Hmm this is a bit like peeling a banana only to throw the banana and eat the peel. Pass by value reduces the true benefit of copy-on-write.

Use immutable pass by reference. Make a copy only if mutability is requested in the thread. This makes concurrent reads lock-free but also cuts down on memory allocations.

I think that what you are calling "immutable pass by reference" is what the OP is calling "pass by value". See, when used abstractly, "pass by value" means that the argument is passed as a value, hence it is immutable and the callee can't mutate it. One way to implement this is by copying the data that represents the value. In the OP's language, and in many other languages that work this way, instead of copying the data, we implement "pass by value" by incrementing the reference count and passing a pointer to the original data. These differing implementations provide the same abstract semantics, but differ in performance.

> 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.