RFR: 8254015: copy_to_survivor_space should use in-hand klass for scanning [v2]

[Kim Barrett]

> Please review this small change to G1 evacuation.  When scanning the oops
> in an instance, the existing code calls obj->oop_iterate_backward(closure).
> That function gets the klass of obj and forwards to another function with
> the obj, closure, and the just obtained klass.  But the caller of
> oop_iterate_backward has already obtained the klass for other reasons.
> Rather than refetching it (including dealing with
> UseCompressedClassPointers), change to use a new oop_iterate_backward
> overload taking the klass.  This saves a small amount of work for non-array
> objects being evacuated.
> 
> Testing:
> tier1
> Generic performance testing didn't find any significant difference.
> Looking at just GCBasher evacuation rate, there may to be a small
> improvement, though it's hard to measure.

Kim Barrett has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev
excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since
the last revision:

 - Merge branch 'master' into iterate_with_klass
 - use in-hand klass

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/504/files
  - new: https://git.openjdk.java.net/jdk/pull/504/files/e48daedf..b0db7f3d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=504&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=504&range=00-01

  Stats: 12938 lines in 283 files changed: 6888 ins; 4836 del; 1214 mod
  Patch: https://git.openjdk.java.net/jdk/pull/504.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/504/head:pull/504

PR: https://git.openjdk.java.net/jdk/pull/504