A method and apparatus for performing pipelined computations that include
cross-iteration computations. The apparatus includes a 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. 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.