Almost all modern CPU have a vector processing unit, making it possible to write faster code for a large category of problems, at the cost of portability - there a re many different instruction sets in the wild! The xsimd library makes it possible to write portable C++ code that targets different architectures and sub-architectures. The specialization choice can be made at compile-time or at runtime, using a provided dispatching mechanism. Intel, ARM, RiscV and Webassembly are supported, and the library has already been adopted by Xtensor, Pythran, Apache Arrow and Firefox.