RFR: 8329597: C2: Intrinsify Reference.clear [v6]

[Tobias Hartmann]

On Mon, 30 Sep 2024 16:59:16 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> [JDK-8240696](https://bugs.openjdk.org/browse/JDK-8240696) added the native method for `Reference.clear`. The original patch skipped intrinsification of this method, because we thought `Reference.clear` is not on a performance sensitive path. However, it shows up prominently on simple benchmarks that touch e.g. `ThreadLocal` cleanups. See the bug for an example profile with `RRWL` benchmarks.
>> 
>> We need to know the actual oop strongness/weakness before we call into C2 Access API, this work models this after existing code for `refersTo0` intrinsics. C2 Access also need a support for `AS_NO_KEEPALIVE` for stores. 
>> 
>> Additional testing:
>>  - [ ] Linux x86_64 server fastdebug, `all`
>>  - [ ] Linux AArch64 server fastdebug, `all`
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Also dispatch to slow-path on other arches

test/micro/org/openjdk/bench/java/lang/ref/ReferenceClear.java line 2:

> 1: //
> 2: // * Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.

Drive-by comment: The `// *` format looks weird.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20139#discussion_r1784292673