When the "I/O Power Reduction" option is turned ON in the MIG GUI, new I/O primitives are used in the RTL code which require multi-cycle path constraints. When the "I/O Power Reduction" option is turned OFF, these multi-cycle constraints are not required.
However the multi-cycle constraints are still being generated for the XDC constraint file used in PlanAhead and Vivado and will cause the following Critical Warnings:
CRITICAL WARNING: [PlanAhead-1387] set_multicycle_path: specified list for option 'from' is an empty list. The constraint will not be applied. Please check to make sure that this is intended. [/project_name/mig_7series_v1_5/example_design/par/proj1.srcs/constrs_1/imports/par/example_top.xdc:305]
These Critical Warnings can be safely ignored as these constraints are only required when "I/O Power Reduction" is enabled.
The following constraints can also be removed from the XDC constraint file to remove the warning message: