Question

我有两个Customize Menus A和B.

我想比较它们并计算相等的元素数量：

例如：

vector<bool>

结果将等于2.

我可以使用_mm_cmpeq_epi8但它只比较16个元素（即我应该将0和1转换为char然后进行比较）。是否可以每次使用SSE（或SIMD指令）比较128个元素？

Answer 1

std::vector<bool>是std::vector类型bool的特化。虽然没有由C ++标准指定，但在大多数实现中std::vector<bool>都是空间有效的，因此它的每个元素都是一个位而不是bool。

std::vector<bool>的行为与其主要模板对应类似，但以下情况除外：

std::vector<bool>不一定连续存储其元素。
为了公开其元素（即各个位），std::vector<bool>使用代理类（即std::vector<bool>::reference）。 class std::vector<bool>::reference下标运算符（即std::vector<bool>）按值返回operator[]的对象。

因此，我不认为使用_mm_cmpeq_epi8类似函数是可移植的，因为std::vector<bool>的存储是实现定义的（即，不保证是连续的）。

另一种可移植的方法是使用常规STL设施，如下例所示：

std::vector<bool> A = {0,1,0,1};
std::vector<bool> B = {0,0,1,1};
std::vector<bool> C(A.size());
std::transform(A.begin(), A.end(), B.begin(), C.begin(), [](bool const &a, bool const &b) { return a == b;});
std::cout << std::count(C.begin(), C.end(), true) << std::endl;

Live Demo

Answer 2

如果您可以假设vector<bool>使用连续的字节大小的元素进行存储，或者您可以考虑使用类似vector<uint8_t>的内容，那么这个示例应该为您提供一个很好的起点：

static size_t count_equal(const vector<uint8_t> &vec1, const vector<uint8_t> &vec2)
{
    assert(vec1.size() == vec2.size());         // vectors must be same size

    const size_t n = vec1.size();
    const size_t max_block_size = 255 * 16;     // max block size before possible overflow

    __m128i vcount = _mm_setzero_si128();
    size_t i, count = 0;

    for (i = 0; i + 16 <= n; )                  // for each block
    {
        size_t m = std::min(n, i + max_block_size);

        for ( ; i + 16 <= m; i += 16)           // for each vector in block
        {
            __m128i v1 = _mm_loadu_si128((__m128i *)&vec1[i]);
            __m128i v2 = _mm_loadu_si128((__m128i *)&vec2[i]);
            __m128i vcmp = _mm_cmpeq_epi8(v1, v2);
            vcount = _mm_sub_epi8(vcount, vcmp);
        }
        vcount = _mm_sad_epu8(vcount, _mm_setzero_si128());
        count += _mm_extract_epi16(vcount, 0) + _mm_extract_epi16(vcount, 4);
        vcount = _mm_setzero_si128();           // update count from current block
    }
    vcount = _mm_sad_epu8(vcount, _mm_setzero_si128());
    count += _mm_extract_epi16(vcount, 0) + _mm_extract_epi16(vcount, 4);
    for ( ; i < n; ++i)                         // deal with any remaining partial vector
    {
        count += (vec1[i] == vec2[i]);
    }
    return count;
}

请注意，这是使用vector<uint8_t>。如果你真的必须使用vector<bool>并且可以保证元素将始终是连续的和字节大小的，那么你只需要将vector<bool>强制转换为const uint8_t *或类似的方式。

测试工具：

#include <cassert>
#include <cstdlib>
#include <ctime>
#include <iostream>
#include <vector>

#include <emmintrin.h>    // SSE2

using std::vector;

static size_t count_equal_ref(const vector<uint8_t> &vec1, const vector<uint8_t> &vec2)
{
    assert(vec1.size() == vec2.size());

    const size_t n = vec1.size();
    size_t i, count = 0;

    for (i = 0 ; i < n; ++i)
    {
        count += (vec1[i] == vec2[i]);
    }
    return count;
}

static size_t count_equal(const vector<uint8_t> &vec1, const vector<uint8_t> &vec2)
{
    assert(vec1.size() == vec2.size());         // vectors must be same size

    const size_t n = vec1.size();
    const size_t max_block_size = 255 * 16;     // max block size before possible overflow

    __m128i vcount = _mm_setzero_si128();
    size_t i, count = 0;

    for (i = 0; i + 16 <= n; )                  // for each block
    {
        size_t m = std::min(n, i + max_block_size);

        for ( ; i + 16 <= m; i += 16)           // for each vector in block
        {
            __m128i v1 = _mm_loadu_si128((__m128i *)&vec1[i]);
            __m128i v2 = _mm_loadu_si128((__m128i *)&vec2[i]);
            __m128i vcmp = _mm_cmpeq_epi8(v1, v2);
            vcount = _mm_sub_epi8(vcount, vcmp);
        }
        vcount = _mm_sad_epu8(vcount, _mm_setzero_si128());
        count += _mm_extract_epi16(vcount, 0) + _mm_extract_epi16(vcount, 4);
        vcount = _mm_setzero_si128();           // update count from current block
    }
    vcount = _mm_sad_epu8(vcount, _mm_setzero_si128());
    count += _mm_extract_epi16(vcount, 0) + _mm_extract_epi16(vcount, 4);
    for ( ; i < n; ++i)                         // deal with any remaining partial vector
    {
        count += (vec1[i] == vec2[i]);
    }
    return count;
}

int main(int argc, char * argv[])
{
    size_t n = 100;

    if (argc > 1)
    {
        n = atoi(argv[1]);
    }

    vector<uint8_t> vec1(n);
    vector<uint8_t> vec2(n);

    srand((unsigned int)time(NULL));

    for (size_t i = 0; i < n; ++i)
    {
        vec1[i] = rand() & 1;
        vec2[i] = rand() & 1;
    }

    size_t n_ref = count_equal_ref(vec1, vec2);
    size_t n_test = count_equal(vec1, vec2);

    if (n_ref == n_test)
    {
        std::cout << "PASS" << std::endl;
    }
    else
    {
        std::cout << "FAIL: n_ref = " << n_ref << ", n_test = " << n_test << std::endl;
    }

    return 0;
}

编译并运行：

$ g++ -Wall -msse3 -O3 test.cpp && ./a.out
PASS

将两个向量<bool>与SSE进行比较

2 个答案: