[fao_]

> But you can know what's doing. Just have it output the .s/.asm file

That's still black box testing. There ~may be~ almost certainly is hidden state that could mean that the optimization you expect to be applied is not (or an optimization is misapplied) under certain circumstances, and you can't/won't discover this in advance due to those conditions.

> A compiler that can compile a whole operating system probably has most of its issues ironed out.

History has shown this to be mostly wrong, given how many critical bugs have been discovered in GCC since it was capable of compiling linux