diff -r b765a6383849 -r b40b0e18a918 jitcraft/Stack.h
--- a/jitcraft/Stack.h	Mon Jul 09 22:25:52 2012 -0700
+++ b/jitcraft/Stack.h	Tue Jul 10 02:11:13 2012 -0700
@@ -56,7 +56,11 @@
 
     Frame *prev_;
     bytecode *prevpc_;
-    Function *fun_;
+    union {
+        Function *fun_;
+        Native *native_;
+        Object *callee_;
+    };
     Code *code_;
     Address lifoPosition_;
 
@@ -70,6 +74,15 @@
     {
     }
 
+    Frame(Frame *prev, bytecode *prevpc, Native *native, Address lifoPosition)
+      : prev_(prev),
+        prevpc_(prevpc),
+        native_(native),
+        code_(NULL),
+        lifoPosition_(lifoPosition)
+    {
+    }
+
     LocalSlot *locals() {
         return reinterpret_cast<LocalSlot *>(this + 1);
     }
@@ -77,13 +90,17 @@
         return locals() + code_->nlocals();
     }
     Function *fun() const {
-        assert(fun_);
+        assert(fun_ && callee_->is(MapKind_Function));
         return fun_;
     }
     Function *maybeFun() const {
-        return fun_;
+        return (fun_ && callee_->is(MapKind_Function)) ? fun_ : NULL;
+    }
+    bool isNative() const {
+        return callee_ && callee_->is(MapKind_Native);
     }
     Code *code() const {
+        assert(code_);
         return code_;
     }
     Frame *prev() const {
@@ -127,9 +144,32 @@
     }
 };
 
+// RAII object for calling a native from C++.
+class NativeArgs
+{
+    friend class Stack;
+
+    Stack *stack_;
+    LocalSlot *argsBase_;
+
+  public:
+    NativeArgs()
+      : stack_(NULL),
+        argsBase_(NULL)
+    {
+    }
+
+    ~NativeArgs();
+
+    intptr_t *args() {
+        return reinterpret_cast<int *>(argsBase_);
+    }
+};
+
 class Stack
 {
     friend class InvokeArgs;
+    friend class NativeArgs;
 
   private:
     static const size_t STACK_SIZE = 1024 * 1024;
@@ -145,6 +185,7 @@
     Frame *popInlineFrame();
 
     bool pushInvokeFrame(Handle<Function> fun, InvokeArgs &args);
+    bool pushNativeFrame(Handle<Native> native, NativeArgs &args, unsigned argc);
 
     Top *top() {
         return &top_;
@@ -154,6 +195,7 @@
 
   private:
     void popInvokeFrame(InvokeArgs &args);
+    void popNativeFrame(NativeArgs &args);
 
   private:
     char *buffer_;