[openmp] Fix libomp.so copy in multi-config generators.

[DavidTruby]

Summary

This changes the copy command for libomp.so to use the output of the target as
the source of the copy, rather than trying to find it based on
${LIBOMP_LIBRARY_DIR}, which appears to be incorrect in multi-config generator
builds.

[DavidTruby]

I'm a little uncomfortable with this line copying things from the build directory to the source directory; I believe the source directory should be able to be immutable and builds should still work. Part of the principle of having out-of-tree builds is that the source tree should remain unmodified.
Regardless, I haven't changed the behaviour here as I don't know why the library gets copied to the source directory. Perhaps someone could clarify that? Until then I am just replicating the existing behaviour but with it also working for multi-config generators.

[protze.joachim]

What would be needed to remove this strange copying step? Do the tests really rely on these directories?
I could run the tests successfully with cmake -DLIBOMP_COPY_EXPORTS=FALSE.

[protze.joachim]

I think this export directory structure is a remainder from the original configure based Intel/OpenMP runtime.
@jlpeyton @AndreyChurbanov is this step still needed?

If Intel relies on this export directory, can we disable by default and you enable it for internal purpose?

[AndreyChurbanov]

In D84148#2161732, @protze.joachim wrote:

I think this export directory structure is a remainder from the original configure based Intel/OpenMP runtime.
@jlpeyton @AndreyChurbanov is this step still needed?

If Intel relies on this export directory, can we disable by default and you enable it for internal purpose?

I think this is historical artifact that can be removed nowadays.
Maybe, to be on the safe side, we can set LIBOMP_COPY_EXPORTS to false by default, as you suggested.
Then later we can remove the copying functionality at all.

[protze.joachim]

@DavidTruby please change the default to false and I'll accept the patch.

[DavidTruby]

Disable libomp export copies by default.

[AndreyChurbanov]

LGTM

[protze.joachim]

Thanks for the update, lgtm

Do you have commit rights to land the patch?

openmp/runtime/CMakeLists.txt

set(LIBOMP_LIBFLAGS "" CACHE STRING
   "Appended user specified linked libs flags. (e.g., -lm)")
 set(LIBOMP_FFLAGS "" CACHE STRING
   "Appended user specified Fortran compiler flags.  These are only used if LIBOMP_FORTRAN_MODULES==TRUE.")
 
 # Should the libomp library and generated headers be copied into the original source exports/ directory
 # Turning this to FALSE aids parallel builds to not interfere with each other.
 # Currently, the testsuite module expects the just built OpenMP library to be located inside the exports/
 # directory.  TODO: have testsuite run under llvm-lit directly.  We can then get rid of copying to exports/
-set(LIBOMP_COPY_EXPORTS TRUE CACHE STRING
+set(LIBOMP_COPY_EXPORTS FALSE CACHE STRING
   "Should exports be copied into source exports/ directory?")
 
 # HWLOC-support
 set(LIBOMP_USE_HWLOC FALSE CACHE BOOL
   "Use Hwloc (http://www.open-mpi.org/projects/hwloc/) library for affinity?")
 set(LIBOMP_HWLOC_INSTALL_DIR /usr/local CACHE PATH
   "Install path for hwloc library")
 

openmp/runtime/cmake/LibompExports.cmake

 else()
   get_target_property(LIBOMP_OUTPUT_DIRECTORY omp LIBRARY_OUTPUT_DIRECTORY)
 endif()
 if(NOT LIBOMP_OUTPUT_DIRECTORY)
   set(LIBOMP_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
 endif()
 add_custom_command(TARGET omp POST_BUILD
   COMMAND ${CMAKE_COMMAND} -E make_directory ${LIBOMP_EXPORTS_LIB_DIR}
-  COMMAND ${CMAKE_COMMAND} -E copy ${LIBOMP_OUTPUT_DIRECTORY}/${LIBOMP_LIB_FILE} ${LIBOMP_EXPORTS_LIB_DIR}
+  COMMAND ${CMAKE_COMMAND} -E copy $<TARGET_FILE:omp> ${LIBOMP_EXPORTS_LIB_DIR}
 )
 
 # Copy Windows import library into exports/ directory post build
 if(WIN32)
   get_target_property(LIBOMPIMP_OUTPUT_DIRECTORY ompimp ARCHIVE_OUTPUT_DIRECTORY)
   if(NOT LIBOMPIMP_OUTPUT_DIRECTORY)
     set(LIBOMPIMP_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
   endif()
   add_custom_command(TARGET ompimp POST_BUILD
     COMMAND ${CMAKE_COMMAND} -E make_directory ${LIBOMP_EXPORTS_LIB_DIR}
     COMMAND ${CMAKE_COMMAND} -E copy ${LIBOMPIMP_OUTPUT_DIRECTORY}/${LIBOMP_IMP_LIB_FILE} ${LIBOMP_EXPORTS_LIB_DIR}
   )
 endif()