Re: [eigen] malloc in Scalar x Matrix Matrix

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

Looks like this:
https://stackoverflow.com/questions/51043842/call-eigen-gemm-over-externally-allocated-data

It has been fixed a year ago on the devel-branch, but apparently not 
backported to 3.3:
http://eigen.tuxfamily.org/bz/show_bug.cgi?id=1562


If you know the sizes of your matrices at compile time, you should write

  const Map<const Matrix<double, 2, 10> > A( _A.data());
  const Map<const Matrix<double, 10, 2> > B( _B.data());
  Map<Matrix2d> C( _C.data());
  double v = 2.0;
  C.noalias() = v * A * B;

If you know can't upgrade to the devel branch, but know that your 
matrices are small (but with unknown size at compile time), you can 
write this as a workaround:

  C.noalias() = v * A.lazyProduct(B);


Generally, if you fail to produce a minimal example from scratch, you 
can start with (a copy of) your code and remove everything which does 
not let the error disappear, hopefully leaving just a few lines.

Cheers,
Christoph


On 09/07/2019 16.15, Henrik Larsson wrote:
> Hi,
>
> I have an expressions of type
> C.noalias() = alpha * A * B;
> where alpha is a double and A, B and C are Maps of double Dynamic
> Matrices of proper sizes.
> I want to avoid any memory allocation but when I try to debug the
> expression using EIGEN_RUNTIME_NO_MALLOC, I get an exception since
> malloc is still used.
> Only if I use the expression _without_ the scalar, i.e.,
> C.noalias() = A * B;
> I don't get an exception. Unfortunately, I do not have a minimal working
> example. A minimal working example actually *works* (without throwing an
> exception) but for reasons I do not understand,
> I get an exception in my actual code, which is more complicated, even if
> I first "remap" everything to the following expression:
>
> const Map<const MatrixXd> A( _A.data(), _A.rows(), _A.cols()); /* size 2
> x 10; _A, _B are templated Maps of RowMajor Dynamic Matrices; _C is a
> normal reference to RowMajor Dynamic Matrix.*/
> const Map<const MatrixXd> B( _B.data(), _B.rows(), _B.cols());  /* size
> 10 x 2*/
> Map<MatrixXd> C( _C.data(), _C.rows(), _C.cols()); /* size 2 x 2*/
> double v = 2.0;
> // C.noalias() = A * B; // no memory allocation
> C.noalias() = v * A * B; // memory allocation
>
> Could you please give me a hint why I get this memory allocation? Since
> I do not have a minimal working example that throws the exception, I
> attach a backtrace of the exception in my actual code.
> It seems that the matrix size of the lhs ("C.noalias()") somehow gets
> lost within the Eigen code. At # 15 of the attached backtrace, the
> matrix size is still known but afterwards it is set to 0 (I cannot print
> the matrix size in #14 due to code inlining).
>
> I use eigen 3.3.7-2 from arch linux.
>
> Best regards,
>
> Henrik

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 Dr.-Ing. Christoph Hertzberg

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