Register spilling

Register spilling is a compiler technique that keeps some values in computer memory when they cannot all remain in the available processor registers. Generated stores preserve a value before its register is reused; later loads recover it when needed. This is part of register allocation, whose choices affect the number of memory accesses.^[ref]

For example, suppose three independently computed values must survive to later instructions, but only two suitable processor registers are available. The allocator may store one value in a stack slot, use its register for another value, and reload it before its next use. The best choice depends on live ranges, use frequency and instruction constraints; recomputing a cheap value can sometimes avoid the store.