http://www.clifford.at/icestorm/
and even featuring a gui app built on top of it! https://icestudio.readthedocs.io/en/latest/
"Project IceStorm
Project IceStorm is a previous project that documented the iCE40 bit-stream format. It will become a part of SymbiFlow. SymbiFlow will support the old Yosys-ArachnePnr-IceStorm flow but will also add a Yosys-VPR-IceStorm flow."
http://www.clifford.at/icestorm/
and even featuring a gui app built on top of it! https://icestudio.readthedocs.io/en/latest/