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Summary: Relax fences for intrusive ptr's refcnt dec op for performance testing. lock needs acquire when the op succeeds and relaxed if the op is not. In addition, the expire call and the following refcnt reads were merged to remove one extra read. incref does not need any fences because the caller should already have a valid reference. use_count follows the same reasoning. decref only needs a release fence to make sure every write op prior to it has finished. When the refcnt goes to zero, there should be a acquire fence to make sure no read op reads stale data before the object is destructed. However, microbenchmark showed that the optimal fence for decref is not performing noticeably better than the current decref with acq-rel, so we keep decref as-is. This change should have no material impact on x86, but for Arm64 (and other CPUs with weak memory models), it should boost performance. Pull Request resolved: https://github.com/pytorch/pytorch/pull/162072 Approved by: https://github.com/swolchok, https://github.com/yfeldblum