> attempting to separate them, like what most if not all compilers do, results in (sometimes very) suboptimal results
Not separating them would have a big disadvantage: all register allocation decisions need to be strictly local, because information about upcoming instructions and their register constraints is not available. Even simple graph coloring algorithms give much better code than algorithms with local decisions only.
In our baseline/unoptimized compiler, we do ISel+RegAlloc(+Encoding) combined in a single step and we get lots of easily avoidable moves and spills. (These typically don't hurt performance that much on modern out-of-order CPUs with store forwarding, but substantially increase the code size.)
Not separating them would have a big disadvantage: all register allocation decisions need to be strictly local, because information about upcoming instructions and their register constraints is not available. Even simple graph coloring algorithms give much better code than algorithms with local decisions only.
In our baseline/unoptimized compiler, we do ISel+RegAlloc(+Encoding) combined in a single step and we get lots of easily avoidable moves and spills. (These typically don't hurt performance that much on modern out-of-order CPUs with store forwarding, but substantially increase the code size.)