I got the opinion it would be much more complex, like deciding if saving a value is faster than reordering, and deciding the best place to request a load. It is also a large instruction word architecture, what means it's mostly similar from the compiler point of view.
The one thing that I think makes the Mill simpler is the no-value values mixed with lazy error conditions. I have no idea if those are powerful enough to compensate the rest.
For spilling: if a desired argument is less than one belt-length away then we directly reference it; between one and two away we reorder; and more than two we spill.
We place a load as soon as the address arguments are statically available. The compiler doesn't have to deal with aliasing, which is a large and bug-ridden part of compiling for other targets.
Lazy-error (aka NaR) means that there are no error dependencies in arithmetic expressions. Current compilers for other targets simply ignore such dependencies, relying on the "undefined behavior" rule. Mill is designed to not produce nasal demons :-)
The one thing that I think makes the Mill simpler is the no-value values mixed with lazy error conditions. I have no idea if those are powerful enough to compensate the rest.