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Are the “modify” operators like +=, |=, &= etc atomic?
I know ++ is atomic (if you perform x++; in two different threads “simultaneously”, you will always end up with x increased by 2, as opposed to x=x+1 with optimization switched off.)
What I wonder is whether variable |= constant, and the likes are thread-safe or do I have to protect them with a mutex?
(…or is it CPU-dependent? In this case, how is it on ARM?)
You are wrong. There is no guarantee that ++ is atomic and neither is there for the compound assignment operators, or indeed for any C++ operation.