Segmentation fault with Eigen library and GMP rationals

[amandini]

Hello,

I am doing some work using the Eigen C++ template library, with large sparse matrices, say 200,000 x 200,000, over the mpq_class provided by the GNU Multiple Precision (GMP) library.

The first problem is that code which compiles without any warnings on the godbolt compiler, a small example of which is given at https://godbolt.org/z/j66jdK7n6, produces the following warnings when compiled on my gentoo system

[sam_]

I see the warning locally as well but not on godbolt. Not sure why yet.

What does -march=native expand to on your system? You can check by running resolve-march-native from app-misc/resolve-march-native. I do see it without but it may be useful to know nonetheless.

Could you give me a testcase which segfaults? Can look more then.

[amandini]

[bstaletic]

I'm not familiar with GMP, but that operator< looks very wrong - sometimes it tells you which of the two inputs is smaller and sometimes it does the opposite.

[GDH-gentoo]

I don't know why the Compiler Explorer doesn't output the warning, but it makes sense. The code in Eigen::internal::AmbiVector<>'s reallocateSparse() member function uses std::memcpy() to copy the value of an object whose type is the type denoted by the class template's _Scalar parameter. I believe that you can have undefined behaviour when using objects of nontrivially copyable types made like that... and GMP's mpq_class is definitely not trivially copyable, it's a class that has nontrivial default and copy constructors, a nontrivial copy assignment operator, and a nontrivial destructor.

I don't know in general if Eigen documents its expectations about the types that are passed as template arguments like this, examples seem to show only arithmetic types like double or at most trivially copyable classes like std::complex<float>. The segfault shown in the backtrace happens in the destructor of some mpq_class object, hard to say if it was properly constructed.
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[amandini]

[amandini]

[GDH-gentoo]

[amandini]

[Hu]

The warning about memcpy is because classes with non-trivial copy constructors often (but not always) have invariants that the copy constructor will maintain, but that a straight memcpy will not. A simple example of that would be std::vector: the copy constructor is nontrivial because the copy gets its own distinct storage, with its own copy of the contained objects. Using memcpy to copy a vector would result in two vector instances that point to the same storage, and both will try to free that storage when destroyed. That free is undefined behavior and may (or may not) cause a crash due to heap corruption.

I would argue that until you can patch Eigen to build cleanly with -Werror=class-memaccess, it is not worth your time investigating the crash further. Eliminating the undefined behavior of copying the non-trivial class is worthwhile, and may (or may not) fix your crash as a side effect.

[GDH-gentoo]

[GDH-gentoo]

[amandini]

[amandini]

[amandini]

[GDH-gentoo]

Interesting, does the GDB backtrace look the same?

Eigen::SparseMatrix<mpq_class, Eigen::RowMajor, int>::insertBackUncompressed() --> mpq_class destructor --> mpq_clear() --> std::free() --> segmentation fault
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[amandini]

[GDH-gentoo]

I see. A little different from your original program, but the segmentation fault still happens when executing Eigen::SparseMatrix<mpq_class, Eigen::RowMajor, int>::insertBackUncompressed()'s code.