SIMD, SIMT, and Flynn's Taxonomy
Every processor design answers two questions: how many instruction streams does it execute, and how many data streams does each instruction touch. Those two answers are the whole of Flynn's taxonomy, and they matter here because "GPU" is not a single point on that map — a GPU's arithmetic units are driven by an execution model, SIMT, that is easy to mistake for ordinary SIMD vectorization and behaves differently in exactly the cases that matter for correctness and performance.