[Bug c++/33599] New: segfault in program compiled by g++ 4.2, corrupted reference

["jacob at math dot jussieu dot fr" <gcc-bugzilla at gcc dot gnu dot org>]

Hello,

I am developing a C++ template library (a matrix library with expression
templates). Upgrading from g++-4.1 to g++-4.2, the programs using this library
run much (4x) faster, but some segfault!

I have fixed for this bug report an archive containing the bare minimum to
reproduce this bug.

To reproduce:

tar xfzv gcc-bug-report.tar.gz
cd gcc-bug-report

Then you can check that it works with g++-4.1:

g++-4.1 test.cpp -o test && ./test

And you can check that it segfaults with g++-4.2:

g++-4.2 test.cpp -o test && ./test

Now let us look into where exactly it segfaults. This happens in
EiMatrixConstRef::_read(), so let us look at the file:
src/internal/MatrixRef.h.

Here is the class EiMatrixConstRef:

template<typename MatrixType> class EiMatrixConstRef
 : public EiObject<typename MatrixType::Scalar, EiMatrixConstRef<MatrixType> >
{
  public:
    typedef typename MatrixType::Scalar Scalar;
    friend class EiObject<Scalar, EiMatrixConstRef>;

    EiMatrixConstRef(const MatrixType& matrix) : m_matrix(matrix)
    {
      std::cout << "contruct ref " << this << " on matrix " << &m_matrix <<
std::endl;
    }
    EiMatrixConstRef(const EiMatrixConstRef& other) : m_matrix(other.m_matrix)
    {
      std::cout << "contruct ref " << this << " from ref " << &other << " on
matrix " << &m_matrix <<
 std::endl;
    }
    ~EiMatrixConstRef() {std::cout << "destruct ref " << this << std::endl;}

    EI_INHERIT_ASSIGNMENT_OPERATORS(EiMatrixConstRef)

  private:
    int _rows() const { return m_matrix.rows(); }
    int _cols() const { return m_matrix.cols(); }

    const Scalar& _read(int row, int col) const
    {
      std::cout << "ref " << this << " reading in matrix " << &m_matrix <<
std::endl;
      return m_matrix._read(row, col);
    }

    const MatrixType& m_matrix;
};

So what happens is that the last call to EiMatrixConstRef::_read() segfaults
because the reference m_matrix is corrupted, i.e. as a pointer it has a bad
value.

This is strange because the cout's that I have added to the construcors and
destructor show that this EiMatrixConstRef object is properly constructed with
a good m_matrix reference, and not destructed since.

So at some point the m_matrix reference gets corrupted. The fact that this
didn't happen with g++-4.1 suggests to me that this might be a bug in g++-4.2.

Of course I might be wrong, I'm not an expert.

Cheers,
Benoit


-- 
           Summary: segfault in program compiled by g++ 4.2, corrupted
                    reference
           Product: gcc
           Version: 4.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jacob at math dot jussieu dot fr


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33599