During the optimization phase of XST synthesis, the following warning is reported:
"WARNING:Xst:615 - < object > not found, property < prop > not attached."
Why does this occur?
During optimization, the signal mentioned by the warning has been renamed or removed from the design. If you have set this property, ensure that the logic surrounding the signal has not been optimized away. Also, check to see if the signal traverses hierarchy; if it does, it is possible that XST renamed it.
If the property is a synthesis property, check the logic surrounding the signal to be sure it remains through optimization, or consider moving the property higher up in the design hierarchy. If the property is an implementation property, consider using the Constraints Editor or UCF to apply the constraint.
In some situations, XST will attach a "keep" property to the signals. This is done for a variety of reasons, such as maintaining connections to black boxes and maintaining replicated signals as they traverse the hierarchy.
If optimization renames these signals, the "keep" properties will not match the new signal name. However, the properties have already done their job, and the messages can be safely ignored.