RFR: 8259862: MutableSpace's end should be atomic [v2]

[Kim Barrett]

> Please review this change to MutableSpace, making its _end member volatile
> and using Atomic operations to access the _top and _end members.  Some
> unused accessor functions that would otherwise need updating are removed.
> 
> Testing:
> mach5 tier1

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 four additional commits since the last revision:

 - describe _top and _end
 - reinstate end_addr() after JDK-8259778
 - Merge branch 'master' into atomic_end
 - make _end volatile and use atomic access

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2323/files
  - new: https://git.openjdk.java.net/jdk/pull/2323/files/a091498c..823879a0

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2323&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2323&range=00-01

  Stats: 13119 lines in 648 files changed: 7935 ins; 2932 del; 2252 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2323.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2323/head:pull/2323

PR: https://git.openjdk.java.net/jdk/pull/2323