[pull] main from WebAssembly:main#274
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pull[bot] merged 2 commits intokkpan11:mainfrom Feb 7, 2025
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We previously reasoned that RMWs that do not change in-memory values could be optimized to just atomic gets because they did not necessarily synchronize with subsequent atomic reads of the same location. This is not correct for seqcst RMWs, though, since subsequent seqcst reads in the total order of seqcst operations are _required_ to read from and synchronize with these RMWs, even though they do not change the in-memory values. Fix the optimization to only work on acquire-release RMWs and RMWs to unshared memory.
We previously reasoned that we had to emit fences when optimizing sequentially consistent RMWs to unshared memory to preserve the RMWs' effect on the total order of sequentially consistent operations. However, that total order alone is insufficient to force any read event to observe some other write event; that requires a synchronization edge to be formed via an atomic write and read pair. RMWs to unshared memory can never be part of such a synchronization edge, so removing them cannot introduce new behaviors. Improve our optimization to stop emitting the unnecessary fences.
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