How to find the horizontal maximum in a 256-bit AVX vector

Question

I have a __m256d vector packed with four 64-bit floating-point values.
I need to find the horizontal maximum of the vector's elements and store the result in a double-precision scalar value;

My attempts all ended up using a lot of shuffling of the vector elements, making the code not very elegant nor efficient. Also, I found it impossible to stay only in the AVX domain. At some point I had to use SSE 128-bit instructions to extract the final 64-bit value. However, I would like to be proved wrong on this last statement.

So the ideal solution will:
1) only use only AVX instructions.
2) minimize the number of instructions. (I am hoping for no more than 3-4 instructions)

Having said that, any elegant/efficient solution will be accepted, even if it doesn't adhere to the above guidelines.

Thanks for any help.

-Luigi

Answer 1

I don't think you can do much better than 4 instructions: 2 shuffles and 2 comparisons.

__m256d x = ...; // input

__m128d y = _mm256_extractf128_pd(x, 1); // extract x[2], and x[3]
__m128d m1 = _mm_max_pd(x, y); // m1[0] = max(x[0], x[2]), m1[1] = max(x[1], x[3])
__m128d m2 = _mm_permute_pd(m1, 1); // set m2[0] = m1[1], m2[1] = m1[0]
__m128d m = _mm_max_pd(m1, m2); // both m[0] and m[1] contain the horizontal max(x[0], x[1], x[2], x[3])

Trivial modification to only work with 256-bit vectors:

__m256d x = ...; // input

__m256d y = _mm256_permute2f128_pd(x, x, 1); // permute 128-bit values
__m256d m1 = _mm256_max_pd(x, y); // m1[0] = max(x[0], x[2]), m1[1] = max(x[1], x[3]), etc.
__m256d m2 = _mm256_permute_pd(m1, 5); // set m2[0] = m1[1], m2[1] = m1[0], etc.
__m256d m = _mm256_max_pd(m1, m2); // all m[0] ... m[3] contain the horizontal max(x[0], x[1], x[2], x[3])

(untested)