Automatic signal fanout management in an FPGA (Xilinx type in this case)

Hi,

I develop Xilinx FPGAs and I am interested in dealing with a maximum fanout
of a signal issued from a FF and going to an increasing number of FF. What
kind of constraint and/or attribute I can set to this signal so that when
the fanout exceeds a certain number I am SURE an extra FF is created. I am
using Leonardo and M5.1i. I have used the max_fanout attribute of leonardo
but it doesn't seem to make much of anything (anyway there is no
replication). Thank you for any suggestion.

JF