RFR: 8329597: C2: Intrinsify Reference.clear [v9]

[Erik Österlund]

On Wed, 9 Oct 2024 08:44:34 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:
>>  - [x] Linux x86_64 server fastdebug, `all`
>>  - [x] Linux AArch64 server fastdebug, `all`
>
> Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 18 commits:
> 
>  - Simplify: just do keep alive checks
>  - Merge branch 'master' into JDK-8329597-intrinsify-reference-clear
>  - More precise bit-unmasks
>  - Reconcile with late barrier expansion in G1
>  - Merge branch 'master' into JDK-8329597-intrinsify-reference-clear
>  - Review comments
>  - Merge branch 'master' into JDK-8329597-intrinsify-reference-clear
>  - Also dispatch to slow-path on other arches
>  - Fix other arches
>  - Tighten up comments in Reference javadoc
>  - ... and 8 more: https://git.openjdk.org/jdk/compare/580eb62d...9f7ad7ab

Looks good.

-------------

Marked as reviewed by eosterlund (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/20139#pullrequestreview-2364968158