Added Rubinius.primitive :vm_global_serial

This is the read-only version of :vm_inc_global_serial,
returning the current value of the serial without modification.

This is useful for implementing caches in Ruby-space that
are to be invalidated when the global serial is incremented.
(That is, when constants are assigned or included into modules).

kernel/bootstrap/rubinius.rb

@@ -19,6 +19,11 @@ def self.extended_modules(obj)
     raise PrimitiveFailure, "Rubinius.extended_modules primitive failed"
   end
 
+  def self.global_serial
+    Rubinius.primitive :vm_global_serial
+    raise PrimitiveFailure, "Rubinius.vm_global_serial primitive failed"
+  end
+
   def self.inc_global_serial
     Rubinius.primitive :vm_inc_global_serial
     raise PrimitiveFailure, "Rubinius.vm_inc_global_serial primitive failed"

vm/builtin/system.cpp

@@ -1323,6 +1323,10 @@ namespace rubinius {
     return RBOOL(obj->kind_of_p(state, mod));
   }
 
+  Object* System::vm_global_serial(STATE, CallFrame* calling_environment) {
+    return Fixnum::from(state->shared().global_serial());
+  }
+
   Object* System::vm_inc_global_serial(STATE, CallFrame* calling_environment) {
     if(state->shared().config.serial_debug) {
       std::cout << "[Global serial increased from " << state->shared().global_serial() << "]" << std::endl;

vm/builtin/system.hpp

@@ -224,6 +224,10 @@ namespace rubinius {
     // Rubinius.primitive+ :vm_object_kind_of
     static Object* vm_object_kind_of(STATE, Object* obj, Module* mod);
 
+    // Return the internal global serial number, used for caching
+    // Rubinius.primitive :vm_global_serial
+    static Object* vm_global_serial(STATE, CallFrame* calling_environment);
+
     // Increment the internal global serial number, used for caching
     // Rubinius.primitive :vm_inc_global_serial
     static Object* vm_inc_global_serial(STATE, CallFrame* calling_environment);