Validating register allocations for straight line programs

Straight line programs are essentially sequences of assignment statements. A general program consists of straight line segments with flow of control between these segments. The interfaces between a straight line segment and the rest of the program place constraints on register allocations for the segment. These constraints may render a register allocation for a straight line program unrealizable. An algorithm is given to determine if a register allocation for a straight line program is realizable. It is shown that the algorithm takes 0(n3) steps, where n is the number of statements in the program. An unrealizable assignment may be made realizable by introducing stores into memory at appropriate points. It is shown how the minimum number of such stores may be found.