[efficax]

An Arc is an Rc that uses an atomic integer for its ref count. This ensures updates to its count are safe between threads, so the Arc can thus be shared between threads. In practice the two become identical assembly, at least on amd64 because most load and stores have memory ordering guarantees, but on other architectures the atomics can in fact be a tad slower. marking the operations as atomic also prevents the compiler from doing instruction reordering that might cause problems