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Fix segmentation fault in AMG if matrix of one rank is empty.

Merged Fix segmentation fault in AMG if matrix of one rank is empty.
fix-amg-empty-matrix-on-one-proc into master
Merged Markus Blatt requested to merge fix-amg-empty-matrix-on-one-proc into master
  1. Dec 14, 2017
    • Markus Blatt's avatar
      Make BCRSMatrix::createBegin mark empty matrices as built. · 8434134d
      Markus Blatt authored 
      For a matrix with no rows the matrix (built in row mode)
would otherwise never marked as built as this happened only
in the operator++ applied to the last row.
      8434134d
    • Markus Blatt's avatar
      Fix segmentation fault in AMG if matrix of one rank is empty. · 1b126e17
      Markus Blatt authored 
      For a matrix with any rows in a parallel run, The subgraph
containing only vertices associated with matrix rows owned by
the process would have 1 vertex although the actual matrix graph
would have none. This is fixed by checking the number of vertices
of the outer graph and act correctly for zero vertices.
For this we need to mark a matrix without rows built in row_wise mode
as built once the CreateIterator is constructed and correct the number
of vertices for a Subgraph of a MatrixGraph with 0 vertices.
      1b126e17