Fastest way to do horizontal SSE vector sum (or other reduction)

Question

Given a vector of three (or four) floats. What is the fastest way to sum them?

Is SSE (movaps, shuffle, add, movd) always faster than x87? Are the horizontal-add ins

Answer 1

You can do it in two HADDPS instructions in SSE3:

v = _mm_hadd_ps(v, v);
v = _mm_hadd_ps(v, v);

This puts the sum in all elements.