Does Vivado Synthesis support null ranges?
What kind of null ranges are supported?
What type of null ranges will be supported in future releases?
Currently, null ranges have limited support in Vivado Synthesis. The complete support for null ranges is expected to be available in the Vivado 2013.1 design tools.
The following null range types are supported in Vivado 2012.4:
In addition to the above null range types supported in 2012.4, the following null range types are being supported starting in Vivado Synthesis 2013.1: