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Computer graphics are powered by graphics APIs (e.g., OpenGL, Direct3D) and their associated shader compilers, which render high-quality images by compiling and optimizing user-written high-level shader programs into GPU machine code. Graphics rendering is extensively used in production scenarios like virtual reality (VR), gaming, autonomous driving, and robotics. Despite the development by industrial manufacturers such as Intel, Nvidia, and AMD, shader compilers — like traditional software — may produce ill-rendered outputs. In turn, these errors may result in negative results, from poor user experience in entertainment to accidents in driving assistance systems.

This paper introduces FSHADER, a metamorphic testing (MT) framework designed specifically for shader compilers to uncover erroneous compilations and optimizations. FSHADER tests shader compilers by mutating input shader programs via four carefully-designed metamorphic relations (MRs). In particular, FSHADER fuses two shader programs via an MR and checks the visual consistency between the image rendered from the fused shader program with the output of fusing individually rendered images. Our study of 13 shader compilers covers five mainstream GPU vendors, including Intel, AMD, Nvidia, ARM, and Apple. We successfully uncover over 16K error-triggering inputs that generate incorrect rendering outputs. We manually locate and characterize buggy optimization places, and developers have confirmed representative bugs.