Fix performance bug in Obj.reachable_words (#10731)

Fixes: #10730

Author: alainfrisch

Changes

@@ -29,6 +29,9 @@ Working version
   (Xavier Leroy and David Allsopp, review by Sébastien Hinderer and
    Damien Doligez)
 
+- #10730, 10731: Fix bug in `Obj.reachable_words` causing a slowdown when called
+  multiple time (Alain Frisch, report by ygrek, review by Xavier Leroy)
+
 ### Code generation and optimizations:
 
 - #10578: Increase the number of integer registers used for

runtime/extern.c

@@ -1135,6 +1135,7 @@ CAMLprim value caml_obj_reachable_words(value v)
   uintnat h = 0;
   uintnat pos;
 
+  obj_counter = 0;
   extern_init_position_table();
   sp = extern_stack;
   size = 0;