1. A computer processor comprising: a plurality of operand storage elements that store operand data values; and execution logic including a number of functional units operably coupled to the plurality of operand storage elements, wherein the execution logic is configured to perform operations that access said operand data values stored in the plurality of operand storage elements; wherein said operand data values include a predefined None operand data value that is indicative of a missing operand value; wherein said operations include a RETIRE operation that specifies a number of operand data values that is intended to be retired in a predefined machine cycle; and wherein, during execution of the RETIRE operation, zero or more None operand data values are selectively retired in the predefined machine cycle based on the number of operand data values specified by the RETIRE operation and the number of operand data values to be retired as a result of execution of other operations by the execution logic in the predefined machine cycle.