Comment by jcparkyn

> Use immutable pass by reference. Make a copy only if mutability is requested in the thread.

This is essentially what Herd does. It's only semantically a pass by value, but the same reference counting optimizations still apply.

In fact, Herd's approach is a bit more powerful than this because (in theory) it can remove the copy entirely if the caller doesn't use the old value any more after creating the thread. In practice, my optimizations aren't perfect and the language won't always detect this.

The big downside is that we have to use atomic reference counts for _everything_. From memory this was about a 5-15% performance hit versus non-atomic counters, though the number might be higher if other bottlenecks were removed.