RFR: 8254028: G1 incorrectly updates scan_top for collection set regions during preparation of evacuation [v2]

[Thomas Schatzl]

> Hi all,
> 
>   can I have reviews for this change that makes values of G1RemSetScanState::_scan_top for regions in the initial
>   collection set consistent with ones in optional collection set?
> So currently G1RemSetScanState::_scan_top is top() for regions in the initial collection set although they will never
> be scanned as enforced when dropping the remsets onto the card table. For the optional collection set, G1 sets scan_top
> manually to NULL when selecting the next few optional regions (using G1RemSet::exclude_region_from_scan()).  When
> debugging this discrepancy recently posed some slight surprise for me, so I would like to change this. Also there is
> some risk that future code that relies on that property will be surprised.  Testing: tier1-4; lots of kitchensink runs
> with JDK-8254164.
> Thanks,
>   Thomas

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  Modified comment

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/557/files
  - new: https://git.openjdk.java.net/jdk/pull/557/files/2ce49cc0..5aa5ca53

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=557&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=557&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/557.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/557/head:pull/557

PR: https://git.openjdk.java.net/jdk/pull/557