It's a tricky situation. CPUs reduce their clock speed when you use vector instructions so you can't enable everything by default. You have to use LTO+PGO or th

It's a tricky situation. CPUs reduce their clock speed when you use vector instructions so you can't enable everything by default. You have to use LTO+PGO or the compiler won't know if your code needs to be vectorised at the expense of slowing down everything else. That's one reason why O3 is sometimes slower than O2 or Os