1. A computer-readable recording medium that stores therein a performance tuning program, the performance tuning program making a computer execute: acquiring a plurality of pieces of input data to be given to a target program; measuring, for each of the input data, a performance index value for each of functions that specify a function of the target program, by giving each of the input data to the target program to execute the target program; combining, for each of the functions, the performance index value of each of the input data; and extracting functions to be candidates for a tuning point from among the functions based on a result of combining at the combining.