Re: [eigen] Some trouble with triangularView

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

2010/9/3 Eamon Nerbonne <eamon.nerbonne@xxxxxxxxx>:
> To speed up covariance computations, I tried to use triangularView to avoid
> computing unnecessary coefficients.  However, that didn't work as expected.
>
> To start with, there seems to be a g++ bug that prevents compiling the
> following template:
>
> #include <Eigen/Core>
> using namespace Eigen;
> template <typename TPoints>
>     inline static Matrix<typename
> TPoints::Scalar,TPoints::RowsAtCompileTime,TPoints::RowsAtCompileTime>
>     CovarianceX(Eigen::MatrixBase<TPoints>const & points) {
>         typedef typename TPoints::Scalar Scalar;
>         typedef Eigen::Matrix<Scalar,TPoints::RowsAtCompileTime,1> TPoint;
>         typedef Matrix<typename
> TPoints::Scalar,TPoints::RowsAtCompileTime,TPoints::RowsAtCompileTime>
> TMatrix;
>         TPoint mean = points.rowwise().sum() * (1.0/points.cols());
>         TPoint diff = TPoint::Zero(points.rows());
>         TMatrix cov = TMatrix::Zero(points.rows(),points.rows());
>
>         for(int i=0;i<points.cols();++i) {
>             diff.noalias() = points.col(i) - mean;
>             cov.triangularView<Eigen::Upper>() += diff * diff.transpose();
> //line A
>         }
>
>         cov.triangularView<Eigen::StrictlyLower>() = cov.transpose(); //line
> B
>         return cov * (1.0/(points.cols()-1.0));
>     }
>
> int main(){return 0;}
>
> In particular, on lines A&B the compiler complains of "expected
> primary-expression before ')' token".  If the function isn't templated it
> compiles (and works).  In MSC, it works.  It looks like specifying a
> template argument here breaks something - anyone know of a workaround?  I
> tried using a templated class with a static function, but that fails
> equally.

This is just the usual c++ syntax issue, argh, we've answered this
question dozens of times, I wonder where we could document it once and
for all so users find it.

You have to add a template keyword:

cov.template triangularView<Eigen::Upper>() += ...

GCC is right to reject this code without template.


>
> Now, even when this works, it's a lot slower than without triangularView,
> which is odd.  At first I thought that's because I can't mix .noalias and
> triangularView, but as far as I can tell, line A is using a lazy product
> despite possible aliasing (isn't that a bug?) so the perf degredation may
> simply be due to the greater code complexity.

no idea... sorry. (no time to investigate now).

Benoit

>
> --eamon@xxxxxxxxxxxx - Tel#:+31-6-15142163
>