Add type Obj.raw_data and functions Obj.raw_field, Obj.set_raw_field

Some OCaml objects contain data that cannot be safely represented
as an OCaml value (type Obj.t).  For example, in no-naked-pointers mode,
this is the case for code pointers inside closures, and for the
"custom operations" pointers inside custom blocks.

This PR introduces a type Obj.raw_data (an alias for nativeint)
to encapsulate this data, and functions
Obj.raw_field / Obj.set_raw_field to read and write the "raw" contents
of fields of blocks.

Note: just like it is wrong to access code pointers and custom operations
using Obj.field / Obj.set_field, it is wrong to access regular fields
possibly containing pointers into the OCaml heap using
Obj.raw_field / Obj.set_raw_field.  The OCaml heap block can be
reclaimed or moved after its address was captured by Obj.raw_field.
Symmetrically, Obj.set_raw_field on a regular field bypasses the
write barrier of the GC.

runtime/obj.c

@@ -73,6 +73,18 @@ CAMLprim value caml_obj_set_tag (value arg, value new_tag)
   return Val_unit;
 }
 
+CAMLprim value caml_obj_raw_field(value arg, value pos)
+{
+  /* Represent field contents as a native integer */
+  return caml_copy_nativeint((intnat) Field(arg, Long_val(pos)));
+}
+
+CAMLprim value caml_obj_set_raw_field(value arg, value pos, value bits)
+{
+  Field(arg, Long_val(pos)) = (value) Nativeint_val(bits);
+  return Val_unit;
+}
+
 CAMLprim value caml_obj_make_forward (value blk, value fwd)
 {
   caml_modify(&Field(blk, 0), fwd);

stdlib/obj.ml

@@ -17,6 +17,8 @@
 
 type t
 
+type raw_data = nativeint
+
 external repr : 'a -> t = "%identity"
 external obj : t -> 'a = "%identity"
 external magic : 'a -> 'b = "%identity"
@@ -34,6 +36,10 @@ external floatarray_set :
 let [@inline always] double_field x i = floatarray_get (obj x : floatarray) i
 let [@inline always] set_double_field x i v =
   floatarray_set (obj x : floatarray) i v
+external raw_field : t -> int -> raw_data = "caml_obj_raw_field"
+external set_raw_field : t -> int -> raw_data -> unit
+                                          = "caml_obj_set_raw_field"
+
 external new_block : int -> int -> t = "caml_obj_block"
 external dup : t -> t = "caml_obj_dup"
 external truncate : t -> int -> unit = "caml_obj_truncate"

stdlib/obj.mli

@@ -20,6 +20,8 @@
 
 type t
 
+type raw_data = nativeint  (* @since 4.12 *)
+
 external repr : 'a -> t = "%identity"
 external obj : t -> 'a = "%identity"
 external magic : 'a -> 'b = "%identity"
@@ -60,6 +62,13 @@ external set_tag : t -> int -> unit = "caml_obj_set_tag"
 val [@inline always] double_field : t -> int -> float  (* @since 3.11.2 *)
 val [@inline always] set_double_field : t -> int -> float -> unit
   (* @since 3.11.2 *)
+
+external raw_field : t -> int -> raw_data = "caml_obj_raw_field"
+  (* @since 4.12 *)
+external set_raw_field : t -> int -> raw_data -> unit
+                                          = "caml_obj_set_raw_field"
+  (* @since 4.12 *)
+
 external new_block : int -> int -> t = "caml_obj_block"
 external dup : t -> t = "caml_obj_dup"
 external truncate : t -> int -> unit = "caml_obj_truncate"