Add age to rb_gc_object_metadata

This will allow ObjectSpace.dump to output the age of the object.

Author: peterzhu2118

gc/default/default.c

@@ -6197,11 +6197,12 @@ rb_gc_impl_object_metadata(void *objspace_ptr, VALUE obj)
 {
     rb_objspace_t *objspace = objspace_ptr;
     size_t n = 0;
-    static ID ID_wb_protected, ID_old, ID_uncollectible, ID_marking, ID_marked, ID_pinned;
+    static ID ID_wb_protected, ID_age, ID_old, ID_uncollectible, ID_marking, ID_marked, ID_pinned;
 
     if (!ID_marked) {
 #define I(s) ID_##s = rb_intern(#s);
         I(wb_protected);
+        I(age);
         I(old);
         I(uncollectible);
         I(marking);
@@ -6218,6 +6219,7 @@ rb_gc_impl_object_metadata(void *objspace_ptr, VALUE obj)
 } while (0)
 
     if (!RVALUE_WB_UNPROTECTED(objspace, obj)) SET_ENTRY(wb_protected, Qtrue);
+    SET_ENTRY(age, INT2FIX(RVALUE_AGE_GET(obj)));
     if (RVALUE_OLD_P(objspace, obj)) SET_ENTRY(old, Qtrue);
     if (RVALUE_UNCOLLECTIBLE(objspace, obj)) SET_ENTRY(uncollectible, Qtrue);
     if (RVALUE_MARKING(objspace, obj)) SET_ENTRY(marking, Qtrue);

test/objspace/test_objspace.rb

@@ -332,11 +332,25 @@ def test_dump_flags
     # Ensure that the fstring is promoted to old generation
     4.times { GC.start }
     info = ObjectSpace.dump("foo".freeze)
-    assert_match(/"wb_protected":true, "old":true/, info)
+    assert_include(info, '"wb_protected":true')
+    assert_include(info, '"age":3')
+    assert_include(info, '"old":true')
     assert_match(/"fstring":true/, info)
     JSON.parse(info) if defined?(JSON)
   end
 
+  def test_dump_flag_age
+    EnvUtil.without_gc do
+      o = Object.new
+
+      assert_include(ObjectSpace.dump(o), '"age":0')
+
+      GC.start
+
+      assert_include(ObjectSpace.dump(o), '"age":1')
+    end
+  end
+
   if defined?(RubyVM::Shape)
     class TooComplex; end