Add weak_references_count to GC.stat

gc/mmtk/mmtk.c

@@ -1410,6 +1410,7 @@ enum gc_stat_sym {
     gc_stat_sym_free_bytes,
     gc_stat_sym_starting_heap_address,
     gc_stat_sym_last_heap_address,
+    gc_stat_sym_weak_references_count,
     gc_stat_sym_last
 };
 
@@ -1428,6 +1429,7 @@ setup_gc_stat_symbols(void)
         S(free_bytes);
         S(starting_heap_address);
         S(last_heap_address);
+        S(weak_references_count);
     }
 }
 
@@ -1463,6 +1465,7 @@ rb_gc_impl_stat(void *objspace_ptr, VALUE hash_or_sym)
         SET(free_bytes, mmtk_free_bytes());
         SET(starting_heap_address, (size_t)mmtk_starting_heap_address());
         SET(last_heap_address, (size_t)mmtk_last_heap_address());
+        SET(weak_references_count, mmtk_weak_references_count());
 #undef SET
 
     if (!NIL_P(key)) {

gc/mmtk/mmtk.h

@@ -129,6 +129,8 @@ void mmtk_declare_weak_references(MMTk_ObjectReference object);
 
 bool mmtk_weak_references_alive_p(MMTk_ObjectReference object);
 
+size_t mmtk_weak_references_count(void);
+
 void mmtk_register_pinning_obj(MMTk_ObjectReference obj);
 
 void mmtk_object_reference_write_post(MMTk_Mutator *mutator, MMTk_ObjectReference object);

gc/mmtk/src/api.rs

@@ -317,6 +317,11 @@ pub extern "C" fn mmtk_weak_references_alive_p(object: ObjectReference) -> bool
     object.is_reachable()
 }
 
+#[no_mangle]
+pub extern "C" fn mmtk_weak_references_count() -> usize {
+    binding().weak_proc.weak_references_count()
+}
+
 // =============== Compaction ===============
 
 #[no_mangle]

gc/mmtk/src/weak_proc.rs

@@ -92,6 +92,10 @@ impl WeakProcessor {
         weak_references.push(object);
     }
 
+    pub fn weak_references_count(&self) -> usize {
+        self.weak_references.lock().unwrap().len()
+    }
+
     pub fn process_weak_stuff(
         &self,
         worker: &mut GCWorker<Ruby>,

test/mmtk/test_stat.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+require_relative "helper"
+
+module MMTk
+  class TestStat < TestCase
+    def test_weak_references_count
+      assert_operator(GC.stat(:weak_references_count), :>, 0)
+
+      EnvUtil.without_gc do
+        before = GC.stat(:weak_references_count)
+        ObjectSpace::WeakMap.new
+        assert_operator(GC.stat(:weak_references_count), :>, before)
+      end
+    end
+  end
+end