Low-power adder circuit

Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for a circuit configured to add multiple inputs. The circuit includes a first adder section that receives a first input and a second input and adds the inputs to generate a first sum. The circuit also includes a second adder section that receives the first and second inputs and adds the inputs to generate a second sum. An input processor of the circuit receives the first and second inputs, determines whether a relationship between the first and second inputs satisfies a set of conditions, and selects a high-power mode of the adder circuit or a low-power mode of the adder circuit using the determined relationship between the first and second inputs. The high-power mode is selected and the first and second inputs are routed to the second adder section when the relationship satisfies the set of conditions.