Happens-before is very relevant because the question is about visibility between threads. The sequencing of statements in c++ is defined by the sequenced-before relation, which is implicitly upgraded to a happens-before relationship. The OP is asking if a write on one thread must become visible to a read on another thread if the latter observes a later time point from a call that is sequenced-before the read than the prior does from a call that is sequenced-after the write. Without a happens-before relationship between the read and the write that simply isn't possible because of trivial hardware optimisations like store buffering. Just because an execution on one thread happens temporally after an execution on another thread does not necessarily mean that the later execution has the same view of memory that the earlier one does.

My point is that unless you consider that observing a time point counts as a type of write there's no way of establishing the release part of the acquire-release relationship that's required for creating the happens-before.

On 11 June 2026 06:51:46 BST, jim x <xmh970252187_at_[hidden]> wrote:
>> What exactly would you be establishing a happens-before relationship with
>> in the original example?
>>
>
>This is the misunderstanding of the OP. The OP doesn't talk about the
>happens-before relationship; instead, the intention can be simplified as
>talking about whether the control flow executing a previous expression
>occurs in no later time point than the time point it executes a later
>expression.
>
>On Thu, Jun 11, 2026 at 1:37 PM Jennifier Burnett via Std-Discussion <
>std-discussion_at_[hidden]> wrote:
>
>> 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.
>> >
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