Relax memory ordering of shared refcount atomics

Feature or enhancement

Proposal:

In the free-threaded build, all operations on ob_ref_shared use sequentially consistent atomics. Full ordering is stronger than the biased reference counting protocol requires, and on ARM64 the ordered instructions are measurably slower under contention. Instead, we could (and I think should) match what C++ shared_ptr implementations do which is relaxed increfs and acquire/release decrefs ^1 ^2 ^3 ^4.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-156135

まず ob_ref_shared の free-threaded 実装を見つけ、リンクされた gh-156135 の作業を確認します。現在の atomic ordering を提案およびそこで引用されている shared_ptr 実装と比較します。ordering の変更が正しく、競合時のパフォーマンスへの影響が検証されれば完了です。