RFR: 8329597: C2: Intrinsify Reference.clear

[Erik Österlund]

On Thu, 11 Jul 2024 15:28:37 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.
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, `all`
>  - [ ] Linux AArch64 server fastdebug, `all`

The reason we did not do this before is that this is not a strong reference store. Strong reference stores with a SATB collector will keep the referent alive, which is typically the exact opposite of what a user wants when they clear a Reference.

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PR Comment: https://git.openjdk.org/jdk/pull/20139#issuecomment-2225266939