I'd move RetData inside of CallFrame and leave only ret_offset, ret_size and should_transfer_value. The rest of the actual attributes we are using are already in CallFrame so there's no need for duplicating those.
I would create a default() for RetData (if it doesn't exist already) that can be used for initializing the first callframe of all, because that one doesn't need RetData.

Note: Maybe there's a reason behind this thing that I'm criticizing that I don't know of, so check if the refactor I suggest is adequate.