Re: [eigen] Eigen containers cannot be vectorized

[Joseph Mirabel <joseph.mirabel@xxxxxxx>]

Hi,

what about this:

#include <Eigen/Dense>

void f(Eigen::Matrix<Eigen::Index, Eigen::Dynamic, Eigen::Dynamic>& A) {
  const Eigen::Index cols = A.cols();
  const Eigen::Index rows = A.rows();
  for (Eigen::Index j = 0; j < cols; ++j) {
    for(Eigen::Index i = 0; i < rows; ++i) {
      A(i, j) = i + j;
    }
  }
}

Best rgds,
Joseph

On 12/08/2016 02:24 PM, Francois Fayard wrote:
> Hi,
> 
> I have found that Eigen containers cannot be vectorized. For instance,
> the following code:
> 
> #include <Eigen/Dense>
> 
> void f(Eigen::Matrix<Eigen::Index, Eigen::Dynamic, Eigen::Dynamic>& A) {
>   for (Eigen::Index j = 0; j < A.cols(); ++j) {
>     for(Eigen::Index i = 0; i < A.rows(); ++i) {
>       A(i, j) = i + j;
>     }
>   }
> }
> 
> is not vectorized by the Intel compiler. It says that the iteration
> count cannot be computed before the execution of the loop.
> 
> It is due to the fact that A is made of:
> Eigen::Index* data_;
> Eigen::Index cols_;
> Eigen::Index rows_;
> The compiler has no way of knowing that data_ does not point to rows_.
> As a consequence, it does not know that the operation A(i, j) = i + j
> does not change the number of rows of A. As a consequence, the loop is
> not countable and cannot be vectorized.
> 
> One way to fix this, would be to store the number of columns as a
> pointer. If cols_ becomes a Eigen::Index* such that the number of
> columns is cols_ - data_, the problems get fixed, and compilers can know
> vectorize the inner loop.
> 
> Best regards,
> 
> François Fayard
> Founder & Consultant - Inside Loop
> Applied Mathematics & High Performance Computing
> Tel: +33 (0)6 01 44 06 93
> Web: www.insideloop.io <http://www.insideloop.io>
> 
> PS: I have found that if you change the type to double, it does not get
> vectorized too, but I have no idea why.

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