code: def get_function_diff(self, function_addr_a, function_addr_b):
          
          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]
docs: :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