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Autovectorization is okay but not that good. Its advantage is just that it can transform any part of the program, when your limited dev time won't touch it all. But it can't focus on the hot loops or know about cache efficiency etc. even if you use PGO. And it's sensitive and unstable wrt code changes which is never good.

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Yeah, and when autovectorization fails it can be really hard to tell why. The diagnostic messages aren't the best (or, at least, they weren't when I was doing this ~4 years ago)

EG: ICC has a AVX version of atan2(), which you can use through intrinsics[0]. But it refused to vectorize even the simplest loop with the function call in it. Even with every possible fast-math flag or every #pragma(please vectorize), it wouldn't do it. I assume due to reduced precision? Nothing I could find online had the answer (again, 4 years ago)

[0] https://software.intel.com/en-us/node/524364

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