RFR: 8256011: Shenandoah: Don't resurrect finalizably reachable objects [v4]

[Aleksey Shipilev]

On Tue, 10 Nov 2020 12:20:14 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> 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
>>  - [x] tier1 +UseShenandoahGC +ShenandoahVerify
>>  - [ ] tier2 +UseShenandoahGC +ShenandoahVerify
>
> Roman Kennke has updated the pull request incrementally with five additional commits since the last revision:
> 
>  - Whitespace changes
>  - Mask decorators in hash/cmp, not in ctor
>  - Use ShBarrierSet::is_strong_access() in CmpP optimization
>  - separate code-paths for phantom- and weak-access in runtime-LRB
>  - Give different name to different LRB runtime calls

This looks fine to me. Let Zhengyu look through this thoroughly.

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Marked as reviewed by shade (Reviewer).

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