[flang] Disable Plugins in out-of-tree builds

[awarzynski]

Summary

https://reviews.llvm.org/D106137 added support for plugins in Flang. The
CMake configuration for plugins requires some LLVM variables that are
not available in out-of-tree builds (e.g. LLVM_SHLIB_OUTPUT_INTDIR).
This has caused the out-of-tree BuildBot worker to start failing:

• https://lab.llvm.org/buildbot/#/builders/175

This patch effectively disables plugins in out-of-tree builds and fixes
the configuration error. In order to support plugins in out-of-tree
builds, we would have to find a way to access the missing CMake
variables from LLVM. This could be implemented at a later time.

[Meinersbur]

LGTM

Such configuration paramters would typically be put into the exports file such that it gets re-imported in find_package.

[awarzynski]

In D107973#2942086, @Meinersbur wrote:

LGTM

Such configuration paramters would typically be put into the exports file such that it gets re-imported in find_package.

Do people mind when you tweak what's being exported? I'm a bit concerned that that could be a longer conversation. And I'm not sure what the future of out-of-tree builds is. Btw, thank you for reviewing!

[Meinersbur]

In D107973#2942163, @awarzynski wrote:

Do people mind when you tweak what's being exported? I'm a bit concerned that that could be a longer conversation. And I'm not sure what the future of out-of-tree builds is. Btw, thank you for reviewing!

I certainly affects compatibility. LLVM/Clang's cmake config was broken/inconsistent for a long time such that people continued to use llvm-config. However, the paths to the .so should be made available to projects linking to LLVM, possibly not as a single path but as generator expression for each shared library.

flang/CMakeLists.txt

 option(FLANG_BUILD_TOOLS
   "Build the Flang tools. If OFF, just generate build targets." ON)
 if (FLANG_BUILD_TOOLS)
   add_subdirectory(tools)
 endif()
 add_subdirectory(runtime)
 
 option(FLANG_BUILD_EXAMPLES "Build Flang example programs by default." OFF)
+if (FLANG_BUILD_EXAMPLES AND FLANG_STANDALONE_BUILD)
+  message(FATAL_ERROR "Examples are not supported in out-of-tree builds.")
+endif()
 add_subdirectory(examples)
 
 if (FLANG_INCLUDE_TESTS)
   add_subdirectory(test)
   if (FLANG_GTEST_AVAIL)
     add_subdirectory(unittests)
   endif ()
 endif()

flang/test/CMakeLists.txt

 # Test runner infrastructure for Flang. This configures the Flang test trees
 # for use by Lit, and delegates to LLVM's lit test handlers.
 
 llvm_canonicalize_cmake_booleans(
   FLANG_BUILD_EXAMPLES
   FLANG_STANDALONE_BUILD
   LLVM_ENABLE_PLUGINS
 )
 
 set(FLANG_TOOLS_DIR ${FLANG_BINARY_DIR}/bin)
 
+# FIXME In out-of-tree builds, "SHLIBDIR" is undefined and passing it to
+# `configure_lit_site_cfg` leads to a configuration error. This is currently
+# only required by plugins/examples, which are not supported in out-of-tree
+# builds.
+if (FLANG_STANDALONE_BUILD)
+  set(PATHS_FOR_PLUGINS "")
+else ()
+  set(PATHS_FOR_PLUGINS "SHLIBDIR")
+endif ()
+
 configure_lit_site_cfg(
   ${CMAKE_CURRENT_SOURCE_DIR}/lit.site.cfg.py.in
   ${CMAKE_CURRENT_BINARY_DIR}/lit.site.cfg.py
   MAIN_CONFIG
   ${CMAKE_CURRENT_SOURCE_DIR}/lit.cfg.py
   PATHS
-  "SHLIBDIR"
+  ${PATHS_FOR_PLUGINS}
 )
 
 configure_lit_site_cfg(
   ${CMAKE_CURRENT_SOURCE_DIR}/Unit/lit.site.cfg.py.in
   ${CMAKE_CURRENT_BINARY_DIR}/Unit/lit.site.cfg.py
   MAIN_CONFIG
   ${CMAKE_CURRENT_SOURCE_DIR}/Unit/lit.cfg.py
 )