It combines the RTL expressions for the instructions by substitution, simplifies the result using algebra, and then attempts to match the result against the machine description. This pass looks for cases where matching constraints would force an instruction to need a reload, and this reload would be a register to register move. It them attempts to change the registers used by the instruction to avoid the move instruction. This pass looks for instructions whose output will not be available by the time that it is used in subsequent instructions. It re-orders instructions within a basic block to try to separate the definition and use of items that otherwise would cause pipeline stalls.
Instruction scheduling is performed twice.
The first time is immediately after instruction combination and the second is immediately after reload. Register class preferencing. The RTL code is scanned to find out which register class is best for each pseudo register. This pass allocates hard registers to pseudo registers that are used only within one basic block. Because the basic block is linear, it can use fast and powerful techniques to do a very good job.