I'm using ISE 8.1.02. I seem to be having trouble making the BYPASS attribute work on nets. As I understand it, it is supposed to go on the net that is the output of an OBUFT. This should cause the TFB/OFB in the ILOGIC block to be activated. FPGA Editor, though, shows no change between building with or without the BYPASS attribute. Putting the BYPASS attribute on any other line (including the inputs for the OBUFT) or object gives me a warning about bad attribute placement.
I don't understand why Xilinx doesn't provide raw primitives for ILOGIC and OLOGIC. It seems to me that would be just as easy as using the IDELAY element, and it would allow me to connect the TFB/OFB directly.
I tried using IOPAD directly. IOPAD doesn't even show up in the documentation as a valid primitive these days. NGDBuild seems to handle it okay, though it does throw an INFO about raising that net to the status of port. It did not, however, help with my BYPASS issue.
Another frustrating issue is that the BYPASS attribute doesn't even show up in the standard documentation. I had to get it from here: