1. A computer-readable recording medium having stored therein a program for causing a computer to execute a digital signature process comprising: determining that a first specific instruction for executing parallel calculations of the same type, each calculation operating on a different piece of data, is generated by combining first and second instructions included in a first code; retrieving, from the first code, a third instruction for calculating data referenced by the first instruction and a fourth instruction for calculating data referenced by the second instruction; and selecting the third and fourth instructions as candidates of instructions to be combined with each other to generate a second specific instruction which is different from the first specific instruction.