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Question

我需要编写矩阵向量和矩阵乘法函数，但我无法绕过SSE命令。

矩阵和向量的维数总是4的倍数。

我设法编写了这样的矢量矢量乘法函数：

void vector_multiplication_SSE(float* m, float* n, float* result, unsigned const int size)
{
    int i;

    __declspec(align(16))__m128 *p_m = (__m128*)m;
    __declspec(align(16))__m128 *p_n = (__m128*)n;
    __declspec(align(16))__m128 *p_result = (__m128*)result;

    for (i = 0; i < size / 4; ++i)
        p_result[i] = _mm_mul_ps(p_m[i], p_n[i]);

    // print the result
    for (int i = 0; i < size; ++i)
    {
        if (i % 4 == 0) cout << endl;
        cout << result[i] << '\t';
    }
}

现在我正在尝试实现矩阵向量乘法。

这是我到目前为止所拥有的：

void multiply_matrix_by_vector_SSE(float* m, float* v, float* result, unsigned const int vector_dims)
{
    int i, j;

    __declspec(align(16))__m128 *p_m = (__m128*)m;
    __declspec(align(16))__m128 *p_v = (__m128*)v;
    __declspec(align(16))__m128 *p_result = (__m128*)result;

    for (i = 0; i < vector_dims; i += 4)
    {
        __m128 tmp = _mm_load_ps(&result[i]);
        __m128 p_m_tmp = _mm_load_ps(&m[i]);

        tmp = _mm_add_ps(tmp, _mm_mul_ps(tmp, p_m_tmp));
        _mm_store_ps(&result[i], tmp);

        // another for loop here? 
    }

    // print the result
    for (int i = 0; i < vector_dims; ++i)
    {
        if (i % 4 == 0) cout << endl;
        cout << result[i] << '\t';
    }
}

此功能看起来完全错误。我的意思是它不仅没有正常工作，而且似乎我正朝着错误的方向前进。

有人可以帮我实现矢量矩阵和矩阵乘法吗？我非常感谢一些示例代码和非常详细的解释

更新

这是我的第二次尝试：

它以Access reading violation例外失败但仍感觉更接近

void multiply_matrix_by_vector_SSE(float* m, float* v, float* result, unsigned const int vector_dims)
{
    int i, j;

    __declspec(align(16))__m128 *p_m = (__m128*)m;
    __declspec(align(16))__m128 *p_v = (__m128*)v;
    __declspec(align(16))__m128 *p_result = (__m128*)result;

    for (i = 0; i < vector_dims; ++i)
    {
        p_result[i] = _mm_mul_ps(_mm_load_ps(&m[i]), _mm_load_ps1(&v[i]));
    }

    // print the result
    for (int i = 0; i < vector_dims; ++i)
    {
        if (i % 4 == 0) cout << endl;
        cout << result[i] << '\t';
    }
}

更新2

void multiply_matrix_by_vector_SSE(float* m, float* v, float* result, unsigned const int vector_dims)
{
    int i, j;
    __declspec(align(16))__m128 *p_m = (__m128*)m;
    __declspec(align(16))__m128 *p_v = (__m128*)v;
    __declspec(align(16))__m128 *p_result = (__m128*)result;

    for (i = 0; i < vector_dims; ++i)
    {
        for (j = 0; j < vector_dims * vector_dims / 4; ++j)
        {
            p_result[i] = _mm_mul_ps(p_v[i], p_m[j]);
        }
    }

    for (int i = 0; i < vector_dims; ++i)
    {
        if (i % 4 == 0) cout << endl;
        cout << result[i] << '\t';
    }
    cout << endl;
}

Answer 1

没有任何技巧或任何东西，矩阵向量乘法只是向量和矩阵行之间的一串点积。您的代码实际上没有这种结构。实际上把它写成点积（未测试）：

for (int row = 0; row < nrows; ++row) {
    __m128 acc = _mm_setzero_ps();
    // I'm just going to assume the number of columns is a multiple of 4
    for (int col = 0; col < ncols; col += 4) {
        __m128 vec = _mm_load_ps(&v[col]);
        // don't forget it's a matrix, do 2d addressing
        __m128 mat = _mm_load_ps(&m[col + ncols * row]);
        acc = _mm_add_ps(acc, _mm_mul_ps(mat, vec));
    }
    // now we have 4 floats in acc and they have to be summed
    // can use two horizontal adds for this, they kind of suck but this
    // isn't the inner loop anyway.
    acc = _mm_hadd_ps(acc, acc);
    acc = _mm_hadd_ps(acc, acc);
    // store result, which is a single float
    _mm_store_ss(&result[row], acc);
}

有一些明显的技巧，例如一次处理多个行，重用向量中的负载，以及创建多个独立的依赖链，以便您可以更好地利用吞吐量。另外一个非常简单的技巧是使用FMA作为mul / add组合，但支持并不是那么普遍。

你可以从中构建矩阵 - 矩阵乘法（如果改变结果的位置），但这不是最优的。

使用SSE的矩阵向量和矩阵 - 矩阵乘法

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