• How can I return early from a function?
• Can I generate a segfault in OCaml?

How can I return early from a function?

Raise an exception. It's probably much more efficient than you expect.

Example from discuss.ocaml Q&A:

let f x y z =
  let exception Return of int in
  try
    if x = 1 then raise (Return 0);
    if y = 2 then raise (Return 1);
    z
  with Return x -> x

Can I generate a segfault in OCaml?

No. For example, given

(* segfault.ml *)
external segfault : unit -> unit = "segfault"
let () = segfault ()

/* segfault_helper.c */
#include <caml/mlvalues.h>
#include <caml/bigarray.h>

CAMLprim value segfault(value unit) {
	int *p = 0;
	*p=1;
	return unit;
}

This does not demonstrate a segfault in OCaml:

$ gcc -I$(opam var lib)/ocaml -c -fPIC segfault_helper.c
$ ocamlopt -o segfault segfault.ml segfault_helper.o
$ ./segfault 
Segmentation fault (core dumped)

Because that's not OCaml. That's OCaml and C.

Likewise this does not demonstrate a segfault in OCaml:

$ ocaml
OCaml version 5.4.0
Enter #help;; for help.

# print_endline (Obj.magic 100);;
Segmentation fault (core dumped)

Because that's not OCaml. That's OCaml and the OCaml compiler implmentation.

By the way, it's not important for the C file to be named _helper, but it can't share the name of the .ml file, or you'll get a bunch of linking errors like:

:(.data+0x28): multiple definition of `camlSegfault.data_end'; segfault.o::(.data+0x28): first defined here
/usr/bin/ld: segfault.o: in function `camlSegfault.data_end':
:(.data+0x30): multiple definition of `camlSegfault.frametable'; segfault.o::(.data+0x30): first defined here