Make compaction available for Oj::Doc

Introduce compaction to `fast.c`, only if `rb_gc_mark_movable` is
available. Otherwise, keep a version that supports ruby <2.7.

Since `rb_data_object_wrap` and `TypedData_Wrap_Struct` is available
in every supported versions (eg >=2.4.10), I've also removed support
for earlier versions.

Result for compaction is great. I've tested it by allocating and
removing arbitrary sized json. For a count of 70 pages, we can see
that we used to need 68 pages after compaction,and now 61 pages. Of
course this test include also lots of noise, but if we count pinned
objects (objects that are marked in C extension as uncollectible),
we can see that for a benchmark with no allocations, we have the
same amount as a benchmark with this commit with thousands of allocations
(499 pinned objects total), and before this, `Oj::Doc` used to generate
5k more objects (5499 pinned objects total).

Hence this is a win, see PR for a graphical result.

Signed-off-by: Ulysse Buonomo <buonomo.ulysse@gmail.com>

CHANGELOG.md

@@ -1,5 +1,9 @@
 # CHANGELOG
 
+## Ongoing
+
+- Added support for `GC.compact` on `Oj::Doc`
+
 ## 3.11.3 - 2021-03-09
 
 - Fixed `respond_to?` method on `Oj::EasyHash`.

Rakefile

@@ -92,4 +92,3 @@ begin
 
 rescue LoadError
 end
-

ext/oj/extconf.rb

@@ -26,8 +26,9 @@
 have_func('rb_time_timespec')
 have_func('rb_ivar_count')
 have_func('rb_ivar_foreach')
+# Support for compaction.
+have_func('rb_gc_mark_movable')
 have_func('stpcpy')
-have_func('rb_data_object_wrap')
 have_func('pthread_mutex_init')
 
 dflags['OJ_DEBUG'] = true unless ENV['OJ_DEBUG'].nil?

ext/oj/fast.c

@@ -19,6 +19,13 @@
 //#define BATCH_SIZE	(4096 / sizeof(struct _leaf) - 1)
 #define BATCH_SIZE 100
 
+// Support for compaction
+#ifdef HAVE_RB_GC_MARK_MOVABLE
+#define mark rb_gc_mark_movable
+#else
+#define mark rb_gc_mark
+#endif
+
 typedef struct _batch
 {
 	struct _batch *next;
@@ -921,7 +928,7 @@ mark_leaf(Leaf leaf)
 		}
 		break;
 	case RUBY_VAL:
-		rb_gc_mark(leaf->value);
+		mark(leaf->value);
 		break;
 
 	default:
@@ -936,10 +943,63 @@ mark_doc(void *ptr)
 	{
 		Doc doc = (Doc)ptr;
 
-		rb_gc_mark(doc->self);
+		mark(doc->self);
 		mark_leaf(doc->data);
 	}
 }
+#ifdef HAVE_RB_GC_MARK_MOVABLE
+static void
+compact_leaf(Leaf leaf)
+{
+	switch (leaf->value_type)
+	{
+	case COL_VAL:
+		if (NULL != leaf->elements)
+		{
+			Leaf first = leaf->elements->next;
+			Leaf e = first;
+
+			do
+			{
+				compact_leaf(e);
+				e = e->next;
+			} while (e != first);
+		}
+		break;
+	case RUBY_VAL:
+		leaf->value = rb_gc_location(leaf->value);
+		break;
+
+	default:
+		break;
+	}
+}
+
+static void
+compact_doc(void *ptr)
+{
+	Doc doc = (Doc)ptr;
+
+	if (doc)
+	{
+		doc->self = rb_gc_location(doc->self);
+		compact_leaf(doc->data);
+	}
+}
+#endif
+
+static const rb_data_type_t oj_doc_type = {
+    "Oj/doc",
+    {
+        mark_doc,
+        free_doc_cb,
+        NULL,
+#ifdef HAVE_RB_GC_MARK_MOVABLE
+        compact_doc,
+#endif
+    },
+    0, 0,
+};
 
 static VALUE
 parse_json(VALUE clas, char *json, bool given, bool allocated)
@@ -988,12 +1048,7 @@ parse_json(VALUE clas, char *json, bool given, bool allocated)
 		}
 	}
 #endif
-	// last arg is free func void* func(void*)
-#ifdef HAVE_RB_DATA_OBJECT_WRAP
-	self = rb_data_object_wrap(clas, doc, mark_doc, free_doc_cb);
-#else
-	self = rb_data_object_alloc(clas, doc, mark_doc, free_doc_cb);
-#endif
+	self = TypedData_Wrap_Struct(clas, &oj_doc_type, doc);
 	doc->self = self;
 	doc->json = json;
 	DATA_PTR(doc->self) = doc;

lib/oj/bag.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 
 module Oj