[samatman]

I think it's critical that we do annotate it as a special multiply.

If wraparound is ok for that particular multiplication, tell the compiler that. As a sibling comment says, this is seldom the case, but it does happen, in particular, expecting byte addition or multiplication to wrap around can be useful.

The actual expectation of the vast majority of arithmetic in a computer program is that the result will be correct in the ordinary schoolyard sense. While developing that program, it should absolutely panic if that isn't the case. "Well defined" doesn't mean correct.

I don't understand your objection to spelling that `val *% GOLDEN_RATIO_32` is. When someone sees that (especially you, later, coming back to your own code) it clearly indicates that wrapping is expected, or at least allowed. That's good.