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4244. Whether the spuriously failed comparison applies to compare_exchange_strong is unclear

Section: 32.5.7.2 [atomics.ref.ops], 32.5.8.2 [atomics.types.operations] Status: Tentatively NAD Submitter: jim x Opened: 2025-04-17 Last modified: 2025-10-20

Priority: Not Prioritized

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Discussion:

Both compare_exchange_strong and compare_exchange_weak share the same specified rule

If and only if the comparison is false then, after the atomic operation, the value in expected is replaced by the value read from the value referenced by *ptr during the atomic comparison.

However, there is a remark for the weak version

A weak compare-and-exchange operation may fail spuriously.

That is, even when the contents of memory referred to by expected and ptr are equal, it may return false and store back to expected the same memory contents that were originally there.

However, we don't explicitly say whether the strong version can have the spuriously failed comparison. The status quo is that we can only infer the point from the name, namely, the strong version should have a stronger guarantee than the weak version.

Suggested resolution:

Explicitly specify whether compare_exchange_strong can have the spurious failed comparison.

[2025-10-20; Reflector poll. Status changed: New → Tentatively NAD.]

"The Effects: explain the behaviour, and don't allow for spurious failures."

"The Remarks: are normative and are explicitly adding permission to fail spuriously. Absent such permission, the specification for strong operations is clear and has no spurious failures."

Proposed resolution: