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c++cclangopenmp

What happens when clang-built program links against both libomp and libgomp?

I have a C/C++ program with various dependencies that come from my Linux distribution.

Some of the dependencies are linked with GCC's OpenMP library libgomp.so.

My program itself has its code compiled with clang's -fopenmp and linked with LLVM's OpenMP library libomp.so.

This means that the program's ldd -v shows both libgomp.so and libomp.so as dependencies that will be loaded on startup.

While my program seems to start correctly, I'm concerned about what happens in this case, with 2 OpenMP runtime libraries linked:

  • Will it cause incorrect results?
  • Will it disable OpenMP parallelism in the dependent libraries that use libgomp?
  • Something else?

Thanks!

(Note that it is currently not possible to tell clang to just use GCC's libgomp; while there's a flag -fopenmp=libgomp that, it simply does nothing, disabling OpenMP, see https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=227438#c2 and https://reviews.llvm.org/D2841)).

(This question is similar to Can/should libiomp5 and libgomp mix? which is about mixing Intel+GCC OpenMP, but that says nothing about clang+GCC.)

Solution

  • The answer is that having more than one OpenMP runtime in a process is not safe. The details are longer than will fit here, so I have written a blog post on CpuFun. See "Is Mixing OpenMP® Runtimes Safe?"