Fix Eigen compilation errors from GCC

[jhacsonmeza]

I've been getting multiple Eigen warnings in GCC 13, which become errors because of the -Werror flag used for compilation on Linux. The same error has been reported on #156.

The first warning is -Werror=maybe-uninitialized:

.../Eigen/src/Core/PlainObjectBase.h:512:17: error: '*(Eigen::DenseCoeffsBase<Eigen::Matrix<double, 3, 1, 0, 3, 1>, 1>::Scalar*)((char*)&svd + offsetof(Eigen::JacobiSVD<Eigen::Matrix<double, 3, 3, 0, 3, 3>, 2>,Eigen::JacobiSVD<Eigen::Matrix<double, 3, 3, 0, 3, 3>, 2>::<unnamed>.Eigen::SVDBase<Eigen::JacobiSVD<Eigen::Matrix<double, 3, 3, 0, 3, 3>, 2> >::m_singularValues.Eigen::Matrix<double, 3, 1, 0, 3, 1>::<unnamed>.Eigen::PlainObjectBase<Eigen::Matrix<double, 3, 1, 0, 3, 1> >::<unnamed>.Eigen::MatrixBase<Eigen::Matrix<double, 3, 1, 0, 3, 1> >::<unnamed>.Eigen::DenseBase<Eigen::Matrix<double, 3, 1, 0, 3, 1> >::<unnamed>.Eigen::DenseCoeffsBase<Eigen::Matrix<double, 3, 1, 0, 3, 1>, 3>::<unnamed>.Eigen::DenseCoeffsBase<Eigen::Matrix<double, 3, 1, 0, 3, 1>, 1>::<unnamed>.Eigen::DenseCoeffsBase<Eigen::Matrix<double, 3, 1, 0, 3, 1>, 0>::<unnamed>))' may be used uninitialized [-Werror=maybe-uninitialized]
  512 |       : Base(), m_storage(other.m_storage) { }
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~

I noticed that this one is also addressed in COLMAP here.

The second warning is -Werror=array-bounds:

.../lib/gcc/x86_64-pc-linux-gnu/13.2.0/include/avxintrin.h:875:19: error: array subscript '__m256d[0]' is partly outside array bounds of 'Eigen::Matrix<double, -1, -1, 0, 1, 1> [1]' [-Werror=array-bounds=]
  875 |   *(__m256d *)__P = __A;
      |   ~~~~~~~~~~~~~~~~^~~~~

This one is a false positive that has been happening since GCC 12 and which the Eigen team is aware of (https://gitlab.com/libeigen/eigen/-/issues/2506). This bug has already been reported to GCC here.

The third warning is -Werror=stringop-overflow:

inlined from 'void Eigen::internal::apply_block_householder_on_the_left(MatrixType&, const VectorsType&, const CoeffsType&, bool) [with MatrixType = Eigen::Block<Eigen::Matrix<double, 3, 1>, -1, -1, false>; VectorsType = Eigen::Block<Eigen::Matrix<double, 3, 1>, -1, -1, false>; CoeffsType = Eigen::Block<Eigen::Matrix<double, 1, 1, 0, 1, 1>, -1, 1, false>]' at /apps/all/Eigen/3.4.0-GCCcore-13.2.0/include/Eigen/src/Householder/BlockHouseholder.h:101:19:
.../lib/gcc/x86_64-pc-linux-gnu/13.2.0/include/avxintrin.h:875:19: error: 'void* __builtin_memcpy(void*, const void*, long unsigned int)' writing 32 or more bytes into a region of size 24 overflows the destination [-Werror=stringop-overflow=]
  875 |   *(__m256d *)__P = __A;
      |   ~~~~~~~~~~~~~~~~^~~~~

I haven't found it reported on the Eigen repo, but maybe this is also a false positive (?).

My proposal to solve these errors is to add the flags -Wno-maybe-uninitialized, -Wno-array-bounds, and -Wno-stringop-overflow to ignore these warnings. Although another alternative would be to remove the -Werror which also worked for me.