A Fast and Concise Parallel Implementation of the 8x8 2D IDCT using Halide

The Inverse Discrete Cosine Transform (IDCT) is commonly used for image and video decoding. Due to the ubiquitous nature of this application area, very efficient implementations of the IDCT transform are of great importance and have lead to the development of highly optimized libraries. The popular libjpeg-turbo library contains 1000s of lines of handwritten assembly code utilizing SIMD instruction sets for a variety of architectures. We present an alternative approach, implementing the 8x8 2D IDCT written in the image processing language Halide - a high-level, functional language that allows for concise, portable, parallel and very efficient code. We show how less than 100 lines of Halide can replace over 1000 lines of code for each architecture in the libjpeg-turbo library to perform JPEG decoding. The Halide implementation is compared for ARMv8 and x86-64 SIMD extensions and shows a 5-25 percent performance improvement over the SIMD code in libjpeg-turbo while also being much easier to maintain and port to new architectures.