RE: [eigen] Eigen containers cannot be vectorized 
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Hi Francois,
Isn't that something where one would rather use (nonstandard) __restrict__?
Best regards Daniel Vollmer  Deutsches Zentrum für Luft und Raumfahrt e.V. (DLR) German Aerospace Center Institute of Aerodynamics and Flow Technology  Lilienthalplatz 7  38108 Braunschweig  Germany Daniel Vollmer  AS C²A²S²E www.DLR.de Von: Francois Fayard [fayard@xxxxxxxxxxxxx]
Gesendet: Donnerstag, 8. Dezember 2016 14:24 An: eigen@xxxxxxxxxxxxxxxxxxx Betreff: [eigen] Eigen containers cannot be vectorized 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 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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