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C++: Strongly Happens Before?

Strongly Happens Before? It started innocently enough. I just wanted to brush up on C++ memory orderings. It’s been a while since I last stared into the abyss of std::atomic, so I figured, why not revisit some good ol’ std::memory_order mayhem?

A is a modification, and B reads the value stored by A We’ll refer to this as the “read-from” relationship in the diagrams going forward, just to keep things visually clearer. According to P0668R5, enforcing a stricter rule would introduce a significant performance penalty on acquire/release synchronization, essentially defeating the purpose of memory_order_acquire and memory_order_release. This confirms that the execution we previously marked as invalid under the “happens-before” interpretation is in fact valid on Power.

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