Re: [eigen]

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

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.

Benoit

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.

Benoit

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

Hello,

 

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

 

{-0.499967339	0.24998367}
S.inverse  =  {
{-6.53215E-05	-0.499967339}

 

 

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

 

 

{-0.499967339	0.24998367}
S.inverse  =  {
{-6.53215E-05	-0.124983670170524}

 

 

What’s wrong?

 

Thank you,

 

Oleg Shirokobrod

 

___________________________________________________________________________
This e-mail is confidential and is for the addressee only.   Please refer to
www.oxinst.com/email-statement for regulatory information.