Hi, I decided to use Eigen2 to solve a particular problem I am working on and came across issues with aliasing. I'm using the latest version 2.0.5. Here's the code saved as eigenbug.cc:

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#include <Eigen/Core>

// import most common Eigen types
USING_PART_OF_NAMESPACE_EIGEN

int main(int, char *[])
{   
  MatrixXf Q(5,5);

  for (int i=0; i<Q.rows(); i++)
    for (int j=0; j<Q.cols(); j++)
      Q(i,j)=i+j*Q.rows();
     
  std::cout << Q << "\n\n";

  MatrixXf R=Q.corner(Eigen::TopLeft, Q.rows()-1,  Q.cols()-1);
  std::cout << R << "\n\n"; // prints correct result

  Q=Q.corner(Eigen::TopLeft, Q.rows()-1, Q.cols()-1);
  std::cout << Q << "\n\n"; // prints incorrect result

  // Causes aliasing:
  R.row(3) = R.col(0);
  std::cout << R << "\n\n";

  return 0;
}


Output:

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raman@lanai:~/Work$ g++ --version
g++ (Ubuntu 4.3.3-5ubuntu4) 4.3.3
Copyright (C) 2008 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

raman@lanai:~/Work$ g++ -I eigen2 eigenbug.cc -o eigenbug && ./eigenbug
0 5 10 15 20
1 6 11 16 21
2 7 12 17 22
3 8 13 18 23
4 9 14 19 24

0 5 10 15
1 6 11 16
2 7 12 17
3 8 13 18

-2.771e-05 4 8 12
-2.771e-05 5 9 13
2 6 10 14
3 7 11 15

0 5 10 15
1 6 11 16
2 7 12 17
0 1 2 0


It looks like the line

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Q=Q.corner(Eigen::TopLeft, Q.rows()-1, Q.cols()-1);


causes aliasing. I can fix it by using an eval() before assignment but I thought that the default behaviour should take care of that. Is this expected behaviour or a bug?

This also occurs when I use any of the other corners (TopRight, BottomLeft, BottomLeft), general blocks using the block() method, or if I copy a column to a row.

Thanks in advance!
Raman

Is this expected behaviour or a bug?

This is expected behaviour. I agree that it's terrible, but there simply is no way to make this work automatically, without giving up the benefit of expression templates in other use cases, and without adding a permanent performance overhead. The only way forward is to make sure that the user knows about these issues and how to deal with them. Since you had the idea of using eval() there, which is exactly the right thing to do, I'm afraid I don't have anything more to teach you