Intel cpu上的SIMD前缀总和

Question

我需要实现一个前缀和算法，并且需要尽可能快。例如：[3, 1,  7,  0,  4,  1,  6,  3]应该给：[3, 4, 11, 11, 15, 16, 22, 25]有没有办法使用SSE SIMD CPU指令来执行此操作？我的第一个想法是递归地将每一对并行求和，直到所有总和都如下计算！//in parallel do for (int i = 0; i < z.length; i++) {    z[i] = x[i << 1] + x[(i << 1) + 1];}为了使算法更清楚一点，z它不是最终的输出，而是用来计算输出的。int[] w = computePrefixSum(z);for (int i = 1; i < ouput.length; i++) {    ouput[i] = (i % 2 == 0) ? (x[i] + ouput[i - 1]) :  w[(i - 1) >> 1];}

Answer 1

您可以利用一些较小的并行机制来获得较大的寄存器长度和较小的和。例如，将16个1字节的值相加（恰好适合一个sse寄存器），仅需要对数2 16的加法和相等的移位数。

数量不多，但比15个依赖的附加项和附加的内存访问快。

__m128i x = _mm_set_epi8(3,1,7,0,4,1,6,3,3,1,7,0,4,1,6,3);

x = _mm_add_epi8(x, _mm_srli_si128(x, 1));

x = _mm_add_epi8(x, _mm_srli_si128(x, 2));

x = _mm_add_epi8(x, _mm_srli_si128(x, 4));

x = _mm_add_epi8(x, _mm_srli_si128(x, 8));

// x == 3, 4, 11, 11, 15, 16, 22, 25, 28, 29, 36, 36, 40, 41, 47, 50

如果总和更长，则可以通过利用指令级并行性并利用指令重新排序来隐藏依赖项。

编辑：类似

__m128i x0 = _mm_set_epi8(3,1,7,0,4,1,6,3,3,1,7,0,4,1,6,3);

__m128i x1 = _mm_set_epi8(3,1,7,0,4,1,6,3,3,1,7,0,4,1,6,3);

__m128i x2 = _mm_set_epi8(3,1,7,0,4,1,6,3,3,1,7,0,4,1,6,3);

__m128i x3 = _mm_set_epi8(3,1,7,0,4,1,6,3,3,1,7,0,4,1,6,3);

__m128i mask = _mm_set_epi8(0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0);

x0 = _mm_add_epi8(x0, _mm_srli_si128(x0, 1));

x1 = _mm_add_epi8(x1, _mm_srli_si128(x1, 1));

x2 = _mm_add_epi8(x2, _mm_srli_si128(x2, 1));

x3 = _mm_add_epi8(x3, _mm_srli_si128(x3, 1));

x0 = _mm_add_epi8(x0, _mm_srli_si128(x0, 2));

x1 = _mm_add_epi8(x1, _mm_srli_si128(x1, 2));

x2 = _mm_add_epi8(x2, _mm_srli_si128(x2, 2));

x3 = _mm_add_epi8(x3, _mm_srli_si128(x3, 2));

x0 = _mm_add_epi8(x0, _mm_srli_si128(x0, 4));

x1 = _mm_add_epi8(x1, _mm_srli_si128(x1, 4));

x2 = _mm_add_epi8(x2, _mm_srli_si128(x2, 4));

x3 = _mm_add_epi8(x3, _mm_srli_si128(x3, 4));

x0 = _mm_add_epi8(x0, _mm_srli_si128(x0, 8));

x1 = _mm_add_epi8(x1, _mm_srli_si128(x1, 8));

x2 = _mm_add_epi8(x2, _mm_srli_si128(x2, 8));

x3 = _mm_add_epi8(x3, _mm_srli_si128(x3, 8));

x1 = _mm_add_epi8(_mm_shuffle_epi8(x0, mask), x1);

x2 = _mm_add_epi8(_mm_shuffle_epi8(x1, mask), x2);

x3 = _mm_add_epi8(_mm_shuffle_epi8(x2, mask), x3);