I was hand-rolling NEON SIMD 15 years ago, and in many cases the compiler (clang/llvm) simply out optimized me. I kept the attempts that were better than what the compiler could already do. That was ARM NEON, quite new at the time, not SSE, and again that was 15 years ago that the compiler could already beat me much of the time. I hate the naive cult coder adage concerning premature optimization, but this might be a situation where you peruse your compiler output before you start writing code in a manner the compiler can for you. In Clang, auto-vectorization is enabled by default at optimization levels -O2 and -O3.