c - How can i optimize my AVX implementation of dot product?

Question

I`ve tried to implement dot product of this two arrays using AVX https://stackoverflow.com/a/10459028. But my code is very slow.

A and xb are arrays of doubles, n is even number. Can you help me?

const int mask = 0x31;
int sum =0;

for (int i = 0; i < n; i++)
{
    int ind = i;
    if (i + 8 > n) // padding
    {
        sum += A[ind] * xb[i].x;
        i++;
        ind = n * j + i;
        sum += A[ind] * xb[i].x;
        continue;
    }

    __declspec(align(32)) double ar[4] = { xb[i].x, xb[i + 1].x, xb[i + 2].x, xb[i + 3].x };
    __m256d x = _mm256_loadu_pd(&A[ind]);
    __m256d y = _mm256_load_pd(ar);
    i+=4; ind = n * j + i;
    __declspec(align(32)) double arr[4] = { xb[i].x, xb[i + 1].x, xb[i + 2].x, xb[i + 3].x };
    __m256d z = _mm256_loadu_pd(&A[ind]);
    __m256d w = _mm256_load_pd(arr);

    __m256d xy = _mm256_mul_pd(x, y);
    __m256d zw = _mm256_mul_pd(z, w);
    __m256d temp = _mm256_hadd_pd(xy, zw);
    __m128d hi128 = _mm256_extractf128_pd(temp, 1);
    __m128d low128 = _mm256_extractf128_pd(temp, 0);
    //__m128d dotproduct = _mm_add_pd((__m128d)temp, hi128);
    __m128d dotproduct = _mm_add_pd(low128, hi128);

    sum += dotproduct.m128d_f64[0]+dotproduct.m128d_f64[1];
    i += 3;
}

Answer 1

There are two big inefficiencies in your loop that are immediately apparent:

(1) these two chunks of scalar code:

__declspec(align(32)) double ar[4] = { xb[i].x, xb[i + 1].x, xb[i + 2].x, xb[i + 3].x };
...
__m256d y = _mm256_load_pd(ar);

and

__declspec(align(32)) double arr[4] = { xb[i].x, xb[i + 1].x, xb[i + 2].x, xb[i + 3].x };
...
__m256d w = _mm256_load_pd(arr);

should be implemented using SIMD loads and shuffles (or at the very least use _mm256_set_pd and give the compiler a chance to do a half-reasonable job of generating code for a gathered load).

(2) the horizontal summation at the end of the loop:

for (int i = 0; i < n; i++)
{
    ...
    __m256d xy = _mm256_mul_pd(x, y);
    __m256d zw = _mm256_mul_pd(z, w);
    __m256d temp = _mm256_hadd_pd(xy, zw);
    __m128d hi128 = _mm256_extractf128_pd(temp, 1);
    __m128d low128 = _mm256_extractf128_pd(temp, 0);
    //__m128d dotproduct = _mm_add_pd((__m128d)temp, hi128);
    __m128d dotproduct = _mm_add_pd(low128, hi128);

    sum += dotproduct.m128d_f64[0]+dotproduct.m128d_f64[1];
    i += 3;
}

should be moved out of the loop:

__m256d xy = _mm256_setzero_pd();
__m256d zw = _mm256_setzero_pd();
...
for (int i = 0; i < n; i++)
{
    ...
    xy = _mm256_add_pd(xy, _mm256_mul_pd(x, y));
    zw = _mm256_add_pd(zw, _mm256_mul_pd(z, w));
    i += 3;
}
__m256d temp = _mm256_hadd_pd(xy, zw);
__m128d hi128 = _mm256_extractf128_pd(temp, 1);
__m128d low128 = _mm256_extractf128_pd(temp, 0);
//__m128d dotproduct = _mm_add_pd((__m128d)temp, hi128);
__m128d dotproduct = _mm_add_pd(low128, hi128);

sum += dotproduct.m128d_f64[0]+dotproduct.m128d_f64[1];