You have a faster CPU. "SSE sdot" and "SSE+tiling" are both faster on your machine. However, "Eigen" is slower. This suggests gpderetta might be right – MSVC is not as good as gcc to optimize Eigen, or conversely, Eigen has not been optimized for MSVC. Nonetheless, MSVC did fully vectorize the inner loop of non-SSE sdot, better than both gcc and clang (EDIT: of the versions I was using; it would be good to try the latest gcc/clang). It seems that explicit SSE vectorization is the most stable. Other implementations of "sdot" depend too much on the behavior of compilers.

Anyway, thanks a lot for this experiment. I rarely use MSVC these days. It is good to know where it stands.