RFR: 8329529: Serial: Move _saved_mark_word out of ContiguousSpace [v2]

[Guoxiong Li]

> Hi all,
> 
> This patch removes the field `ContiguousSpace::_saved_mark_word` and its related methods, adds the corresponding fields to `DefNewGeneration` and `TenuredGeneration`, and moves the method `ContiguousSpace::oop_since_save_marks_iterate` to `Generation`. Many usages are adjusted as well.
> 
> An alternative way is add a class which extends the `ContiguousSpace` to place `_saved_mark_word` and its related methods. But this way would increase the class hierarchy. Not really know whether it deserves to do that.
> 
> The tests `make test-tier1_gc` passed locally. Thanks for taking the time to review.
> 
> Best Regards,
> -- Guoxiong

Guoxiong Li 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 four additional commits since the last revision:

 - Remove saved mark word of eden-space and from-space.
 - Merge branch 'master' into SAVED_MARK_WORD
 - Set saved mark word out of oop iteration.
 - JDK-8329529

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18603/files
  - new: https://git.openjdk.org/jdk/pull/18603/files/946bbe0c..8ef588b5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18603&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18603&range=00-01

  Stats: 1077 lines in 98 files changed: 194 ins; 454 del; 429 mod
  Patch: https://git.openjdk.org/jdk/pull/18603.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18603/head:pull/18603

PR: https://git.openjdk.org/jdk/pull/18603