Method and apparatus for choosing register classes and/or instruction categories

An improved method, apparatus, and computer instructions for generating instructions to process multiple similar expressions. Parameters are identified for the expressions in the original instructions, to form a set of identified parameters typically including the operations performed, the types of data used, and the data sizes. Each type of execution unit that can execute the instructions needed to process the expressions using the set of identified parameters is identified, wherein a set of identified execution unit types is formed. An execution unit type from the set of identified execution unit types is selected to meet a performance goal. The new instructions are generated for the selected execution unit type to process the expressions, and the original instructions for the expressions are discarded.