Method and apparatus for nested instruction looping using implicit predicates

A method and apparatus for executing a nested program loop on a vector processor, the loop comprising outer-pre, inner and outer-post portions. An input stream unit of the vector processor provides a data value to a data path and sets an associated data validity tag to 'valid' once per outer loop iteration, as indicated by an inner counter of the input stream unit. The tag is set to 'invalid' in other iterations. Functional units of the vector processor operate on data values in the data path, each functional unit producing a valid result if the data validity tags associated with inputs data values are set to 'valid'. An output stream unit of the vector processor sinks a data value from the data path once per outer loop iteration if an associated data validity tag indicates that the data value is valid.