Garmaine Staff asked 1 year ago
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I am trying to use Vector<T> from System.Numerics in C#.

I have been given a C++ approach to the problem I am trying to solve but I am not very good at C++ and have problem to understand the logic and wonder if anyone have the knowledge of How to convert this code to C#.

My original post that describes this problem and from where I got this code is: Micro Optimization of a 4-bucket histogram of a large array or list

Perhaps without going into to much details of what the code does. Maybe the code will be self explanatory where I try to use Vector to histogram values from a List<int>/std::vector<int> as fast as possible where in this example simultaneously processes 8 indexes at the same time using SIMD.

#include <immintrin.h>
#include <vector>

static inline
__m128i hsum_xpose(const __m256i counts[4])  // UNTESTED and not 100% sure this is right
{
    // AMD Zen1 might benefit from reducing inputs to 128 first
    // but on Zen2 and Intel 256-bit in-lane shuffles are no cheaper than 128
    __m256i sum01 = _mm256_hadd_epi32(counts[0], counts[1]);
    __m256i sum23 = _mm256_hadd_epi32(counts[2], counts[3]);
    __m256i sum0123 = _mm256_hadd_epi32(sum01, sum23);      // in-lane hsums

    __m128i sum_high = _mm256_extracti128_si256(sum0123, 1);  // add high and low lanes
    // add upper 128 bits of sum to its lower 128 bits
    __m128i result = _mm_add_epi32(sum_high, _mm256_castsi256_si128(sum0123));
    return result;
}

void count_elements_avx2(const std::vector<int> &input,  unsigned output_counts[4])
{
    __m256i  counts[4] = { _mm256_setzero_si256() };  // 4 vectors of zeroed counters
                  // each vector holds counts for one bucket, to be hsummed at the end

    size_t size = input.size();
    for(size_t i = 0 ; i<size ; i+=8) {  // 8x 32-bit elements per vector
        __m256i v = _mm256_loadu_si256((const __m256i*)&input[i]);  // unaligned load of 8 ints
        for (int val = 0 ; val < 3; val++) {
           // C++ compilers will unroll this with 3 vector constants and no memory access
            __m256i match = _mm256_cmpeq_epi32(v, _mm256_set1_epi32(val));  // 0 or all-ones aka -1
            counts[val] = _mm256_sub_epi32(counts[val], match);   // x -= -1 or 0 conditional increment
        }
    }


    // transpose and sum 4 vectors of 8 elements down to 1 vector of 4 elements
    __m128i summed_counts = hsum_xpose(counts);   // helper function defined in Godbolt link
    _mm_storeu_si128((__m128i*)output_counts, summed_counts);

    output_counts[3] = size - output_counts[0]
                       - output_counts[1] - output_counts[2];

    // TODO: handle the last size%8 input elements; scalar would be easy
}