Re: [eigen]

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This is now fixed.

The problem was not specific to inverse(), but existed for all ReturnByValue-enabled function. For example, doing x = y.solve(x) would probably have exhibited the same aliasing issues.

The fix was to make ReturnByValue evaluate-before-assigning. If you want the old behavior (slightly more efficient), you can use .noalias() to tell Eigen not to care about aliasing, like this:

  x.noalias() = y.inverse();  /// don't do this if y aliases x !!

Since this is the behavior for products, it felt natural, API-wise, to do the same for other "multiplicative" operations such as inverse(), and more generally, solve() and friends.


2010/5/30 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>
aaahh... this had me puzzled for a while, but what you really have here is an aliasing problem. With the devel branch, when you do S = S.inverse(), S starts being overwritten with its inverse and then read again to compute the rest of the inverse... i need to have a look at the code to see if we're getting any speed gain from that, a priori i'd say let's work like in 2.0, taking care of aliasing, and if later there is demand for a super optimized noalias variant, we can then add it with an explicit name like a.noalias()  = a.inverse(), like for products.


2010/5/28 SHIROKOBROD Oleg <Oleg.Shirokobrod@xxxxxxxxxx>



I’ve met a problem with development branch. Here you are the case.

                                   { -2.0                                     -1.0 }

I have a 2x2 matrix S = {                                                   }

                                   { 0.00026130287408377900     -2.0 }


and I want to inverse it. This is the code I compiled with MSVS 2008


#include <Eigen\Core>

#include <Eigen\LU>


Eigen::Matrix<double, 2, 2> S;

S << -2.0 , -1.0 , 0.00026130287408377900 , -2.0;

S = S.inverse();


Eigen2 gives the right answer




S.inverse  =  {






Eigen3 (development branch I got on 26.05) gives a wrong answer





S.inverse  =  {






What’s wrong?


Thank you,


Oleg Shirokobrod


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