test-parallel-ug-mpi-2 fails with clang

When compiled with clang-3.8 and libstdc++-6.2 (Ubuntu 16.10) the test test-parallel-ug fails for 2 mpi ranks with the error

--------------------------------------------------------------------------
mpirun noticed that process rank 1 with PID 12585 on node patton exited on signal 4 (Illegal instruction).
--------------------------------------------------------------------------

Notice that this is not for master but for core/dune-grid!101 (merged) because UG does not work with clang otherwise.

I have a similar issue with g++-5 and test-parallel-ug. It also doesn't work with 2 processors. The output is

[...]
11: -------------------------------------------------------
11: Primary job  terminated normally, but 1 process returned
11: a non-zero exit code.. Per user-direction, the job has been aborted.
11: -------------------------------------------------------
11: --------------------------------------------------------------------------
11: mpiexec detected that one or more processes exited with non-zero status, thus causing
11: the job to be terminated. The first process to do so was:
11: 
11:   Process name: [[57763,1],1]
11:   Exit code:    1
11: --------------------------------------------------------------------------
1/2 Test #11: test-parallel-ug-mpi-2 ...........***Failed    1.44 sec
test 12
    Start 12: test-loadbalancing

@carsten.graeser Could you provide a backtrace and disassembly to see which instruction fails?

I wouldn't be surprised if this was caused by unaligned access.

Here's the output from a gdb backtrace

Reading symbols from test-parallel-ug...done.
(gdb) run
Starting program: /home/graeser/dune_cmake/build-debug-clang/dune-grid/dune/grid/test/test-parallel-ug 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff1b68700 (LWP 3246)]
This is process 1 of 2, PID 3232 .
Testing parallel UGGrid for 2D
DimX=2, DimY=1, DimZ=1
DimX=2, DimY=1, DimZ=1

Thread 1 "test-parallel-u" received signal SIGILL, Illegal instruction.
ComputeNodeBorderPrios (obj=0x12f5950 "\004\200\003`\002")
    at /home/graeser/dune_cmake/dune-uggrid/parallel/dddif/priority.cc:176
176       if (me!=min_proc)
(gdb) backtrace 
#0  ComputeNodeBorderPrios (obj=0x12f5950 "\004\200\003`\002") at /home/graeser/dune_cmake/dune-uggrid/parallel/dddif/priority.cc:176
#1  0x00000000008108f8 in UG::D2::IFExecLoopObj (LoopProc=0x7f6ef0 <ComputeNodeBorderPrios(char*)>, obj=0x12f81c0, nItems=5) at /home/graeser/dune_cmake/dune-uggrid/parallel/ddd/if/ifuse.cc:282
#2  0x00000000007d00f6 in UG::D2::DDD_IFAExecLocal (aIF=8, aAttr=32, ExecProc=0x7f6ef0 <ComputeNodeBorderPrios(char*)>) at /home/graeser/dune_cmake/dune-uggrid/parallel/ddd/if/ifcmd.ct:227
#3  0x00000000007f6e93 in UG::D2::SetBorderPriorities (theGrid=0x12e73f0) at /home/graeser/dune_cmake/dune-uggrid/parallel/dddif/priority.cc:523
#4  0x00000000007f1f3a in UG::D2::ConstructConsistentMultiGrid (theMG=0x12d0e80) at /home/graeser/dune_cmake/dune-uggrid/parallel/dddif/gridcons.cc:426
#5  0x00000000008125a0 in UG::D2::TransferGridFromLevel (theMG=0x12d0e80, level=0) at /home/graeser/dune_cmake/dune-uggrid/parallel/dddif/trans.cc:855
#6  0x00000000009ba8f4 in UG::D2::lbs (argv=0x7fffffffc9d0 "0", theMG=0x12d0e80) at /home/graeser/dune_cmake/dune-uggrid/parallel/dddif/lb.cc:439
#7  0x000000000082f9ed in Dune::UG_NS<2>::lbs (argv=0x7fffffffc9d0 "0", theMG=0x12d0e80) at /home/graeser/dune_cmake/dune-grid/dune/grid/uggrid/ugwrapper.hh:1033
#8  0x000000000082f92a in Dune::UGGrid<2>::loadBalance (this=0x1082230, minlevel=0) at /home/graeser/dune_cmake/dune-grid/dune/grid/uggrid/uggrid.cc:462
#9  0x00000000005baaaf in Dune::UGGrid<2>::loadBalance<LoadBalance::LBDataHandle<Dune::UGGrid<2>, std::vector<Dune::FieldVector<double, 2>, std::allocator<Dune::FieldVector<double, 2> > >, 2> > (this=0x1082230, dataHandle=...)
    at /home/graeser/dune_cmake/dune-grid/dune/grid/uggrid.hh:499
#10 0x00000000005b1dca in LoadBalance::test<Dune::UGGrid<2> > (grid=...) at /home/graeser/dune_cmake/dune-grid/dune/grid/test/test-parallel-ug.cc:441
#11 0x00000000005aed2b in testParallelUG<2> (simplexGrid=false, localRefinement=false, refinementDim=0, refineUpperPart=false) at /home/graeser/dune_cmake/dune-grid/dune/grid/test/test-parallel-ug.cc:494
#12 0x00000000005ae659 in main (argc=1, argv=0x7fffffffdc68) at /home/graeser/dune_cmake/dune-grid/dune/grid/test/test-parallel-ug.cc:642
(gdb)

I note that ComputeNodeBorderPrios misses a return statement which apparently can lead to clang generating illegal instructions.

@carsten.graeser Could you add a return 0 at the end of ComputeNodeBorderPrios? And probably also to ComputeVectorBorderPrios and ComputeEdgeBorderPrios (right below the first one).

At least clang has chosen illegal instructions instead of nasal demons.

This indeed fixes the problem. Implemented in !29 (merged).

Moved from core/dune-grid#33 (moved)

Status changed to closed by commit b5e12845

Mentioned in commit b88127d3

Mentioned in merge request !30 (merged)

Mentioned in commit a3653a36