The present invention provides methods, apparatus, and systems to remove a redundant, sign extension instruction from a program and to improve the execution efficiency of the program. In an example embodiment, a conversion program for controlling a computer for the conversion of an execution program, especially a compiler, permits the computer to perform: a function for analyzing a sign extension instruction issued in order to perform the sign extension of a value defined in the execution program; and a function for, in accordance with analysis results, removing a predetermined sign extension instruction from those included in the execution program.