We have played around with having "hints" in the original source code that will be preserved through the IR, but the vast majority of the optimizations and scheduling decisions we need to do are happening once it has been converted into machine code by the backend. I may be slightly off (I'm the dumb CEO/hardware architect), but I'm fairly sure 85%-90% of our compiler work has been done on the backend... I just remember being told some tweaks were made to clang due to some weird issue.