Comment by gnufx

Note that the code GCC generates for reductions (dot product, anyhow) will not be scalar with an option like -Ofast. That enables loop vectorization but also use of non-standard-conforming SIMD for the non-associative operations. It likely does as well as simple use of intrinsics. Level 1 operations are only worth worrying about much with small arrays, as they're ultimately memory-bound.