RFR: 8256011: Shenandoah: Don't resurrect finalizably reachable objects [v12]

[Roman Kennke]

> In the weak-LRB we currently return referents when it is 'marked', that is when it's either reachable strongly or through a finalizable object. This means a finalizable object can be resurrected by Reference.get(), which is wrong. Only truly strongly reachable objects should be returned by Reference.get() during weak-reference-processing. 
> 
> I had to reconsider the way we call into runtime-LRBs from generated code for these reasons:
>  - We need to distinguish phantom, weak and strong reference strength, and native vs in-heap access. Those are two orthogonal dimensions
>  - We can have strong and phantom native referents, and strong and weak in-heap referents
>  - Native referents are never compressed
> 
> Note that this depends on PR#1140.
> 
> Testing:
>  - [x] hotspot_gc_shenandoah (x86_64, x64_32, aarch64)
>  - [x] tier1 +UseShenandoahGC +ShenandoahVerify
>  - [x] tier2 +UseShenandoahGC +ShenandoahVerify

Roman Kennke has updated the pull request incrementally with two additional commits since the last revision:

 - Avoid null-check and skip cset-check on non-strong refs in C1 LRB stub
 - Call weak-LRB with ON_WEAK_OOP_REF, not ON_UNKNOWN_OOP_REF (cosmetic)

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1109/files
  - new: https://git.openjdk.java.net/jdk/pull/1109/files/29347682..83c07cb8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1109&range=11
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1109&range=10-11

  Stats: 26 lines in 2 files changed: 6 ins; 7 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1109.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1109/head:pull/1109

PR: https://git.openjdk.java.net/jdk/pull/1109