ECAx: Balancing Error Correction Costs in Approximate Accelerators

Approximate computing has emerged as a design paradigm amenable to error-tolerant applications. It enables trading the quality of results for efficiency improvement in terms of delay, power, and energy consumption under user-provided tolerable quality degradation. Approximate accelerators have been proposed to expedite frequently executing code sections of error-resilient applications while meeting a defined quality level. However, these accelerators may produce unacceptable errors at run time if the input data changes or dynamic adjustments are made for a defined output quality constraint. State-of-the-art approaches in approximate computing address this issue by correctly re-computing those accelerator invocations that produce unacceptable errors; this is achieved by using the host processor or an alternate exact accelerator, which is activated on-demand. Nevertheless, such approaches can nullify the benefits of approximate computing, especially when input data variations are high at run time and errors due to approximations are above a tolerable threshold. As a robust and general solution to this problem, we propose ECAx, a novel methodology to explore low-overhead error correction in approximate accelerators by selectively correcting most significant errors, in terms of their magnitude, without losing the gains of approximations. We particularly consider the case of approximate accelerators built with approximate functional units such as approximate adders. Our novel methodology reduces the required exact re-computations on the host processor, achieving up to 20% performance gain compared to state-of-the-art approaches.