Cross-Layer Approximate Hardware Synthesis for Runtime Configurable Accuracy

Approximate computing trades off computation accuracy against energy efficiency. The extent of approximation tolerance, however, significantly varies with a change in input characteristics and applications. We propose a novel cross-layer approach for the synthesis of runtime accuracy-configurable hardware that minimizes energy consumption at area expense. To that end, first, we explore instantiating multiple hardware blocks in the architecture with different fixed approximation levels. These blocks can be selected dynamically and thus allow to configure the accuracy during runtime. They benefit from having fewer transistors and also synthesis relaxations in contrast to state-of-the-art gating mechanisms that only switch off a group of paths of the circuit. Our cross-layer approach combines instantiating such blocks in the architecture with area-efficient gating mechanisms that reduce toggling activity, creating a fine-grained design-time knob on energy versus area. We present a systematic methodology to explore this joint design space and find energy-area optimal solutions as a function of required accuracies, their utilization in the workload, together with hardware parameters: dynamic power savings, area of the hardware block, and leakage of the technology. Examining total energy savings for a range of circuits under different workloads and accuracy tolerances shows that our method finds Pareto-optimal solutions providing up to 32% and 60% energy savings compared to state-of-the-art accuracy-configurable gating mechanism and an exact hardware block, respectively, at 2× area cost.