Current status

The current FPGA interchange implementation never sets placement relative constraints. This will mean that carry chain structures will have a hard timing statifying the DedicatedInterconnect and waste a lot of time in placement strict legalization. nextpnr's relatively constraint system should be usable for this case, with at least 1 exception.

Potential solutions

During packing, chains of cells that have dedicated interconnect need to be detected and those chain structures identified. In the simple case, once the chain is found settings the cell placement constraint will be enough, see https://github.com/YosysHQ/nextpnr/blob/692d7dc26ddf21e2d38dd16aecef652ab4c0d5e3/common/nextpnr_types.h#L168-L175

There is an exception case that is not solvable with the current system from nextpnr, and that has to do with 7-series carry chain that extend across CMTs. It is worth noting that the symbiflow-arch-defs implementation has the same deficiency, so ignoring this case is potentially fine. The specific case in question is what happens if a carry chain extends across the BRKH_CLB tile type that appears between CMTs. In this case the constr_y will be 2 instead of 1. To solve this would likely require either a placement constraint system change or an Arch API change.