Question

我想通过AVX2指令对a[i] = a[i-1] +c进行矢量化。由于依赖性，它似乎无法矢量化。我已经矢量化了，想在这里分享答案，看看这个问题是否有更好的答案，或者我的解决方案是好的。

Answer 1

我已经实现了以下功能来进行矢量化，看起来还可以！加速比gcc -O3快2.5倍这是解决方案：

// vectorized
inline void vec(int a[LEN], int b, int c)
{
    // b=1 and c=2 in this case
    int i = 0;
    a[i++] = b;//0 --> a[0] = 1
    //step 1:
    //solving dependencies vectorization factor is 8
    a[i++] = a[0] + 1*c; //1  --> a[1] = 1 + 2  = 3
    a[i++] = a[0] + 2*c; //2  --> a[2] = 1 + 4  = 5
    a[i++] = a[0] + 3*c; //3  --> a[3] = 1 + 6  = 7
    a[i++] = a[0] + 4*c; //4  --> a[4] = 1 + 8  = 9
    a[i++] = a[0] + 5*c; //5  --> a[5] = 1 + 10 = 11
    a[i++] = a[0] + 6*c; //6  --> a[6] = 1 + 12 = 13
    a[i++] = a[0] + 7*c; //7  --> a[7] = 1 + 14 = 15
    // vectorization factor reached
    // 8 *c will work for all 
    //loading the results to an vector
    __m256i dep1, dep2; //  dep = { 1,   3,  5, 7,  9,  11, 13, 15 }
    __m256i coeff = _mm256_set1_epi32(8*c); //coeff = { 16, 16, 16, 16, 16, 16, 16, 16 }

    for(; i<LEN-1; i+=16){

        dep1 = _mm256_load_si256((__m256i *) &a[i-8]);
        dep1 = _mm256_add_epi32(dep1, coeff);
        _mm256_store_si256((__m256i *) &a[i], dep1);    

        dep2 = _mm256_load_si256((__m256i *) &a[i]);
        dep2 = _mm256_add_epi32(dep2, coeff);
        _mm256_store_si256((__m256i *) &a[i+8], dep2);  

    }
}

如何用AVX2矢量化[i] = a [i-1] + c

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