What does C5002 1204 auto-vectorizer reason code mean?

Question

I'm a student and I'm learning for techniques of vectorization. I'm trying to make compiler vectorize a function which multiplies two matricies (each matrix consists of elements that are matricies of equal size). The code looks as follows:

#define f_dim1 2000
#define f_dim2 240
#define s_dim1 240
#define s_dim2 2000

#define i_dim1 4
#define i_dim2 4

void automaticallyBuilt(float* firstMatrix, float* secondMatrix, float* result) {
    for (int i = 0; i < f_dim2; i++) { // rows in frist matrix
        for (int j = 0; j < s_dim1; j++) { // columns in second matrix
            for (int o = 0; o < f_dim1; o++) { // row element of first matrix = column element of second
                for (int k = 0; k < i_dim2; k++) { // rows in inner matrix
                    for (int l = 0; l < i_dim1; l++) { // columns in inner matrix
                        for (int h = 0; h < i_dim2; h++) { // row element of inner = column element of inner
                            *(result + i*s_dim1*i_dim2*i_dim1 + j*i_dim2*i_dim1 + k*i_dim1 + l) +=
                                *(firstMatrix + i*f_dim1*i_dim2*i_dim1 + o*i_dim2*i_dim1 + k*i_dim1 + h) *
                                *(secondMatrix + o*s_dim1*i_dim2*i_dim1 + j*i_dim2*i_dim1 + h*i_dim1 + l);
                            // smth like result[i][j][k][l] += firstMatrix[i][o][k][h] * secondMatrix[o][j][h][l];
                        }
                    }
                }
            }
        }
    }
}

In attempt to make compiler vectorize it I modified this code as follows:

#define f_dim1 2000
#define f_dim2 240
#define s_dim1 240
#define s_dim2 2000

#define i_dim1 4
#define i_dim2 4

void automaticallyVectorized(float* firstMatrix, float*  secondMatrix, float* result) {
    for (int i = 0; i < f_dim2; i++) { // rows in frist matrix
        for (int o = 0; o < f_dim1; o++) { // row element of first matrix = column element of second
            for (int j = 0; j < s_dim1; j++) { // columns in second matrix
                for (int k = 0; k < i_dim2; k++) { // rows in inner matrix
                    for (int h = 0; h < i_dim2; h++) { // row element of inner = column element of inner
                        float firstMatrixInnerRowElement = *(firstMatrix + i*f_dim1*i_dim2*i_dim1 + o* i_dim2*i_dim1 + k*i_dim1 + h);
                        float* resultInnerRow = result + i*s_dim1*i_dim2*i_dim1 + j*i_dim2*i_dim1 + k*i_dim1;
                        float* secondMatrixInnerColumnElementRow = secondMatrix + o*s_dim1*i_dim2*i_dim1 + j*i_dim2*i_dim1 + h*i_dim1;
                        for (int l = 0; l < i_dim1; l++) { // columns in inner matrix
                            resultInnerRow[l] += firstMatrixInnerRowElement * secondMatrixInnerColumnElementRow[l];
                        }
                    }
                }
            }
        }
    }
}

During build compiler emits the following message:

code.cpp(161) : info C5002: Loop not vectorized due to reason: "1204"

Code 1204 is not mentioned here. Furthermore, I haven't found any information about it with Google. I tried to use __restrict modifier but had no luck.

I use Visual Studio 2019, but I tried to build it with VS 2017 with the same results.

Can anybody explain what this reason code means? I don't believe that nobody faced this issue before.

Answer 1

This reason code means that loops are nested too deeply

As Eljay suggested, I posted an issue on github using the link at the bottom of the documentation page. Here is the link to the answer.