Towards a Domain-Extensible Compiler: Optimizing an Image Processing Pipeline on Mobile CPUs

Halide and many similar projects have demonstrated the great potential of domain specific optimizing compilers. They enable programs to be expressed at a convenient high-level, while generating high-performance code for parallel architectures. As domains of interest expand towards deep learning, probabilistic programming and beyond, it becomes increasingly clear that it is unsustainable to redesign domain specific compilers for each new domain. In addition, the rapid growth of hardware architectures to optimize for poses great challenges for designing these compilers. In this paper, we show how to extend a unifying domain-extensible compiler with domain-specific as well as hardware-specific optimizations. The compiler operates on generic patterns that have proven flexible enough to express a wide range of computations. Optimizations are not hard-coded into the compiler but are expressed as user-defined rewrite rules that are composed into strategies controlling the optimization process. Crucially, both computational patterns and optimization strategies are extensible without modifying the core compiler implementation. We demonstrate that this domain-extensible compiler design is capable of expressing image processing pipelines and well-known image processing optimizations. Our results on four mobile ARM multi-core CPUs, often used for image processing tasks, show that the code generated for the Harris operator outperforms the image processing library OpenCV by up to 16x and achieves performance close to - or even up to 1.4 x better than - the state-of-the-art image processing compiler Halide.