:param function_addr_a: The address of the first function (in the first binary)
        :param function_addr_b: The address of the second function (in the second binary)
        :returns: the FunctionDiff of the two functions

def get_function_diff(self, function_addr_a, function_addr_b):
        """
        :param function_addr_a: The address of the first function (in the first binary)
        :param function_addr_b: The address of the second function (in the second binary)
        :returns: the FunctionDiff of the two functions
        """
        pair = (function_addr_a, function_addr_b)
        if pair not in self._function_diffs:
            function_a = self.cfg_a.kb.functions.function(function_addr_a)
            function_b = self.cfg_b.kb.functions.function(function_addr_b)
            self._function_diffs[pair] = FunctionDiff(function_a, function_b, self)
        return self._function_diffs[pair]