1. A code generation method, 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, the first specific instruction using a first register to store the different pieces of data; retrieving, from the first code by tracing the first code from the first and second instructions to a beginning, a third instruction for calculating data referenced by the first instruction and a fourth instruction for calculating data referenced by the second instruction; 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 and is to be executed before the first specific instruction, the second specific instruction using a second register to store an execution result of the second specific instruction; determining whether to combine the third and fourth instructions to generate the second specific instruction by determining whether data included in the second register is to be transferred to the first register when the first specific instruction is to be executed after execution of the second specific instruction; and determining a combination of the first and second instructions by scanning an instruction queue included in the first code from the end to the beginning of the instruction queue.