Variable-latency signed addition on FPGAs

Variable-latency, or speculative, addition is an effective technique to implement fast adders working on very long operands. Most approaches to speculative addition are either based on the assumption that operands have equiprobable independent bits, which is rarely the case in real applications due to sign-extension, or they can handle the case of signed numbers at the price of a considerable area overhead. Furthermore, many existing approaches require ad-hoc schemes preventing the reuse of standard adders typically available as optimized library components in many technologies, most notably Field-Programmable Gate Arrays. This paper introduces an innovative scheme for speculative addition that effectively addresses both problems, yielding fast and low-area circuits able to handle sign-extended numbers speculatively and only made of optimized carry-propagation adders based on fast carry circuitry as basic building blocks.