[Offload] Correctly reject building on unsupported architectures (#92276)

Summary:
Previously we had this `LIBOMPTARGET_ENABLED` variable which controlled
including `libomptarget`. This is now redundant since it's controlled by
`LLVM_ENABLE_RUNTIMES`. However, this had the extra effect of not
building it when given unsupported targets. THis was lost during the
move to `offload`. This patch moves this logic back and makes the
`offload` target just quit without doing anything if used on an
unsupported architecture.

#91881
#91819

---------

Co-authored-by: Sylvestre Ledru <[email protected]>

Author: jhuber6

offload/CMakeLists.txt

@@ -17,26 +17,16 @@ if ("${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_CURRENT_SOURCE_DIR}")
   project(offload C CXX ASM)
 endif()
 
-set(ENABLE_LIBOMPTARGET ON)
-# Currently libomptarget cannot be compiled on Windows or MacOS X.
-# Since the device plugins are only supported on Linux anyway,
-# there is no point in trying to compile libomptarget on other OSes.
-# 32-bit systems are not supported either.
-if (APPLE OR WIN32 OR NOT "cxx_std_17" IN_LIST CMAKE_CXX_COMPILE_FEATURES OR NOT CMAKE_SIZEOF_VOID_P EQUAL 8)
-  set(ENABLE_LIBOMPTARGET OFF)
-endif()
-
-option(OPENMP_ENABLE_LIBOMPTARGET "Enable building libomptarget for offloading."
-       ${ENABLE_LIBOMPTARGET})
-if (OPENMP_ENABLE_LIBOMPTARGET)
-  # Check that the library can actually be built.
-  if (APPLE OR WIN32)
-    message(FATAL_ERROR "libomptarget cannot be built on Windows and MacOS X!")
-  elseif (NOT "cxx_std_17" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
-    message(FATAL_ERROR "Host compiler must support C++17 to build libomptarget!")
-  elseif (NOT CMAKE_SIZEOF_VOID_P EQUAL 8)
-    message(FATAL_ERROR "libomptarget on 32-bit systems are not supported!")
-  endif()
+# Check that the library can actually be built.
+if(APPLE OR WIN32 OR WASM)
+  message(WARNING "libomptarget cannot be built on Windows and MacOS X!")
+  return()
+elseif(NOT "cxx_std_17" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
+  message(WARNING "Host compiler must support C++17 to build libomptarget!")
+  return()
+elseif(NOT CMAKE_SIZEOF_VOID_P EQUAL 8)
+  message(WARNING "libomptarget on 32-bit systems is not supported!")
+  return()
 endif()
 
 if(OPENMP_STANDALONE_BUILD)

openmp/CMakeLists.txt

@@ -97,18 +97,6 @@ set(OPENMP_TEST_FLAGS "" CACHE STRING
 set(OPENMP_TEST_OPENMP_FLAGS ${OPENMP_TEST_COMPILER_OPENMP_FLAGS} CACHE STRING
   "OpenMP compiler flag to use for testing OpenMP runtime libraries.")
 
-set(ENABLE_LIBOMPTARGET ON)
-# Currently libomptarget cannot be compiled on Windows or MacOS X.
-# Since the device plugins are only supported on Linux anyway,
-# there is no point in trying to compile libomptarget on other OSes.
-# 32-bit systems are not supported either.
-if (APPLE OR WIN32 OR WASM OR NOT "cxx_std_17" IN_LIST CMAKE_CXX_COMPILE_FEATURES
-    OR NOT CMAKE_SIZEOF_VOID_P EQUAL 8 OR ${CMAKE_SYSTEM_NAME} MATCHES "AIX")
-  set(ENABLE_LIBOMPTARGET OFF)
-endif()
-
-option(OPENMP_ENABLE_LIBOMPTARGET "Enable building libomptarget for offloading."
-       ${ENABLE_LIBOMPTARGET})
 option(OPENMP_ENABLE_LIBOMP_PROFILING "Enable time profiling for libomp." OFF)
 
 # Header install location