Paul (@pauldreik) has come up with system-specific atomic-safe copy functions (#767). It seems to be based on empirical work by Erik Rigtorp https://rigtorp.se/isatomic/ for x64 systems. There is a caveat: even if the operations are indeed atomic, we still need our users (e.g., Google v8) to agree that it is indeed atomic. The x64 isa is quite varied and it would be nice to have an authoritative reference.

We are currently working with faster 64-bit copies, that should work well portably. It has decent speeds.