• Codingame / github - avx v8f simple and short
• std-simd - portable, zero-overhead C++ types for explicitly data-parallel programming
• Vc library - in maintenance mode - used by Krita
• Highway- Google C++ library that provides portable SIMD/vector intrinsics.

see also:

• std::valarray - Some C++ standard library implementations use expression templates to implement efficient operations on std::valarray… Only rarely are valarrays optimized any further, as in e.g. Intel Integrated Performance Primitives.