Manual loop unrolling VS. using #pragma unroll

I suspect in the case of the (automatic) unrolled loop those lines become cascaded so that the evaluation of one line is added to the result of the previous line. When you manually unroll it you probably ended up with more of a binary tree structure to how all the subterms were added together which might account for the smaller footprint. As for the throughput speedup I'm not sure why that is without seeing the code surrounding it but it's possible the pipeline depth is shorter in the manually unrolled case and attributing to that.