There is a reason proprietary backends would always be necessary. Hardware companies are very touchy about the patents, and not publishing an ISA-related work

There is a reason proprietary backends would always be necessary. Hardware companies are very touchy about the patents, and not publishing an ISA-related work may often be the only way to protect from the trolls. Such companies would often encourage (privately or publicly) clean room 3rd party backends, and I'm pretty sure that the one we're discussing here is exactly of this kind. But they would be scared to death to allow any of their employees to even send a tiny patch to a compiler backend, if it could expose some microarchitecture knowledge that was not published.