General Description: FPGA Express will write out an IOB attribute on every flip-flop in a Virtex design. This causes the 'map -pr' option to be obsolete.
解决方案
Change the IOB attribute setting to either TRUE or FALSE in the FPGA Express constraints editor. The UCF will also override this particular constraint.
If you want no attribute to be placed on the flip-flop, the only known workaround is to manually edit the EDIF file to remove the IOB attributes.