The code generator doesn't need to put the argument count in eax for...

src/codegen-ia32.cc

 // Copyright 2006-2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 4436 matching lines @@
 
   // Push the arguments ("left-to-right").
   int arg_count = args->length();
   for (int i = 0; i < arg_count; i++) {
     Load(args->at(i));
   }
 
   if (function == NULL) {
     // Call the JS runtime function.
     Handle<Code> stub = ComputeCallInitialize(arg_count);
-
-    Result num_args = allocator()->Allocate(eax);
-    ASSERT(num_args.is_valid());
-    __ Set(num_args.reg(), Immediate(args->length()));
-    Result answer = frame_->CallCodeObject(stub, RelocInfo::CODE_TARGET,
-                                           &num_args, arg_count + 1);
+    Result answer =
+        frame_->CallCodeObject(stub, RelocInfo::CODE_TARGET, arg_count + 1);
     frame_->RestoreContextRegister();
     frame_->SetElementAt(0, &answer);
   } else {
     // Call the C runtime function.
     Result answer = frame_->CallRuntime(function, arg_count);
     frame_->Push(&answer);
   }
 }
 
 
@@ skipping 2519 matching lines @@
 
   // Slow-case: Go through the JavaScript implementation.
   __ bind(&slow);
   __ InvokeBuiltin(Builtins::INSTANCE_OF, JUMP_FUNCTION);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal