[jdk17u] RFR: 8271862: C2 intrinsic for Reference.refersTo() is often not used

[Aleksey Shipilev]

On Fri, 10 Sep 2021 08:50:27 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Clean backport to make JDK 16 introduced feature more compelling with a workaround.
> 
> Additional testing:
>  - [x] Linux x86_64 fastdebug, `RefersTo` tests pass
> 
> `RefersTo` tests:
> 
> 
> ./test/hotspot/jtreg/gc/TestReferenceRefersTo.java
> ./test/hotspot/jtreg/gc/TestReferenceRefersToDuringConcMark.java
> ./test/hotspot/jtreg/gc/shenandoah/TestReferenceRefersToShenandoah.java
> ./test/hotspot/jtreg/compiler/intrinsics/TestReferenceRefersTo.java
> ./test/jdk/java/lang/ref/ReferenceRefersTo.java

Got the push approval.

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PR: https://git.openjdk.java.net/jdk17u/pull/62