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*mkl_dcsrmm* multiplies a CSR sparse matrix by a dense matrix, storing the result in a dense matrix.

From what I can tell, this function expects the B and C dense matrices to be in row-major order, as opposed to the column-major order used in most other BLAS routines. I've attached a test case which demonstrates that, given the same B and C inputs in the same format, *mkl_dcsrmm* produces completely different results than *dgemm. dgemm* calculates C = alpha*A*B + beta*C (as expected), but *mkl_dcsrmm* appears to calculate C' = alpha*A*B' + beta*C' instead.

This seems to make it very difficult to mix sparse functions with normal BLAS function. Must I transpose my matrices before and after using the sparse functions?

I tried using *mkl_dcs cmm* instead, but it seems to do the same thing. In fact, I think that

*mkl_dcsrmm*and

*mkl_dcscmm*are identical except that they invert the meaning of the

*transa*parameter.

Do all of the sparse matrix-matrix multiply functions expect the dense parts to be row-major? If so, what is the best practise for using these beside traditional column-major BLAS functions?

(I've found that this question has been asked before, but not answered.)

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I'm going to answer my own question.

There appears to some pretty important undocumented behavior in *mkl_dcsrmm. *(At least I can't find it documented anywhere!)

If the sparse matrix uses zero-based indexing (as indicated by the *matdescra* array) then *mkl_dcsrmm *assumes that the B and C matrices are row major. If the sparse matrix uses one-based indexing it assumes that the B and C matrices are column major.

I can easily change my sparse matrices to be one-based so that *mkl_dcsrmm *will work with column-major inputs.

The documentation at http://software.intel.com/sites/products/documentation/hpc/mkl/mklman/GUID-78C55D9B-86FF-4A9F-B5D5-D2F61B9314FC.htm#GUID-78C55D9B-86FF-4A9F-B5D5-D2F61B9314FC really ought to explain this important detail. While you're fixing the doc, there are couple of places where it looks like "m-" has been replaced by an em dash (e.g.* pntre(—1)* should be *pntre(m-1)*). Also the description for 'c' is incorrect. It says "On entry, the leading..." but should say "On entry **with transa= 'N' or 'n'**, the leading..."

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Peter B. wrote:

(I've found that this question has been asked before, but not answered.)

It has been answered here http://software.intel.com/en-us/forums/topic/365430.

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