https://bugs.openjdk.java.net/browse/JDK-8200233

Webrev:
http://cr.openjdk.java.net/~tschatzl/8200233/webrev/

Testing:
hs-tier-1

Thanks,
  Thomas