Register allocation

The next step of the code generator is register allocation. As we saw in the previous example, some of the registers being used were virtual registers. Register allocation is the task of assigning physical registers to these virtual registers. In LLVM, the virtual registers can be infinite in number, but the numbers of physical registers are limited depending on the target. So, by register allocation, we aim at maximizing the number of physical registers being assigned to virtual registers. We must note that the physical registers are limited in number, so it is not always possible that all the virtual registers can be assigned a physical register. If there is no physical register available at some point and we need a physical register for a variable, we might move a variable that is present in physical register to main memory and thus assign the freed register to the variable we want. This process of moving a variable from physical register to memory is called spilling...