RFR: 8361376: Regressions 1-6% in several Renaissance in 26-b4 only MacOSX aarch64 [v2]

[Martin Doerr]

On Thu, 24 Jul 2025 18:51:22 GMT, Dean Long <dlong at openjdk.org> wrote:

>> This PR removes the recently added lock around set_guard_value, using instead Atomic::cmpxchg to atomically update bit-fields of the guard value.  Further, it takes a fast-path that uses the previous direct store when at a safepoint.  Combined, these changes should get us back to almost where we were before in terms of overhead.  If necessary, we could go even further and allow make_not_entrant() to perform a direct byte store, leaving 24 bits for the guard value.
>
> Dean Long has updated the pull request incrementally with one additional commit since the last revision:
> 
>   remove NMethodEntryBarrier_lock

src/hotspot/share/gc/shared/barrierSetNMethod.cpp line 113:

> 111:   }
> 112: 
> 113:   MACOS_AARCH64_ONLY(ThreadWXEnable wx(WXWrite, Thread::current()));

This looks also ok as alternative to my proposal.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26399#discussion_r2248631958