RFR: 8284161: Implementation of Virtual Threads (Preview) [v4]

[Alan Bateman]

> This is the implementation of JEP 425: Virtual Threads (Preview); TBD which JDK version to target.
> 
> We will refresh this PR periodically to pick up changes and fixes from the loom repo.
> 
> Most of the new mechanisms in the HotSpot VM are disabled by default and require running with `--enable-preview` to enable.
> 
> The patch has support for x64 and aarch64 on the usual operating systems (Linux, macOS, and Windows). There are stubs (calling Unimplemented) for zero and some of the other ports. Additional ports can be contributed via PRs against the fibers branch in the loom repo.
> 
> There are changes in many areas. To reduce notifications/mails, the labels have been trimmed down for now to hotspot, serviceability and core-libs. We'll add the complete set of labels when the PR is further along.
> 
> The changes include a refresh of java.util.concurrent and ForkJoinPool from Doug Lea's CVS. These changes will probably be proposed and integrated in advance of this PR.
> 
> The changes include some non-exposed and low-level infrastructure to support the (in draft) JEPs for Structured Concurrency and Scope Locals. This is to make life a bit easier and avoid having to separate VM changes and juggle branches at this time.

Alan Bateman has updated the pull request incrementally with one additional commit since the last revision:

  Refresh

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8166/files
  - new: https://git.openjdk.java.net/jdk/pull/8166/files/f53f0d4e..ff1ef712

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8166&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8166&range=02-03

  Stats: 1517 lines in 74 files changed: 1034 ins; 311 del; 172 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8166.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8166/head:pull/8166

PR: https://git.openjdk.java.net/jdk/pull/8166