They don't have to be global state - they can be updating thread local state, for example, in what is essentially dynamic scope. Depending on the OS, the effici

They don't have to be global state - they can be updating thread local state, for example, in what is essentially dynamic scope. Depending on the OS, the efficiency of such TLS accesses can be quite good - on Linux, for example, it's just an extra pointer indirect or two with a segment selector (assuming the code is statically linked; it's very slightly more expensive in a solib).