A Wallace tree structure such as that used in a digital signal processor (DSP) is arranged to sum vectors. The structure has a number of adder stages, each of which may have half adders with two input nodes, and full adders with three input nodes. The structure is designed with reference to the vectors to be summed. The number of full- and half-adders in each stage and the arrangement of vector inputs depends upon their characteristics. An algorithm calculates the possible tree structures and input arrangements, and selects an optimum design having a small final stage ripple adder after the last stage of the Wallace tree structure, the design being based upon the characteristics of the vector inputs. This leads to reduced propagation delay and a reduced amount of semiconductor material for implementation of the DSP.