Try only updating hash value references

I'm afraid the keys to this hash are just integers, and those integers
may look like VALUE pointers when they are not.  Since we don't mark the
keys to this hash, it's probably safe to say that none of them have
moved, so we shouldn't try to update the references either.

gc.c

@@ -7769,6 +7769,41 @@ hash_foreach_replace(st_data_t key, st_data_t value, st_data_t argp, int error)
     return ST_CONTINUE;
 }
 
+static int
+hash_replace_ref_value(st_data_t *key, st_data_t *value, st_data_t argp, int existing)
+{
+    rb_objspace_t *objspace = (rb_objspace_t *)argp;
+
+    if (gc_object_moved_p(objspace, (VALUE)*value)) {
+        *value = rb_gc_location((VALUE)*value);
+    }
+
+    return ST_CONTINUE;
+}
+
+static int
+hash_foreach_replace_value(st_data_t key, st_data_t value, st_data_t argp, int error)
+{
+    rb_objspace_t *objspace;
+
+    objspace = (rb_objspace_t *)argp;
+
+    if (gc_object_moved_p(objspace, (VALUE)value)) {
+        return ST_REPLACE;
+    }
+    return ST_CONTINUE;
+}
+
+static void
+gc_update_tbl_refs(rb_objspace_t * objspace, st_table *tbl)
+{
+    if (!tbl || tbl->num_entries == 0) return;
+
+    if (st_foreach_with_replace(tbl, hash_foreach_replace_value, hash_replace_ref_value, (st_data_t)objspace)) {
+        rb_raise(rb_eRuntimeError, "hash modified during iteration");
+    }
+}
+
 static void
 gc_update_table_refs(rb_objspace_t * objspace, st_table *tbl)
 {
@@ -8020,7 +8055,7 @@ gc_update_object_references(rb_objspace_t *objspace, VALUE obj)
         }
         if (!RCLASS_EXT(obj)) break;
         if (RCLASS_IV_TBL(obj)) {
-            gc_update_table_refs(objspace, RCLASS_IV_TBL(obj));
+            gc_update_tbl_refs(objspace, RCLASS_IV_TBL(obj));
         }
         update_class_ext(objspace, RCLASS_EXT(obj));
         update_const_tbl(objspace, RCLASS_CONST_TBL(obj));