[gpderetta]

wouldn't a concurrent change without synchronization be UB anyway? Also parent wants to cache the address, not the value (but you have to cache the value if you want to optimize manually)

[skitter]

Why would it be UB? All objects are behind (thin) pointers, which can be overwritten atomically.

[asa400]

Not necessarily UB, but absolutely "spooky action" nondeterministic race conditions that make things difficult to understand.