It's not an -O2 thing. Rather it's a -march thing. -O2 in gcc has vectorization flags set which will use avx if the target CPU supports it. It is less aggres

It's not an -O2 thing. Rather it's a -march thing. -O2 in gcc has vectorization flags set which will use avx if the target CPU supports it. It is less aggressive on vectorization than -O3.