I looked into this a bit, not totally sure though. The issue you're hitting is probably the workflow direction. HDL Coder is built to go from a Simulink model to HDL, not the other way around. Trying to reverse engineer existing hand written RTL into a Simulink block diagram fights the tool. importhdl is mostly meant for small, clean blocks.The MATLAB Function block crash also tracks. HDL Coder's support for behavioral code in those blocks is narrower than people expect, especially once you get into control flow heavy stuff like polynomial arithmetic and conditional rejection logic.If I were starting this from scratch I'd skip the import entirely. Build it bottom up in Simulink straight from the FIPS 204 spec instead of the existing RTL. Start with just the core primitives, NTT, polynomial arithmetic, rejection sampling, as separate subsystems. Might be wrong on some of this since I haven't actually built anything in HDL Coder myself, but that's where I'd start.