1. A method for processing of expressions in a data processing system, comprising the steps of: parsing a data processing instruction using a parsing template, which parsing template defines elements of a first type of expression that can be evaluated to control a conditional data processing operation, to identify elements of an expression of said first type including expression parameter values; selecting an evaluation template representing said first type of expression in a form that is optimized for evaluation; inserting parameter values identified by the parsing step into the evaluation template to generate optimized evaluation code; and using the optimized evaluation code to evaluate said expression.