1. A non-transitory 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 that includes a plurality of functions; measuring a performance index value for each of the plurality of functions by executing the target program using each of the plurality of pieces of input data; calculating a combined performance index value for each of the plurality of functions by adding performance index values that are weighted based on attribute information of the input data; extracting functions to be candidates for a tuning point from among the plurality of functions based on the combined performance index value; and calculating reliability of a result of the calculating, the reliability being a total number of the input data handled by the calculating of the combined performance index value.