1. A data processor for pipelined computation comprising: a first functional unit having at least one input and an output, each input being operable to receive an input data value and an associated input data validity tag indicative of the validity of the input data value and the output being operable to provide an output data value and an associated output data validity tag indicative of the validity of the output data value; wherein the first functional unit is operable in a first mode in which an output data value from the first functional unit is valid if all of the input data values are valid, and in a second mode in which the output data value from the first functional unit is valid if any of the input data values is valid.