I have a packed vector of four 64-bit floating-point values.
I would like to get the sum of the vector's elements.
With SSE (and using 32-bit floats) I could just do the following:
v_sum = _mm_hadd_ps(v_sum, v_sum);
v_sum = _mm_hadd_ps(v_sum, v_sum);
Unfortunately, even though AVX features a _mm256_hadd_pd instruction, it differs in the result from the SSE version. I believe this is due to the fact that most AVX instructions work as SSE instructions for each low and high 128-bits separately, without ever crossing the 128-bit boundary.
Ideally, the solution I am looking for should follow these guidelines:
1) only use AVX/AVX2 instructions. (no SSE)
2) do it in no more than 2-3 instructions.
However, any efficient/elegant way to do it (even without following the above guidelines) is always well accepted.
Thanks a lot for any help.
-Luigi Castelli