Backport lazy from OCaml 5 to runtime4 (#2038)

Author: xclerc

backend/cmm_helpers.ml

@@ -826,7 +826,8 @@ let get_header ptr dbg =
      data race on headers. This saves performance with ThreadSanitizer
      instrumentation by avoiding to instrument header loads. *)
   Cop
-    ( (if Config.runtime5 then mk_load_immut Word_int else mk_load_mut Word_int),
+    ( mk_load_mut Word_int,
+      (* CR xclerc: consider whether that could be changed to mk_load_immut *)
       [Cop (Cadda, [ptr; Cconst_int (-size_int, dbg)], dbg)],
       dbg )
 

middle_end/flambda2/terms/flambda_primitive.mli

@@ -327,9 +327,14 @@ type unary_primitive =
   | Obj_dup  (** Corresponds to [Obj.dup]; see the documentation in obj.mli. *)
   | Get_header
       (** Get the header of a block. This primitive is invalid if provided with
-          an immediate value.
+          an immediate value. It should also not be used to read tags above
+          [No_scan_tag].
           Note: The GC color bits in the header are not reliable except for
-          checking if the value is locally allocated *)
+          checking if the value is locally allocated
+          Invariant: never read the tag of a possibly-lazy value from
+          ocamlopt-generated code. Tag reads that are allowed to be lazy tags
+          (by the type system) should always go through caml_obj_tag, which is
+          opaque to the compiler. *)
   | Atomic_load of Block_access_field_kind.t
 
 (** Whether a comparison is to yield a boolean result, as given by a particular