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Bug 1736

Summary:	Column access of some IndexedView won't compile
Product:	Eigen	Reporter:	Antoine Bussy <bussyantoine>
Component:	Core - expression templates	Assignee:	Nobody <eigen.nobody>
Status:	RESOLVED FIXED
Severity:	Compilation Problem	CC:	bussyantoine, chtz, gael.guennebaud, jacob.benoit.1
Priority:	Normal	Keywords:	test-needed
Version:	3.4 (development)
Hardware:	All
OS:	All
Whiteboard:
Bug Depends on:
Bug Blocks:	814

Description Antoine Bussy 2019-08-07 11:34:28 UTC

Hi,

The following code won't compile on clang-7 (I also tried gcc-9 and clang-8 on https://godbolt.org/):

#include <Eigen/Core>
#include <array>

auto indexed_view_col()
{
    auto const m = Eigen::Matrix4d::Random().eval();
    auto ok  = m(Eigen::all, Eigen::seq(1, Eigen::last)).col(0).eval();
    auto ok2 = m(Eigen::all, Eigen::seq(0, 1)).col(0).eval();
    auto err = m(Eigen::all, std::array<Eigen::Index,2>{0,1}).col(0).eval();
    auto err2= m(Eigen::all, Eigen::Vector<Eigen::Index,2>(0, 1)).col(0).eval();
}

The compilers don't seem to find the unary version of coeff(), and indeed, it isn't in IndexedView.h (whereas it is present in Block.h). I implemented it similarly to Block's:

  EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE
  CoeffReturnType coeff(Index index) const
  {
    return coeff((m_xpr.rows() == 1 ? 0 : index),
                 (m_xpr.rows() == 1 ? index : 0));
  }

  EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE
  Scalar& coeffRef(Index index)
  {
    return coeffRef((m_xpr.rows() == 1 ? 0 : index),
                 (m_xpr.rows() == 1 ? index : 0));
  }

and it seems to solve my problem. but I'm not familiar with Eigen's internals at all, so the root cause might be elsewhere?

Comment 1 Christoph Hertzberg 2019-08-07 17:18:03 UTC

The actual problem here is that `.coeff(Index)` is called -- even though only in a false-branch, i.e., never executed.
With C++17 this would be trivial to workaround using

   if constexpr (ForwardLinearAccess) {...}

Your patch should work of course (since it will never actually be called). But perhaps we should add assertions that the expression is a vector at compile time (actually, also to the corresponding Block::coeff methods)


Ideally, m(X, Y).col(i) could be simplified to m(X,Y[i]) -- similar for any other block-operations.

Comment 2 Antoine Bussy 2019-08-08 09:23:00 UTC

Thanks for the answer, adding constexpr in CoreEvaluator.h is indeed much cleaner :)

Concerning m(X,Y[i]), I can't really use this simplification in my code since m(X,Y) is the input of a function that works on matrices.

For a non-C++17 workaround, it is possible to emulate the if constexpr with a small amount of boilerplate:

EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE
CoeffReturnType coeff_impl(std::true_type, Index index) const
{ 
	return m_argImpl.coeff(m_linear_offset.value() + index); 
}

EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE
CoeffReturnType coeff_impl(std::false_type, Index index) const
{ 
	return coeff(RowsAtCompileTime == 1 ? 0 : index, RowsAtCompileTime == 1 ? index : 0);
}

EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE
CoeffReturnType coeff(Index index) const
{
	return coeff_impl(std::bool_constant<ForwardLinearAccess>{}, index);
}

Of course, it would be necessary to reimplement std::bool_constant, but this is trivial.

Comment 3 Gael Guennebaud 2019-09-11 13:41:52 UTC

Thank you for reporting this issue and suggesting a fix.

https://bitbucket.org/eigen/eigen/commits/f4920c83f9c4/

Comment 4 Nobody 2019-12-04 18:44:04 UTC

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