OCaml 4 has "Already Scanned" optimisation to avoid rescanning stack frames that haven't changed. See

In OCaml 5, since the runtime maintains the stacks, the stack limits are generous: #11238. Hence, the already scanned optimisation is much more useful than in OCaml 5 than in OCaml 4. So much so that it may be worth coming with a scheme to enable this feature in all the architectures and not just in those where bits can be stolen return addresses. One possibility is to have an extra word in every frame for the already scanned information. Since only a single bit is necessary for the already scanned information, the rest of the bits may be used for other purposes.

CC @stedolan and @jasigal, who has some OCaml programs that may benefit from this optimisation.