1. An integrated circuit comprising: a plurality of configurable logic blocks; configurable interconnect resources programmably interconnecting the plurality of configurable logic blocks; a plurality of configurable digital signal processing (DSP) elements, including a first DSP element and a second DSP element, the second DSP element including: an element output port; and multiplexing circuitry having: a feedback port connected to the element output port; a cascade input port connected to the slice output port of the first DSP element; a multiplexing-circuitry output port; and a select port receiving mode control signals; wherein the multiplexing circuitry connects at least one of the feedback port and the cascade input port to the multiplexing-circuitry output port in response to the mode control signals.