A method and apparatus for the elimination of prolog and epilog instructions in a vector processor. To eliminate the prolog, a functional unit of the vector processor has at least one input for receiving an input data value tagged with a data validity tag and an output for outputting an intermediate result tagged with a data validity tag. The data validity tags indicate the validity of the data. Before a loop is executed, the data validity tags are set to indicate that the associated data values are invalid. During execution of the loop body a functional unit checks the validity of input data. If all of the input data values are valid the functional operation is performed, the corresponding data validity tag set to indicate that the result is valid. If any of the input data values is invalid, the data validity tag of the result is set to indicate that the result is invalid. To eliminate the epilog, an iteration counter is associated with each sink unit of the vector processor. When a specified number of data values have been produced by a particular sink, no more data values are produced by that sink. The instructions for the pipelined loop body may be repeated, without alteration, to eliminate prolog and epilog instructions.