[Flang-RT][Offload] Always use LLVM-built GTest (#143682)

The Offload and Flang-RT had the ability to compile GTest themselves.
But in bootstrapping builds, LLVM_LIBRARY_OUTPUT_INTDIR points to the
same location as the stage1 build. If both are building GTest, they
everwrite each others `libllvm_gtest.a` and `libllvm_test_main.a` which
causes #143134.

This PR removes the ability for the Offload/Flang-RT runtimes to build
their own GTest and instead relies on the stage1 build of GTest. This
was already the case with LLVM_INSTALL_GTEST=ON configurations. For
LLVM_INSTALL_GTEST=OFF configurations, we now also export gtest into the
buildtree configuration. Ultimately, this reduces combinatorial
explosion of configurations in which unittests could be built
(LLVM_INSTALL_GTEST=ON, GTest built by Offload, GTest built by Flang-RT,
GTest built by Offload and also used by Flang-RT).

GTest and therefore Offload/Runtime unittests will not be available if
the runtimes are configured against an LLVM install tree. Since llvm-lit
isn't available in the install tree either, it doesn't matter.

Note that compiler-rt and libc also use GTest in non-default
configrations. libc also depends on LLVM's GTest build (and would
error-out if unavailable), but compiler-rt builds it completely
different.

Fixes #143134

2 files changed, 15 insertions, 11 deletions

diff --git a/offload/CMakeLists.txt b/offload/CMakeLists.txt
index 0a441c3bc578..d49069f6eb42 100644
--- a/offload/CMakeLists.txt
+++ b/offload/CMakeLists.txt
@@ -380,15 +380,5 @@ add_subdirectory(liboffload)
 # Add tests.
 if(OFFLOAD_INCLUDE_TESTS)
   add_subdirectory(test)
-
-  # Add unit tests if GMock/GTest is present
-  if(NOT LLVM_THIRD_PARTY_DIR)
-    set(LLVM_THIRD_PARTY_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../third-party")
-  endif()
-  if(EXISTS ${LLVM_THIRD_PARTY_DIR}/unittest AND NOT TARGET llvm_gtest)
-    add_subdirectory(${LLVM_THIRD_PARTY_DIR}/unittest ${CMAKE_CURRENT_BINARY_DIR}/third-party/unittest)
-  endif()
-  if(TARGET llvm_gtest)
-    add_subdirectory(unittests)
-  endif()
+  add_subdirectory(unittests)
 endif()
diff --git a/offload/unittests/CMakeLists.txt b/offload/unittests/CMakeLists.txt
index 1eae785b8765..388d15f834b1 100644
--- a/offload/unittests/CMakeLists.txt
+++ b/offload/unittests/CMakeLists.txt
@@ -1,6 +1,20 @@
 add_custom_target(OffloadUnitTests)
 set_target_properties(OffloadUnitTests PROPERTIES FOLDER "Tests/UnitTests")
 
+if (CMAKE_CROSSCOMPILING)
+  # TODO: It is possible that LLVM_GTEST_RUN_UNDER defines an emulator or
+  #       ssh remote command invocation; for this case provide an option to
+  #       enable unittests.
+  message(STATUS "Offload unittests disabled because we are cross-compiling")
+  return ()
+endif ()
+
+if (NOT TARGET llvm_gtest)
+  message(WARNING "Offload unittests disabled due to GTest being unavailable; "
+                  "Try LLVM_INSTALL_GTEST=ON for the LLVM build")
+  return ()
+endif ()
+
 function(add_offload_test_device_code test_filename test_name)
   set(SRC_PATH ${CMAKE_CURRENT_SOURCE_DIR}/${test_filename})
   set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)