Abstract: A first two-input adder computes the sum of one wider and one narrower input by combining a conventional adder for the low-order bits with an incrementer and selection logic for the high-order bits. A second three-input adder computes the sum of one wider and two narrower inputs in a similar way: the low-order bits are computed with a conventional carry save adder (CSA) followed by a carry propagate adder (CPA), while the high-order bits are computed with an incrementer and selection logic. The first and second circuits are combined to form a third arithmetic circuit that takes four input operands, the first of which is wider than the other three, and speculatively computes two results: (1) the sum of the first and second input operands; and (2) the sum of the first, third, and fourth input operands. This combined circuit contains all of the elements of the first two circuits, but shares a single incrementer. A degenerate case of the third circuit occurs when the second and third inputs are common.