General Description: A case has been seen involving a design that has a mixture of LOC constraints coming from an MFP file written by the Floorplanner, and an NCF written by Synplicity. There are several carry chains in the design that end with an XORCY whose CI pin is driven by the carry chain and whose LI pin is driven by a LUT.
The LUT is LOC'd (MFP) to the specific site needed to satisfy the carry chain. The XORCY has a Range LOC where the range contains the site need for the carry chain. The packer should be putting the LUT and XORCY in the same slice, but it is not. Since the LUT is LOC'd to the specific site needed by the carry chain, the slice with the XORCY can't go there, and the carry chain is unplacable. The following error is reported:
Placer Error: ERROR:Place:871 - Some of the slices in the carry chain containing the slice inst_core/inst_uttxums/UTTX.madd_2_0(1) have LOC constraints that make it impossible to place the chain legally. Please either fix or remove the constraints.
There may be a related Floorplanner bug wherein only the LUT is being LOC'd.
This problem will be fixed in the next major release after 4.1i.
Meanwhile, the work-around is to modify the NCF constraint so that the XORCY has a LOC to the specific site needed.