[OpenMP] Fix an OpenMP Windows build problem

[vadikp-intel]

Summary

When built as part of LLVM, CMAKE is generating incorrect locations references for the first build step's artifacts being used in regenerating the Windows import library in the second step. The fix is to feed a dummy non-buildable, rather than buildable, source to CMAKE to satisfy its source requirement in the second step removing the need to reference the first step's artifacts.

[vadikp-intel]

Removed redundant comments.

[vadikp-intel]

Updated with a better fix.

[mstorsjo]

LGTM, thanks! This is indeed the neatest solution so far!

openmp/runtime/src/CMakeLists.txt

304

Can't we get rid of the omp dependency here at this point too, since there's no technical need to build it after the main library has been built?

[vadikp-intel]

removed a now redundant dependency

openmp/runtime/src/CMakeLists.txt

if(MSVC)
     set(LIBOMPIMP_GENERATED_DEF_FILE ${LIBOMP_LIB_NAME}.imp.def)
     add_custom_target(libompimp-needed-def-file DEPENDS ${LIBOMPIMP_GENERATED_DEF_FILE})
     add_custom_command(
       OUTPUT  ${LIBOMPIMP_GENERATED_DEF_FILE}
       COMMAND ${PERL_EXECUTABLE} ${LIBOMP_TOOLS_DIR}/generate-def.pl ${LIBOMP_GDFLAGS} -D NAME=${LIBOMP_LIB_FILE} -D NOORDINALS
           -o ${LIBOMPIMP_GENERATED_DEF_FILE} ${CMAKE_CURRENT_SOURCE_DIR}/dllexports
       DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/dllexports ${LIBOMP_TOOLS_DIR}/generate-def.pl
     )
-    # while this is merely generating an import library off the one generated with the runtime dll,
-    # kmp_global.cpp will satisfy the librarian's apparent ask to see the actual exported DATA items
-    set_source_files_properties(${LIBOMP_GENERATED_IMP_LIB_FILENAME} PROPERTIES GENERATED TRUE EXTERNAL_OBJECT TRUE)
-    add_library(${LIBOMP_IMP_LIB_TARGET} STATIC ${LIBOMP_GENERATED_IMP_LIB_FILENAME} kmp_global.cpp)
+    # while this merely generates an import library off a def file, CMAKE still requires it to have a "source" so feed it a dummy one, 
+    # making it a .txt which CMAKE will filter out from the librarian (a .cpp will make lib.exe punt trying to resolve the .def symbols)
+    add_library(${LIBOMP_IMP_LIB_TARGET} STATIC kmp_dummy.txt)
     set_target_properties(${LIBOMP_IMP_LIB_TARGET} PROPERTIES
-        PREFIX "" SUFFIX "" OUTPUT_NAME "${LIBOMP_IMP_LIB_FILE}" LINKER_LANGUAGE C
+        PREFIX "" SUFFIX "" OUTPUT_NAME "${LIBOMP_IMP_LIB_FILE}" LINKER_LANGUAGE ${LIBOMP_LINKER_LANGUAGE}
         STATIC_LIBRARY_OPTIONS "${CMAKE_LINK_DEF_FILE_FLAG}${CMAKE_CURRENT_BINARY_DIR}/${LIBOMPIMP_GENERATED_DEF_FILE}")
-    add_dependencies(${LIBOMP_IMP_LIB_TARGET} omp libompimp-needed-def-file)
+    add_dependencies(${LIBOMP_IMP_LIB_TARGET} libompimp-needed-def-file)
   endif()
 endif()
 
 # Building the Fortran module files
 # One compilation step creates both omp_lib.mod and omp_lib_kinds.mod
 if(${LIBOMP_FORTRAN_MODULES})
   configure_file(${LIBOMP_INC_DIR}/omp_lib.h.var omp_lib.h @ONLY)
   configure_file(${LIBOMP_INC_DIR}/omp_lib.f90.var omp_lib.f90 @ONLY)

openmp/runtime/src/kmp_dummy.txt

+/*
+ * kmp_dummy.txt -- dummy "source" file used in regenerating the Windows import library
+ */
+
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// end of file //