[gblargg]

My compiler knows SIMD. However, knowing the limitations of SIMD might help avoid a calculation that can't be optimized to use it.

[Jtarii]

Your compiler can vectorise trivial things, once your code gets complicated it will no longer vectorise.

[tmtvl]

Once the code is complicated you don't need to make it more complicated lest it becomes unmaintainable.

[spider-mario]

It really doesn’t take much complexity at all for autovectorisation to fail.