A computer program compiler generates object code to be executed by a RISC type of CPU, whereby in effect each bit operational expression appearing in the source code, such as an AND operation on two bit variables, is converted to an operation whereby the respective values of the bit variables which are operands of that expression are judged, and a specific binary value is assigned as the result of the bit operational expression based upon the judgement results, to thereby substantially reduce the number of RISC object code instructions corresponding to such an operation.