Comment by oldmanhorton

Both ahead of time compilers and JIT compilers often perform autovectorization of tight loops. The problem is that lots of hot loops are not necessarily simple loops, and in particular a lot of source code is written in a way which uses sequential dependencies that can’t be modeled in SIMD code. Aside from undefined behavior in C/C++, most compilers will fail to autovectorize because doing so would very slightly change the behavior of your code in a very hard to understand way.