add read barrier to value.p

API changes:
- value.p must be accessed via mrb_value_p macro
- value.p must be mutated via MRB_SET_VALUE_P macro

src/class.c

@@ -1438,7 +1438,7 @@ undef_method(mrb_state *mrb, struct RClass *c, mrb_sym a)
     mrb_name_error(mrb, a, "undefined method '%S' for class '%S'", mrb_sym2str(mrb, a), mrb_obj_value(c));
   }
   else {
-    MRB_SET_VALUE(m, MRB_TT_PROC, value.p, 0);
+    MRB_SET_VALUE_P(m, MRB_TT_PROC, 0);
     mrb_define_method_vm(mrb, c, a, m);
   }
 }

src/etc.c

@@ -176,7 +176,7 @@ mrb_obj_id(mrb_value obj)
   case  MRB_TT_FILE:
   case  MRB_TT_DATA:
   default:
-    return MakeID(obj.value.p);
+    return MakeID(mrb_value_p(obj));
   }
 }
 

src/object.c

@@ -29,7 +29,7 @@ mrb_obj_eq(mrb_state *mrb, mrb_value v1, mrb_value v2)
     return (mrb_float(v1) == mrb_float(v2));
 
   default:
-    return (v1.value.p == v2.value.p);
+    return (mrb_value_p(v1) == mrb_value_p(v2));
   }
 }
 

src/vm.c

@@ -37,7 +37,7 @@ void abort(void);
 #define SET_NIL_VALUE(r) MRB_SET_VALUE(r, MRB_TT_FALSE, value.i, 0)
 #define SET_INT_VALUE(r,n) MRB_SET_VALUE(r, MRB_TT_FIXNUM, value.i, (n))
 #define SET_SYM_VALUE(r,v) MRB_SET_VALUE(r, MRB_TT_SYMBOL, value.sym, (v))
-#define SET_OBJ_VALUE(r,v) MRB_SET_VALUE(r, (((struct RObject*)(v))->tt), value.p, (v))
+#define SET_OBJ_VALUE(r,v) MRB_SET_VALUE_P(r, (((struct RObject*)(v))->tt), (v))
 #ifdef MRB_NAN_BOXING
 #define SET_FLT_VALUE(mrb,r,v) r.f = (v)
 #elif defined(MRB_WORD_BOXING)