Fix a GC bug in local stack scanning (ocaml-flambda#17)

The stack pointer was computed relative to the wrong local arena

runtime/roots_nat.c

@@ -522,7 +522,7 @@ static void do_local_allocations(caml_local_arenas* loc,
       if (marked_local) {
         int ix = get_local_ix(loc, *p);
         struct caml_local_arena a = loc->arenas[ix];
-        intnat newsp = (char*)p - (a.base + a.length);
+        intnat newsp = (char*)*p - (a.base + a.length);
         if (sp <= newsp) {
           /* forwards pointer, common case */
           CAMLassert(ix <= arena_ix);

testsuite/tests/typing-local/localgcbug.ml

@@ -0,0 +1,20 @@
+(* TEST
+   flags += "-extension local"
+   * native *)
+
+type n = Z | S of n
+
+let rec gen_locals (local_ n) depth _ = local_
+  if depth = 0
+  then
+    S n
+  else
+    let s = S n in
+    let m = gen_locals s (depth - 1) (ref 42) in
+    let _ = gen_locals m (depth - 1) (ref 42) in
+    S n
+
+let () =
+  match gen_locals Z 21 (ref 42) with
+  | S Z -> print_endline "ok"
+  | _ -> assert false

testsuite/tests/typing-local/localgcbug.reference

@@ -0,0 +1 @@
+ok