Rename JumpToBuiltin to JumpToRuntime. Add comment to ResolveBuiltin.

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
===================================================================
--- src/x64/macro-assembler-x64.h	(revision 2967)
+++ src/x64/macro-assembler-x64.h	(working copy)
@@ -536,14 +536,14 @@
   void CallRuntime(Runtime::FunctionId id, int num_arguments);
 
   // Tail call of a runtime routine (jump).
-  // Like JumpToBuiltin, but also takes care of passing the number
+  // Like JumpToRuntime, but also takes care of passing the number
   // of arguments.
   void TailCallRuntime(const ExternalReference& ext,
                        int num_arguments,
                        int result_size);
 
-  // Jump to the builtin routine.
-  void JumpToBuiltin(const ExternalReference& ext, int result_size);
+  // Jump to a runtime routine.
+  void JumpToRuntime(const ExternalReference& ext, int result_size);
 
 
   // ---------------------------------------------------------------------------
@@ -603,8 +603,16 @@
                       Label* done,
                       InvokeFlag flag);
 
-  // Get the code for the given builtin. Returns if able to resolve
-  // the function in the 'resolved' flag.
+  // Prepares for a call or jump to a builtin by doing two things:
+  // 1. Emits code that fetches the builtin's function object from the context
+  //    at runtime, and puts it in the register rdi.
+  // 2. Fetches the builtin's code object, and returns it in a handle, at
+  //    compile time, so that later code can emit instructions to jump or call
+  //    the builtin directly.  If the code object has not yet been created, it
+  //    returns the builtin code object for IllegalFunction, and sets the
+  //    output parameter "resolved" to false.  Code that uses the return value
+  //    should then add the address and the builtin name to the list of fixups
+  //    called unresolved_, which is fixed up by the bootstrapper.
   Handle<Code> ResolveBuiltin(Builtins::JavaScript id, bool* resolved);
 
   // Activation support.