RFR: 8329597: C2: Intrinsify Reference.clear [v3]

[Vladimir Kozlov]

On Fri, 19 Jul 2024 15:52:14 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 incrementally with one additional commit since the last revision:
> 
>   Amend the test case for guaranteing it works under different compilation regimes

There is coming JEP for later G1 barriers expansion similar to ZGC. Will you still need this intrinsic after it? I assume Shenandoah will follow G1 later.

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

PR Comment: https://git.openjdk.org/jdk/pull/20139#issuecomment-2379910959