Obj.reachable_words is extremely slow when called multiple times

[ygrek]

The reason seems to be that obj_counter is never reset to zero, so extern_resize_position_table is called on every invocation.

Here is a dirty proof (another way to observe is to run similar program and put breakpoint on extern_resize_position_table):

$ ocamlopt q_stubs.c q.ml -o q
$ ./q
0) obj_counter 111 reachable_words 333
1) obj_counter 222 reachable_words 333
2) obj_counter 333 reachable_words 333
3) obj_counter 444 reachable_words 333
4) obj_counter 555 reachable_words 333
5) obj_counter 666 reachable_words 333
6) obj_counter 777 reachable_words 333
7) obj_counter 888 reachable_words 333
8) obj_counter 999 reachable_words 333
9) obj_counter 1110 reachable_words 333

$ cat q.ml 
external obj_counter : unit -> int = "ml_obj_counter"
let () =
  let a = Array.init 10 (fun _ -> List.init 111 (fun _ -> 'a')) in
  Array.iteri (fun i l -> Printf.printf "%d) obj_counter %d reachable_words %d\n%!" i (obj_counter ()) (Obj.reachable_words @@ Obj.repr l)) a
$ cat q_stubs.c 
#include "caml/memory.h"
uintnat reference;
value ml_obj_counter(value v) {
  return Val_int(*(uintnat *)((char *)&reference - 0xc0928 + 0xbc178));
}

magic numbers are addresses of reference and obj_counter as shown by nm - quick way to access obj_counter w/o rebuilding ocaml