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openmpintel-ipp

Can/should libiomp5 and libgomp mix?

We are compiling an application that uses OpenMP. We are using gcc 4.4, with -fopenmp. The app also uses IPP, which includes its own version of OpenMP (libiomp5). (Note: we are disabling IPP's internal threading by calling ippSetNumThread(1). According to Intel's documentation, this should avoid conflicts with other threading libraries. However, linking with IPP still links in libiomp5.so.)

Since libiomp5.so is already linked in, we have not been linking with libgomp.so (gcc's version of OpenMP). For a long time this has worked, but after a seemingly inconsequential change we started seeing very odd OpenMP-related crashes on one of four platforms we support (the other three platforms still work fine).

I can make the crashes go away if I link in libgomp.so as well as libiomp5.so.

I have a couple questions about this:

  1. Is linking with both these libraries safe? It seems like they would both define the same symbols.
  2. Is there a way to tell what version of OpenMP libiomp5.so supports? With gcc 4.4, libgomp.so should be at OpenMP v3.0. I can't find any information in Intel's documentation about the OpenMP version of libiomp5.so.
Solution

  • Since no one has answered for a few days, I'll just report what I've found out independently:

    Is linking with both these libraries safe? No. Here's the most useful page I found on this topic: http://software.intel.com/sites/products/documentation/studio/composer/en-us/2011/compiler_c/optaps/common/optaps_par_openmp_multiple_compilers.htm

    Intel recommends that if you are going to be mixing IPP's internal OpenMP threading with your own OpenMP threading, you link to libiomp5 instead of your compiler's OpenMP library. The current version of libiomp5 provides "source compatibility and object-level interoperability" with gcc's OpenMP, but only if you are using gcc "4.42" (sic; I assume they mean 4.4.2) or later.

    Is there a way to tell what version of OpenMP libiomp5.so supports? Yes. Set the environment variable KMP_VERSION=1, then run your application. You'll get some debugging output printed by libiomp5 to your console. If you are using IPP v7 or later, one line will be something like

    Intel(R) OMP API version: 3.0 (200805)

    If you are using IPP 6, it won't tell you the API version, but it will tell you when it was built and with which version of the Intel compiler. Then you can check and see what version of OpenMP that compiler supported. (11.0 was the first version of the Intel compiler to support OpenMP v3.0.)