When opening an implemented design, I see the following messages.
I did not see this in Vivado 2014.4 from same sources.
While importing this netlist, 1 undefined cell was found and converted to a black box.
Make sure you have loaded all intended module definitions before proceeding.
Black boxes can be populated by using the Add Sources operation and re-running Synthesis
INFO: [Opt 31-120] Instance xyz has been optimized to an empty box cell during sweep but it has constraints that prevent its removal. Empty box cells do not impact the implementation flow but they have no functional relevance.
Resolution: If this is not expected, please check for DONT_TOUCH properties or timing constraint set on the empty box cell or on nets connected to the cell. If found, remove the relevant DONT_TOUCH property or timing constraint and re-run opt_design.