Physical optimization (phys_opt_design) automatically replicates the high fanout
net drivers based on slack and placement information, and usually significantly improves
timing. Xilinx recommends that you drive high fanout nets with a
fabric register (FD*), which is easier to replicate and relocate during physical
optimization.
In some cases, the default phys_opt_design command
does not replicate all critical high fanout nets. Use a different directive to increase
the command effort: Explore, AggressiveExplore or AggressiveFanoutOpt. Also, when a high
fanout net becomes critical during routing, you can add an iteration of phys_opt_design to force replication on specific nets before
trying to route the design again. For example:
phys_opt_design -force_replication_on_nets [get_nets [list netA netB netC]]