[libomptarget]Add a trap for external omptarget from LLVM

[ye-luo]

Summary

Old LLVM installation may expose its internal omptarget CMake target when being used by find_package(LLVM) and caused issues in the CMake of libomptarget that is being built. Trap the issue early.

[tianshilei1992]

LGTM

openmp/libomptarget/DeviceRTL/CMakeLists.txt

36

This two lines can go in a NFC patch w/o review.

openmp/libomptarget/DeviceRTL/CMakeLists.txt

 
 if (NOT LIBOMPTARGET_BUILD_DEVICERTL_BCLIB)
   libomptarget_say("Not building DeviceRTL: Disabled by LIBOMPTARGET_BUILD_DEVICERTL_BCLIB")
   return()
 endif()
 
 if (LLVM_DIR)
   # Builds that use pre-installed LLVM have LLVM_DIR set.
+  # A standalone or LLVM_ENABLE_RUNTIMES=openmp build takes this route
   find_program(CLANG_TOOL clang PATHS ${LLVM_TOOLS_BINARY_DIR} NO_DEFAULT_PATH)
   find_program(LINK_TOOL llvm-link PATHS ${LLVM_TOOLS_BINARY_DIR}
     NO_DEFAULT_PATH)
   find_program(OPT_TOOL opt PATHS ${LLVM_TOOLS_BINARY_DIR} NO_DEFAULT_PATH)
   if ((NOT CLANG_TOOL) OR (NOT LINK_TOOL) OR (NOT OPT_TOOL))
     libomptarget_say("Not building DeviceRTL. Missing clang: ${CLANG_TOOL}, llvm-link: ${LINK_TOOL} or opt: ${OPT_TOOL}")
     return()
   else()
     libomptarget_say("Building DeviceRTL. Using clang: ${CLANG_TOOL}, llvm-link: ${LINK_TOOL} and opt: ${OPT_TOOL}")
   endif()
 elseif (LLVM_TOOL_CLANG_BUILD AND NOT CMAKE_CROSSCOMPILING AND NOT OPENMP_STANDALONE_BUILD)
   # LLVM in-tree builds may use CMake target names to discover the tools.
+  # A LLVM_ENABLE_PROJECTS=openmp build takes this route
   set(CLANG_TOOL $<TARGET_FILE:clang>)
   set(LINK_TOOL $<TARGET_FILE:llvm-link>)
   set(OPT_TOOL $<TARGET_FILE:opt>)
   libomptarget_say("Building DeviceRTL. Using clang from in-tree build")
 else()
   libomptarget_say("Not building DeviceRTL. No appropriate clang found")
   return()
 endif()

openmp/libomptarget/cmake/Modules/LibomptargetGetDependencies.cmake

if (OPENMP_STANDALONE_BUILD)
   # Complete LLVM package is required for building libomptarget
   # in an out-of-tree mode.
   find_package(LLVM REQUIRED)
   message(STATUS "Found LLVM ${LLVM_PACKAGE_VERSION}")
   message(STATUS "Using LLVM in: ${LLVM_DIR}")
   list(APPEND LIBOMPTARGET_LLVM_INCLUDE_DIRS ${LLVM_INCLUDE_DIRS})
   list(APPEND CMAKE_MODULE_PATH ${LLVM_CMAKE_DIR})
   include(AddLLVM)
+  if(TARGET omptarget)
+    message(FATAL_ERROR "CMake target 'omptarget' already exists. "
+                        "Use an LLVM installation that doesn't expose its 'omptarget' target.")
+  endif()
 else()
   # Note that OPENMP_STANDALONE_BUILD is FALSE, when
   # openmp is built with -DLLVM_ENABLE_RUNTIMES="openmp" vs
   # -DLLVM_ENABLE_PROJECTS="openmp", but openmp build
   # is actually done as a standalone project build with many
   # LLVM CMake variables propagated to it.
   list(APPEND LIBOMPTARGET_LLVM_INCLUDE_DIRS
     ${LLVM_MAIN_INCLUDE_DIR} ${LLVM_BINARY_DIR}/include