[lworld] RFR: 8376171: [lworld] Enable storing the hash code in the mark word of value objects [v3]

[Tobias Hartmann]

> Since [JDK-8247299](https://bugs.openjdk.org/browse/JDK-8247299), value objects no longer use `markWord::always_locked_pattern` (see description of [JDK-8247298](https://bugs.openjdk.org/browse/JDK-8247298) for more details). Therefore, the limitations around storing the hash code in the mark word that are mentioned in [JDK-8244975](https://bugs.openjdk.org/browse/JDK-8244975) no longer exist. Let's enable this optimization.
> 
> Running `System.identityHashCode(obj)` in a loop and passing various primitive boxes leads to a **17-18x speedup** on my machine. I'm also seeing a **7% improvement** in the score of the SPECjvm2008 serial benchmark but we are still a bit behind baseline without `--enable-preview`. More improvements to come.
> 
> It's worth noting that value objects will lose their buffer object when stored in a flat container or sometimes when being scalarized ([JDK-8372268](https://bugs.openjdk.org/browse/JDK-8372268) will improve this). The hash then needs to be re-computed, which is expensive.
> 
> I'm working on ways to further improve this, see [JDK-8252185](https://bugs.openjdk.org/browse/JDK-8252185).
> 
> Thanks,
> Tobias

Tobias Hartmann has updated the pull request incrementally with one additional commit since the last revision:

  Use CAS to set mark

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

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1954/files
  - new: https://git.openjdk.org/valhalla/pull/1954/files/e4744a94..3f15d890

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1954&range=02
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1954&range=01-02

  Stats: 9 lines in 1 file changed: 4 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/valhalla/pull/1954.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1954/head:pull/1954

PR: https://git.openjdk.org/valhalla/pull/1954