On some Virtex designs, MAP posts the following warning message:
"WARNING: MapLib:149 - Failed to find a MUXCY that associates with XORCY symbol "rx_bufctl_2/C3812" (output signal=rx_bufctl_2/syn2962). Cannot recognize the standard carry chain structure."
Typically, an XORCY is used in combination with a MUXCY that has shared input signals. It is perfectly valid to use it without a corresponding MUXCY, but this configuration is considered suspect enough to post a warning message in case this configuration is accidental.