You're right, only two loops. It is still messy, though. And "it has SIMD" is not really a sufficient explanation for the messiness. I think of -O3 as a kind o

You're right, only two loops. It is still messy, though. And "it has SIMD" is not really a sufficient explanation for the messiness. I think of -O3 as a kind of "throw everything at the wall and see what sticks". You're pressing a button that makes your code big, messy, and fast (although not always big, not always messy, and not always fast). You can spend as much time as you like explaining the reasoning behind these code transformations, but in the end, there are very good reasons why people don't use -O3 as the default optimization level.