[breaking] Change the prototype of [caml_atomic_cas_field].

This is a breaking change because this function was (unfortunately)
exposed outside CAML_INTERNALS, and is used by exactly one external
user, you guessed it:
  https://github.com/ocaml-multicore/multicore-magic/blob/360c2e829c9addeca9ccaee1c71f4ad36bb14a79/src/Multicore_magic.mli#L181-L185
  https://github.com/ocaml-multicore/multicore-magic/blob/360c2e829c9addeca9ccaee1c71f4ad36bb14a79/src/unboxed5/multicore_magic_atomic_array.ml#L36-L43

We chose to change the prototype to remain consistent with the naming
convention for the new caml_atomic_*_field primitives, which will be
added to support atomic record fields.

User code can easily adapt to this new prototype we are using, but not
in a way that is compatible with both old and new versions of
OCaml (not without some preprocessing at least).

Another option would be to expose

    int caml_atomic_cas_field(value obj, intnat fld, value, value)
    value caml_atomic_cas_field_boxed(value obj, value vfld, value, value)

but no other group of primitives in the runtime uses this _boxed
terminology, they instead use

    int caml_atomic_cas_field_unboxed(value obj, intnat fld, value, value)
    value caml_atomic_cas_field(value obj, value vfld, value, value)

and this would again break compatiblity -- it is not easier to convert
code to that two-version proposal, and not noticeably more efficient.

So in this case we decided to break compatibility (of an obscure,
experimental, undocumented but exposed feature) in favor of
consistency and simplificity of the result.

Author: gasche

runtime/caml/memory.h

@@ -43,7 +43,7 @@ CAMLextern void caml_alloc_dependent_memory (mlsize_t bsz);
 CAMLextern void caml_free_dependent_memory (mlsize_t bsz);
 CAMLextern void caml_modify (volatile value *, value);
 CAMLextern void caml_initialize (volatile value *, value);
-CAMLextern int caml_atomic_cas_field (value, intnat, value, value);
+CAMLextern value caml_atomic_cas_field (value, value, value, value);
 CAMLextern value caml_check_urgent_gc (value);
 
 /* [caml_stat_*] functions below provide an interface to the static memory

runtime/memory.c

@@ -345,18 +345,19 @@ CAMLprim value caml_atomic_exchange (value ref, value v)
   return ret;
 }
 
-CAMLexport int caml_atomic_cas_field (
-  value obj, intnat field, value oldval, value newval)
+CAMLexport value caml_atomic_cas_field (
+  value obj, value vfield, value oldval, value newval)
 {
+  intnat field = Long_val(vfield);
   if (caml_domain_alone()) {
     /* non-atomic CAS since only this thread can access the object */
     volatile value* p = &Field(obj, field);
     if (*p == oldval) {
       *p = newval;
       write_barrier(obj, field, oldval, newval);
-      return 1;
+      return Val_true;
     } else {
-      return 0;
+      return Val_false;
     }
   } else {
     /* need a real CAS */
@@ -365,15 +366,15 @@ CAMLexport int caml_atomic_cas_field (
     atomic_thread_fence(memory_order_release); /* generates `dmb ish` on Arm64*/
     if (cas_ret) {
       write_barrier(obj, field, oldval, newval);
-      return 1;
+      return Val_true;
     } else {
-      return 0;
+      return Val_false;
     }
   }
 }
 CAMLprim value caml_atomic_cas (value ref, value oldval, value newval)
 {
-  return Val_bool(caml_atomic_cas_field(ref, 0, oldval, newval));
+  return caml_atomic_cas_field(ref, Val_long(0), oldval, newval);
 }
 
 CAMLprim value caml_atomic_fetch_add (value ref, value incr)

runtime/obj.c

@@ -173,9 +173,9 @@ CAMLprim value caml_obj_add_offset (value v, value offset)
 CAMLprim value caml_obj_compare_and_swap (value v, value f,
                                           value oldv, value newv)
 {
-  int res = caml_atomic_cas_field(v, Int_val(f), oldv, newv);
+  value res = caml_atomic_cas_field(v, f, oldv, newv);
   caml_check_urgent_gc(Val_unit);
-  return Val_int(res);
+  return res;
 }
 
 CAMLprim value caml_obj_is_shared (value obj)