I wonder whether you could use Sail to make a compiler that figures out how to compile things. Then you could re-target it just by giving it a description of the new processor instead of a description of the old one.
If you don't care about compiler performance, auto-generating a compiler backend from scratch is already highly non-trivial. Sail's constrained nature may help a little but not that much, as you essentially need to auto-understand an interpreter for a Turing-complete language. If you do want performance, the problem becomes much harder still. One of the biggest problems in compiler performance is the memory model: what reordering is permitted, what is prohibited to the optimiser? Sail does not address this. Instead, (like all similar languages) it 'handles' main memory access by two uninterpreted functions which are (roughly) "memory read" and "memory write" That's a polite way of saying: Sail (like all similar languages) leaves this particular problem to the user.
There are open research questions here.
Save Intel's compiler, do any rely on implementation details? They rely on cache and fetch details, but only in broad, nebulous ways that seem very portable.
There may be open research questions but it seems to me the biggest thing is paying someone to do it.
I think you a word while reading.