Failure memory order cannot be release or acq_rel.

[Seija K.]

It is undefined behavior and clang now enforces this.

[Alexey Proskuryakov]

https://github.com/WebKit/WebKit/pull/22049

[Mark Lam]

(In reply to Seija K. from comment #0)
> It is undefined behavior and clang now enforces this.

Can you please point us to the C++ spec where it says that using release or acq_rel for the failure memory order is undefined behavior?  Thanks.

[Radar WebKit Bug Importer]

<rdar://problem/120047049>

[Mark Lam]

(In reply to Mark Lam from comment #2)
> Can you please point us to the C++ spec where it says that using release or
> acq_rel for the failure memory order is undefined behavior?  Thanks.

From https://en.cppreference.com/w/cpp/atomic/atomic/compare_exchange:
```
If failure is stronger than success or(until C++17) is one of std::memory_order_release and std::memory_order_acq_rel, the behavior is undefined.
```

Also, from that link, there's a table that says for `compare_exchange_strong`'s "load operation" in the failure mode, to use:
```
* std::memory_order_acquire if order is std::memory_order_acq_rel
* std::memory_order_relaxed if order is std::memory_order_release
* otherwise order
```

[EWS]

Committed 272461@main (d2ba4a07e209): <https://commits.webkit.org/272461@main>

Reviewed commits have been landed. Closing PR #22049 and removing active labels.