RFR: 8242847: G1 should not clear mark bitmaps with no marks [v2]

[Ivan Walulya]

> Hi all,
> 
> Please review this change to bound the range of bitmap clearing for each region using the liveness data collected during marking. For the Concurrent Undo Mark cycle, the liveness information (next_top_at_mark_start and live_words) is in sync wrt. the _next_mark_bitmap that needs clearing. Hence, we use these details to clear only bitmaps for regions that were dirtied and need clearing i.e. only clear between [bottom, ntams), and only clear bitmaps for regions that had at least one bit set (i.e. have some live data).
> 
> Testing: Tier 1-3.

Ivan Walulya has updated the pull request incrementally with one additional commit since the last revision:

  thomas review

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5213/files
  - new: https://git.openjdk.java.net/jdk/pull/5213/files/6ab1f44b..0ce053a3

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5213&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5213&range=00-01

  Stats: 50 lines in 1 file changed: 18 ins; 8 del; 24 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5213.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5213/head:pull/5213

PR: https://git.openjdk.java.net/jdk/pull/5213