A Cross-Layer Gate-Level-to-Application Co-Simulation for Design Space Exploration of Approximate Circuits in HEVC Video Encoders

A cross-layer design space exploration (DSE) method based on a proposed co-simulation technique is presented herein. The proposed method is demonstrated evaluating the impacts on both coding efficiency and power dissipation of applying distinct approximate logic operators in a s $\mu {\mathrm{ m}}$ of absolute differences (SAD) kernel that accelerates an H.265/HEVC (high-efficiency video coding) encoder. The proposed method simulates the gate-level circuit dynamically inside the application, with realistic results of the impact of the adder-tree approximate logic implementation on both quality and encoder bit-rate results. A comprehensive DSE is shown herein, with 13 types of 6 classes of approximate adders in the SAD accelerator hardware blocks. Over 3,000 logic variants of approximations at gate-level were developed. Actual video sequences as inputs to the x265 software encoder are co-simulated, to dynamically capture the video motion-estimation (ME) behavior in the presence of logic approximations. While the prior art that only estimates the impact of the approximate logic on power, area, and quality on static designs with statistical assumptions, which are agnostic to the actual algorithm data-dependent behavior in the application, our method explores accurately the trade-off between power dissipation and coding efficiency dynamically over the entire HEVC encoding. Our approach shows that the lower-part-or and error-tolerant adder I approximate adders, as well as truncation-to-zero deliver better compression-power trade-offs, with substantial differences from the static analysis.