1. A method in a data processing system for processing expressions, the method comprising: responsive to receiving expressions, identifying parameters for the expressions to form a set of identified parameters; identifying each type of execution unit with an instruction category from an existing set of instruction categories, wherein the execution unit can execute instructions needed to process the expressions using the set of identified parameters, wherein a set of identified execution unit types is formed; generating a list of acceptable types of registers associated with the set of identified execution types to form a set of registers; constructing lists of stores involving adjacent memory; for each list of stores, selecting an adjacent subset; setting all acceptable register class flags to true; examining instructions in lists of store and updating acceptable register kind flags; selecting a combination of an execution unit type from the set of identified execution unit types and an acceptable register from the set of registers, to meet a performance goal; and generating replacement instructions for the combination to process the expressions.