Both parts (compiler backends & fuzzing) are immensely complicated stuff on their own. I can only imagine the trickery and head scratching that comes with the combination.

I’ve worked on simpler approximations of both [0][1] before and can attest even the simple parts are hard to get working reliably.

[0]: https://github.com/danobi/btrfs-fuzz

[1]: https://github.com/iovisor/bpftrace