My design has a trimming issue that I have traced to a feedback path involving a MUX and LUT logic. If I apply an "S" property to the feedback net, all of the trimming is blocked.
Is there a problem with my design, or was the original trimming invalid?
Feedback paths are a problem area for the trimming algorithm. The "S" property blocks the invalid trimming and maintains the valid logical circuit.
Multiple similar cases have been found and fixed involving feedback paths. This problem has been fixed in the latest 10.1 Service Pack available at:
http://www.xilinx.com/support/download/
The first service pack containing the fix is 10.1 Service Pack 3.