Comment by godelski

Definitely not true.

You aren't going to beat the compiler if you have to consider a wide range of inputs and outputs but that isn't a typical setting and you can actually beat them. Even in general settings this can be true because it's still a really hard problem for the compiler to infer things you might know. That's why C++ has all those compiler hints and why people optimize with gcc flags other than -O.

It's often easy to beat Blas in matrix multiplication if you know some conditions on that matrix. Because Blas will check to find the best algo first but might not know (you could call directly of course and there you likely won't win, but you're competing against a person not the compiler).

Never over estimate the compiler. The work the PL people do is unquestionably useful but they'll also be the first to say you can beat it.

You should always do what Knuth suggested (the often misunderstood "premature optimization" quote) and get the profile.