linux.conf.au
Toolchain Hackers...

    case CALL_EXPR:
      {
 	if (TREE_CODE (TREE_OPERAND (exp, 0)) == ADDR_EXPR
 	    && (TREE_CODE (TREE_OPERAND (TREE_OPERAND (exp, 0), 0))
 		== FUNCTION_DECL)
 	    && DECL_BUILT_IN (TREE_OPERAND (TREE_OPERAND (exp, 0), 0))
 	    && (DECL_BUILT_IN_CLASS (TREE_OPERAND (TREE_OPERAND (exp, 0), 0))
 		== BUILT_IN_FRONTEND))
 	  return c_expand_builtin (exp, target, tmode, modifier);
 	else
 	  abort();
      }
      break;

    default:
      abort ();
    }

  abort ();
  return NULL;
}