Linking against Intel-OpenMP

[mratsim]

System information (version)

• OpenCV => master - #a9c8a52
• Operating System / Platform => Linux
• Compiler => GCC 7 + NVCC

Detailed description

Currently OpenCV cannot be linked easily to Intel OpenMP by passing -liomp5 -lpthread -L<icc_dir>/lib as mentioned in Intel User Guide - using the OpenMP Libraries due to CMake.

Also following limitations in the current MKL-DNN with TBB, both Intel MKL and MKL-DNN are built with OpenMP on my system.

As I'm linking OpenCV to MKL for the BLAS and LAPACK functionalities, it's probably best to also use OpenMP for OpenCV.

Following the recommendation of MKL-DNN I'm trying to link against Intel OpenMP but I'm stuck.

Steps to reproduce

• Install Intel MKL
• Install MKL-DNN
• Try to build OpenCV with WITH_OPENMP=ON
• Check the parallel lib during cmake config, it will be pthread

[alalek]

What MKL version is used?

[mratsim]

MKL 2018.3.222 downloaded and installed with this script: https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=intel-parallel-studio-xe#n64

[mratsim]

I think #12771 should have been kept open while this one should be closed.

Especially due to point 4

4. Where is MKL used? As mentioned here MKL_WITH_TBB failed to found the latest TBB 2018 on Windows #11383, BLAS library are not needed anymore for DNN in OpenCV 3.3 but I would expect that LAPACK functionality is still needed given that the option WITH_LAPACK is still there but even the PCA is not using LAPACK for either SVD or covariance matrix + eigenvecs computation. Checking the linked libraries/dependencies yields "Unused shared library":
   

If MKL is not needed we can just remove BLAS and LAPACK related options from the cmake scripts. Though being able to choose the OpenMP implementation still remains.