RFR: 8256011: Shenandoah: Don't resurrect finalizably reachable objects [v16]

[Aleksey Shipilev]

On Thu, 12 Nov 2020 20:14:12 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 (x86_64, x64_32, aarch64)
>>  - [x] tier1 +UseShenandoahGC +ShenandoahVerify
>>  - [x] tier2 +UseShenandoahGC +ShenandoahVerify
>
> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Asserts against impossible combinations of weak/phantom vs in-native/in-heap

Okay, looks good.

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Marked as reviewed by shade (Reviewer).

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