Constraint-Aware Multi-Technique Approximate High-Level Synthesis for FPGAs

Numerous approximate computing (AC) techniques have been developed to reduce the design costs in error-resilient application domains, such as signal and multimedia processing, data mining, machine learning, and computer vision, to trade-off computation accuracy with area and power savings or performance improvements. Selecting adequate techniques for each application and optimization target is complex but crucial for high-quality results. In this context, Approximate High-Level Synthesis (AHLS) tools have been proposed to alleviate the burden of hand-crafting approximate circuits by automating the exploitation of AC techniques. However, such tools are typically tied to a specific approximation technique or a difficult-to-extend set of techniques whose exploitation is not fully automated or steered by optimization targets. Therefore, available AHLS tools overlook the benefits of expanding the design space by mixing diverse approximation techniques toward meeting specific design objectives with minimum error. In this work, we propose an AHLS design methodology for FPGAs that automatically identifies efficient combinations of multiple approximation techniques for different applications and design constraints. Compared to single-technique approaches, decreases of up to 30% in mean squared error and absolute increases of up to 6.5% in percentage accuracy were obtained for a set of image, video, signal processing and machine learning benchmarks.