4309. How are two seq_cst operations ordered in the single total order if these two operations are unsequenced?

Section: 32.5.4 [atomics.order] Status: New Submitter: jim x Opened: 2025-08-06 Last modified: 2025-08-09

Priority: Not Prioritized

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Discussion:

Consider this snippet code:

std::atomic<int> flag = {0};
std::atomic<int> turn = {0};

if(flag + turn){}

The loads of flag and turn as the operands of + are unsequenced according to [intro.execution] p10.

Except where noted, evaluations of operands of individual operators and of subexpressions of individual expressions are unsequenced.

However, 32.5.4 [atomics.order] p4 says:

There is a single total order S on all memory_order::seq_cst operations, including fences, that satisfies the following constraints.

Then, it says that:

First, if A and B are memory_order::seq_cst operations and A strongly happens before B, then A precedes B in S.

According to the first sentence, the load of flag and the load of turn do have an order in the single total order S since they are both memory_order::seq_cst operations. However, since they are unsequenced, the second sentence does not apply to them. So, what's the order of them in S? Is the order of them in S unspecified?

Suggested Resolution:

We may want to say the order of such operations is indeterminate in the single total order. That is, either A precedes B or B precedes A, but it is unspecified which.

Proposed resolution: