You're not wrong - in that ALL optimizations are inter-dependent. But, for reasons that I think are rather obvious, it's really hard to do them optimally, all a

You're not wrong - in that ALL optimizations are inter-dependent. But, for reasons that I think are rather obvious, it's really hard to do them optimally, all at once. So typically you'd do things like "register-pressure-aware" selection & scheduling, repeated optimization steps (like, do a register-pressure-aware first scheduling, RA, then schedule again) etc. It's all just heuristics, in the end. > since it can only make the latter easier. Not necessarily. For a simple case, yes; but in general, committing to early to reuse a virtual register may have very bad performance implications, since it limits code mobility, freedom to select a particular register, and may increase register pressure in critical sections of the code (say you have "A conflicts with B, B with C and C with D"; you can put all of these in 2 registers: A=R0, B=R1, C=R0, D=R1. But if you committed too early to unify the lifetimes of A and D - eg. for instruction selection purposes - now you need 3 registers. Which is fine, and likely the best solution if you actually end up having 3 registers available - but very likely sub-optimal if you only had 2).