RFR: 8261492: Shenandoah: reconsider forwardee accesses memory ordering [v6]

[Aleksey Shipilev]

> Shenandoah carries forwardee information in object's mark word. Installing the new mark word is effectively "releasing" the object copy, and reading from the new mark word is "acquiring" that object copy.
> 
> For the forwardee update side, Hotspot's default for atomic operations is memory_order_conservative, which emits two-way memory fences around the CASes at least on AArch64 and PPC64. This seems to be excessive for Shenandoah forwardee updates, and "release" is enough.
> 
> For the forwardee load side, we need to guarantee "acquire". We do not do it now, reading the markword without memory semantics. It does not seem to pose a practical problem today, because GC does not access the object contents in the new copy, and mutators get this from the JRT-called stub that separates the fwdptr access and object contents access by a lot. It still should be cleaner to "acquire" the mark on load to avoid surprises.
> 
> Additional testing:
>  - [x] Linux x86_64 `hotspot_gc_shenandoah`
>  - [x] Linux AArch64 `hotspot_gc_shenandoah`
>  - [x] Linux AArch64 `tier1` with Shenandoah

Aleksey Shipilev 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 two additional commits since the last revision:

 - Merge branch 'master' into JDK-8261492-shenandoah-forwardee-memord
 - 8261492: Shenandoah: reconsider forwardee accesses memory ordering

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2496/files
  - new: https://git.openjdk.java.net/jdk/pull/2496/files/36e2da27..4953a6dd

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2496&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2496&range=04-05

  Stats: 20377 lines in 568 files changed: 8720 ins; 9594 del; 2063 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2496.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2496/head:pull/2496

PR: https://git.openjdk.java.net/jdk/pull/2496