Rat's Register Allocator

rat's compiler backend uses a priority bin-packing register allocator that assigns virtual registers to physical registers or stack slots by processing live ranges in order of importance. The allocator improved upon the previous linear scan approach by reducing code from 1,392 to 584 lines while producing better quality code, and elegantly handles calling conventions without explicit rules through its design. The allocation process involves five steps: identifying live ranges, marking fixed registers, coalescing copies, assigning registers by priority, and spilling to stack when necessary.