Using -O3 is a very sensible way to use vector instructions, in practice. It’s not universally good. It’s just that if you take naïve C code that does bulk ope

Using -O3 is a very sensible way to use vector instructions, in practice. It’s not universally good. It’s just that if you take naïve C code that does bulk operations on arrays, and compile it at -O3, you might get a very nice improvement from the auto-vectorizer. With some additional restrict keyword annotations, you can sometimes improve the performance significantly. Then there are non-standard tricks you can use, like __builtin_assume_aligned(). This is something of an arcane art. Sometimes it just doesn’t work at all, sometimes it’s not obvious how to write your code so that the autovectorizer can work well, and getting the autovectorizer to work requires enabling various code transformation passes that will often make code worse. However, it’s still worth using, because when your function does autovectorize well, it saves you a ton of work trying to vectorize it manually.