8231630: Optimize boot loader with no bootclasspath append entry

Optimize SystemDictionary::load_instance_class for boot loader. Immediately return NULL if the requested class is either in the unnamed package or unnamed module, or in a module not defined to the boot loader, when there is no bootclasspath append entry.

Reviewed-by: martin, aeubanks, dthomson, redestad, lfoltan, iklam

src/hotspot/share/classfile/classLoader.hpp

@@ -237,6 +237,8 @@ class ClassLoader: AllStatic {
   CDS_ONLY(static ClassPathEntry* app_classpath_entries() {return _app_classpath_entries;})
   CDS_ONLY(static ClassPathEntry* module_path_entries() {return _module_path_entries;})
 
+  static bool has_bootclasspath_append() { return _first_append_entry != NULL; }
+
  protected:
   // Initialization:
   //   - setup the boot loader's system class path

src/hotspot/share/classfile/systemDictionary.cpp

@@ -1432,6 +1432,11 @@ InstanceKlass* SystemDictionary::load_instance_class(Symbol* class_name, Handle
         // a named package within the unnamed module.  In all cases,
         // limit visibility to search for the class only in the boot
         // loader's append path.
+        if (!ClassLoader::has_bootclasspath_append()) {
+           // If there is no bootclasspath append entry, no need to continue
+           // searching.
+           return NULL;
+        }
         search_only_bootloader_append = true;
       }
     }