There is provided methods and apparatus to improve runtime by computer programs at compilation time. A compiler analyzes code to be translated into machine executable instructions to identify overloaded functional units of the target processor, and replaces instructions scheduled on the overloaded functional unit to an idle unit using functionally equivalent operations on the idle unit. The replacement instructions may be taken from an instruction replacement library comprising function calls that implement functionality of a functional unit of the target processor on another functional unit of the target processor.