[pcwalton]

> This is relatively easy to do in Go compared to other languages because packages must form a DAG.

That doesn't make sense to me. The complicated part is storing the IR in packages in a form that can be read back into the compiler later. That's needed to do inlining at all. Once you have that done, doing LTO is trivial: you just slurp your IR for all modules linked together into the compiler and emit a single binary. (You can be fancier, like ThinLTO does, but again, the effort needed to do ThinLTO is independent of whether you have cyclic dependencies or not.)