RFR: 8256999: Add C2 intrinsic for Reference.refersTo and PhantomReference::refersTo [v2]

[Vladimir Ivanov]

On Wed, 25 Nov 2020 23:35:14 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> JDK-8188055 added the function Reference.refersTo. For performance, the supporting native methods Reference.refersTo0 and PhantomReference.refersTo0 should be intrinsified by C2.
>> 
>> Initial patch was prepared by @fisk.
>> 
>> Tested hs-tier1-4. Added new compiler tests to test intrinsics.
>> 
>> Ran new test with Shenandoah. Found only one issue. As result I disable  PhantomReference::refersTo intrinsic for COOP+ Shenandoah combination. Someone from Shenandoah team have to test changes if that is enough.
>
> Vladimir Kozlov 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 three additional commits since the last revision:
> 
>  - Merge branch 'master' into JDK-8256999
>  - Added ZLoadBarrierElided = 0 definition.
>    Removed is_exact argument in load_field_from_object().
>    Added Shenandoah support for narrow phantom accesses.
>  - 8256999: Add C2 intrinsic for Reference.refersTo and PhantomReference::refersTo

Marked as reviewed by vlivanov (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/1425