COMPILER FOR OPTIMIZING PROGRAM

A compiler system and method for calculating a value to be assigned to a variable for optimizing a program. The apparatus includes a subrange analysis unit for analyzing, for an instruction to assign a value to a variable in the program, a range of the value being assignable to the variable by the instruction, as a subrange of the variable in a case where instruction is executed; a determination unit for determining if the execution result of the program changes if the instruction assigns any value in the subrange of the variable to the variable on the basis of the analyzed subrange of the variable; and a replacement unit for replacing the instruction to assign the value to the variable with an instruction to assign a constant value in the subrange of the variable to the variable.