General Description: During optimization, XST reports the following HDL Advisor message:
"WARNING:Xst:387 - The KEEP property attached to the net <name> may hinder timing optimization."
The KEEP property is used to maintain an internal node through synthesis and implementation. If this property is located at a node that might otherwise be optimized by XST, XST will issue this warning. This frequently occurs if the node is within combinatorial logic that could be potentially reduced to a smaller number of LUTs, thus reducing the delay of that path.
Examine the location of the KEEP property and the timing of paths through that point to determine whether it is logical for the KEEP property to remain.
If no KEEP constraints have been inserted into the HDL code, you are encountering a XST known issue; XST is printing the warning messages for no apparent reason.
If this is the case, you may safely ignore the warning message. This problem will be fixed in software version 5.1i.