Problem description: A case has been seen where map trimming led to the incorrect merger of an FDCE and XORCY. This causes the carry chains in the RPM to be non-aligned, and therefore unroutable. (This is a regression from 2.1i behavior.)
Root Cause ------------ In the user's design, a FDCE with RLOC drives a LUT1_L and the LUT1_L drives a XORCY through a LOCALBUF.
FDCE ---> LUT1_L ---> LOCALBUF ---> XORCY
The mapper trimmed out both LUT1_L and LOCALBUF and put a "FAST" property to the input pin of the XORCY during the mapping process. Therefore, the packer legally merged the FDCE and the XORCY into the same SLICE, which caused this unroutable situation, as the XORCY is part of the carry chain.
2.1i doesn't cause this situation because the packer didn't process the "FAST" property correctly. The local output packer didn't merge the FDCE and the XORCY together as they should be. Thus, the unroutable situation is avoided.