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davydden1

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12-04-2018
09:10 PM

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Question about ldb in mkl_?omatcopy for transpose of column-major matrices

Dear all,

I need to do out-of-place transpose of the matrix (actually I have plenty of them if various sizes/shapes). I have a question about ldb parameter in mkl_?omatcopy. Documentation says that if a matrix is in column major format and I am doing transpose, then ldb must be at least equal to number of columns in B. Is it indeed the case or it’s just a typo?

My matrices are stored with minimum leading dimension — number of rows for column major format. So that means I am out of luck using this function if number of columns in B is more than number of rows? That is unfortunate as I would expect this function to handle such cases internally in the best possible way.

Denis.

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davydden1

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12-05-2018
05:46 AM

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On the related note, I have issues using this function on macOS for anything but square matrices (it simply does not work). Given that I store everything in column major format and the leading dimension is number of rows, I can not see where I could mis-use this function, see below, compiled with:

clang++ -std=c++11 -m64 -I/opt/intel/mkl/include example.cc -o example -L/opt/intel/mkl/lib -Wl,-rpath,/opt/intel/mkl/lib -lmkl_intel_lp64 -lmkl_sequential -lmkl_core -lpthread -lm -ldl

// compile with: // clang++ -std=c++11 -m64 -I/opt/intel/mkl/include example.cc -o example -L/opt/intel/mkl/lib -Wl,-rpath,/opt/intel/mkl/lib -lmkl_intel_lp64 -lmkl_sequential -lmkl_core -lpthread -lm -ldl #include <iostream> #include <vector> #include <mkl.h> void print(const std::vector<double> &A, const unsigned int m, const unsigned int n) { for (unsigned int i = 0; i < m; ++i) { for (unsigned int j = 0; j < n; ++j) std::cout << A[i+j*m] << " "; std::cout << std::endl; } } bool compare(const std::vector<double> & A, const std::vector<double> &B, const unsigned int Am, const unsigned int An) { bool ret = true; for (unsigned int i = 0; i < Am; ++i) for (unsigned int j = 0; j < An; ++j) ret &= (A[i+j*Am] == B[j+i*An]); return ret; } void test(const unsigned int m, const unsigned int n) { std::cout << "Test " << m << "x" << n << std::endl; const auto &Am = n; const auto &An = m; const auto &Bm = m; const auto &Bn = n; std::vector<double> A(Am*An,0.); std::vector<double> B(Bm*Bn,0.); for (unsigned int i = 0; i < Am*An; ++i) A= 1+i; mkl_domatcopy('C', 'T', Bm, Bn, 1., A.data(), Am, B.data(), Bm); std::cout << "Input:" << std::endl; print(A,Am,An); std::cout << "Output:" << std::endl; print(B,Bm,Bn); std::cout << "Correct: " << compare(A,B,Am,An) << std::endl<< std::endl; } int main(int argc, char **argv) { test(11,27); test(15,4); test(10,10); }

Gennady_F_Intel

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12-05-2018
08:29 AM

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We would recommend you take a look at the domatcopy.c example (mklroot\examples\transc\source\ dir)

davydden1

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12-05-2018
09:26 AM

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Hi Gennady,

Thanks for the prompt reply. I quickly translated your example to C++ and noticed the following. The mkl_domatcopy is being called with rows=2 and cols=4 (3rd and 4th argument). However, the matrix B (which is clear both from in-code comments and its dst_stride=2) is 4x2. That is, it has 4 rows and 2 columns and is stored in row-major format (stride=2).

However the MKL documentation states:

rows The number of rows in matrix B (the destination matrix).

cols The number of columns in matrix B (the destination matrix).

To me this indicates that MKL's documentation is confusing and not consistent with what is actually meant by rows/cols. Probably, what is actually meant is the number of rows/cols which define a submatrix of A, on which the operation will be applied.

As another proof, If I swap rows/cols in my code above, everything works for all cases!

I would recommend you to reword the documentation to avoid confusion for other users.

Regards, Denis.

Ye_C_1

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02-11-2019
04:42 AM

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I have the same question.

I think the manual is wrong, the rows is the row number of destination matrix B without operation.

But I am not sure.

davydden wrote:Hi Gennady,

Thanks for the prompt reply. I quickly translated your example to C++ and noticed the following. The mkl_domatcopy is being called with rows=2 and cols=4 (3rd and 4th argument). However, the matrix B (which is clear both from in-code comments and its dst_stride=2) is 4x2. That is, it has 4 rows and 2 columns and is stored in row-major format (stride=2).

However the MKL documentation states:

rows The number of rows in matrix B (the destination matrix).

cols The number of columns in matrix B (the destination matrix).

To me this indicates that MKL's documentation is confusing and not consistent with what is actually meant by rows/cols. Probably, what is actually meant is the number of rows/cols which define a submatrix of A, on which the operation will be applied.

As another proof, If I swap rows/cols in my code above, everything works for all cases!

I would recommend you to reword the documentation to avoid confusion for other users.

Regards, Denis.

Pamela_H_Intel

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10-08-2019
09:58 AM

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Folks,

It looks like the documentation is incorrect. I will verify with the MKL developers and get back to this thread..

Pamela

Pamela_H_Intel

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10-25-2019
04:30 PM

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Here is the corrected info. We will update the official documentation. Thank you for pointing out this issue!

According to our examples the description of parameters should be as follows:

rows - the number of rows in matrix A

cols - the number of columns in matrix A

alpha - this parameter scales the input matrix by alpha.

a - array of size at least lda*rows in case of Row-major ordering (ordering = 'R'). And lda*cols in case of Column-major ordering (ordering = 'C')

lda - If ordering = 'R' or 'r', lda represents the number of elements in array a between adjacent rows of matrix A; lda must be at least equal to cols.

If ordering = 'C' or 'c', lda represents the number of elements in array a between adjacent columns of matrix A; lda must be at least equal to rows.

b - If trans == 'R' or 'N' then it is array of size at least ldb*rows in case of Row-major ordering (ordering = 'R'). And ldb*cols in case of Column-major ordering (ordering = 'C').

If trans == 'T' or 'C' then it is array of size at least ldb*cols in case of Row-major ordering (ordering = 'R'). And ldb*rows in case of Column-major ordering (ordering = 'C').

ldb - If ordering = 'R' or 'r', lda represents the number of elements in array a between adjacent rows of matrix B; ldb must be at least equal to cols if trans=='R' or 'N'. And rows if trans=='C' or 'T'

If ordering = 'C' or 'c', lda represents the number of elements in array a between adjacent columns of matrix B; ldb must be at least equal to rows if trans=='R' or 'N'. And cols if trans=='C' or 'T'

The same changes should be applied to ?omatcopy2 routine + the following:

stridea - If ordering = 'R' or 'r', stridea represents the number of elements in array 'a' between adjacent columns of matrix A. stridea must be at least 1.

If ordering = 'C' or 'c', stridea represents the number of elements in array 'a' between adjacent rows of matrix A. stridea must be at least 1.

strideb - If ordering = 'R' or 'r', strideb represents the number of elements in array 'b' between adjacent columns of matrix B. strideb must be at least 1.

If ordering = 'C' or 'c', strideb represents the number of elements in array 'b' between adjacent rows of matrix B. strideb must be at least 1.

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