Abstract: An optimizer (100) comprises a memory (110) and a processor (130). The memory stores a program (200) to be optimized and optimization software (301). Controlled by the optimization software, the processor (120) (a) determines local vectors (“local”) in instructions of the program (200) which indicate the use of resources by the instructions (use-vectors, exh-vectors); (b) scans the program (200) for Single-Entry-Single-Exit (SESE) structures (U, T, V, S); and (c) determines SESE vectors from the local vectors. The SESE vectors indicate the use of resources by the SESE structures and can be combined by the optimizer to obtain a program vector. When some instructions are modified, then optimizer (100) only re-calculates the SESE vector of the corresponding SESE and re-combines the old SESE vector with the modified SESE vector to determine a new program vector.