Improve error message in DenseMatrix::Eigensystem

[benzwick]

Attempting to compute the eigensystem of a dense matrix without LAPACK results in the following error:

DenseMatrix::Eigensystem
Aborted

which is not very informative.

Could we add some extra information to this line in densemat.cpp:

   mfem_error("DenseMatrix::Eigensystem");

e.g. like this:

   mfem_error("DenseMatrix::Eigensystem: requires MFEM built with LAPACK");

to make it immediately obvious why this failed?

Maybe the instructions for using LAPACK could also be added to the instructions on the website.

[benzwick]

On line 1167 there is also this one:

   mfem_error("DenseMatrix::Eigensystem for generalized eigenvalues");

On line 3372 (and a few other places) there is this message which could be used for the similar errors above:

   mfem_error("DenseMatrixEigensystem::Eval(): Compiled without LAPACK");

There are also some other errors in densemat.cpp that could output more information like "incorrect matrix dimensions" or "dimension mismatch", for example here:

   if (A.Height() != ABt.Height() || B.Height() != ABt.Width() ||
       A.Width() != B.Width())
   {
      mfem_error("MultABt(...)");
   }

[barker29]

In my view these suggestions are very reasonable. Could you collect them in a pull request and submit it?

[benzwick]

Yes, I will create a PR for this.

Somewhat related, can someone please explain to me what the MFEM_CONTRACT_VAR macro is used for. It is defined here as:

#define MFEM_CONTRACT_VAR(x) if (false && (&x)+1){}

and used a lot in densemat.cpp.

[barker29]

If I understand correctly, MFEM_CONTRACT_VAR is a hack to suppress an unused variable warning in optimized build. If you use a variable in asserts but nothing else, the preprocessor will strip out all the uses and you get the warning, but if you use MFEM_CONTRACT_VAR then the optimizer will still remove the statement (it doesn't do anything) but not trigger the warning.

[tzanio]

Discussion continued in #1685.