8337213: Shenandoah: Add verification for class mirrors

Reviewed-by: rkennke, wkemper

src/hotspot/share/gc/shenandoah/shenandoahAsserts.cpp

@@ -250,6 +250,25 @@ void ShenandoahAsserts::assert_correct(void* interior_loc, oop obj, const char*
                     file, line);
     }
   }
+
+  // Do additional checks for special objects: their fields can hold metadata as well.
+  // We want to check class loading/unloading did not corrupt them.
+
+  if (java_lang_Class::is_instance(obj)) {
+    Metadata* klass = obj->metadata_field(java_lang_Class::klass_offset());
+    if (klass != nullptr && !Metaspace::contains(klass)) {
+      print_failure(_safe_all, obj, interior_loc, nullptr, "Shenandoah assert_correct failed",
+                    "Instance class mirror should point to Metaspace",
+                    file, line);
+    }
+
+    Metadata* array_klass = obj->metadata_field(java_lang_Class::array_klass_offset());
+    if (array_klass != nullptr && !Metaspace::contains(array_klass)) {
+      print_failure(_safe_all, obj, interior_loc, nullptr, "Shenandoah assert_correct failed",
+                    "Array class mirror should point to Metaspace",
+                    file, line);
+    }
+  }
 }
 
 void ShenandoahAsserts::assert_in_correct_region(void* interior_loc, oop obj, const char* file, int line) {

src/hotspot/share/gc/shenandoah/shenandoahVerifier.cpp

@@ -212,6 +212,21 @@ private:
       fwd_reg = obj_reg;
     }
 
+    // Do additional checks for special objects: their fields can hold metadata as well.
+    // We want to check class loading/unloading did not corrupt them.
+
+    if (java_lang_Class::is_instance(obj)) {
+      Metadata* klass = obj->metadata_field(java_lang_Class::klass_offset());
+      check(ShenandoahAsserts::_safe_oop, obj,
+            klass == nullptr || Metaspace::contains(klass),
+            "Instance class mirror should point to Metaspace");
+
+      Metadata* array_klass = obj->metadata_field(java_lang_Class::array_klass_offset());
+      check(ShenandoahAsserts::_safe_oop, obj,
+            array_klass == nullptr || Metaspace::contains(array_klass),
+            "Array class mirror should point to Metaspace");
+    }
+
     // ------------ obj and fwd are safe at this point --------------
 
     switch (_options._verify_marked) {