An acquire barrier is meaningless in this context. An acquire barrier needs either a release operation or atomic operation preceded by a release barrier to have any semantics beyond relaxed. What exactly would you be establishing a happens-before relationship with in the original example? Assuming the call to now() is an implicit acquire barrier changes nothing about the semantics because there's no corresponding release for it to establish a happens-before relationship with. The first thread doesn't even have any operations other than the call to now() that the acquire barrier would apply to

On 11 June 2026 01:37:42 BST, Thiago Macieira via Std-Discussion <std-discussion_at_[hidden]> wrote:
>On Wednesday, 10 June 2026 16:24:30 Pacific Daylight Time Simon Cooke wrote:
>> If you don't want that, you need acquire semantics, which will prevent the
>> write from down after the timestamp() call in #2
>
>This is the essence of the argument: reading from the monotonic clock should
>be an acquire barrier. The Standard does not say it is, but I think it should.
>