I saw an Intel video on youtube that explains how to use in the .qsf :
set_global_assignment -name PRESERVE_FOR_DEBUG_ENABLE ON
and in the RTL code
attribute preserve_for_debug of my_signal_keep : signal is true;
But they never mention if this is only for Quartus pro only (and not standard).
As I use a cyclone V FPGA, I'm forced to use Quartus Std, and I have the impression this is not available there.
I wanted to use it because my keep attribute was not working (the net was removed):
attribute keep of my_net_to_keep : signal is true;
but I got it kept by changing this option in the QSF (from ON value):
set_global_assignment -name REMOVE_REDUNDANT_LOGIC_CELLS OFF