Reapply "Skipping host target exports during cross-compilation" (#210… (#210496)

This reverts commit 42eb7db188. This
reland 3fbb037d03 (#209922).

Under CMake 4+, calling add_library(... SHARED IMPORTED) on a target
platform that lacks dynamic linking support triggers a fatal error. This
becomes an issue when building LLVM and runtimes for baremetal targets
like armv6m-none-eabi. This patch adds the option
"LLVM_OMIT_EXPORTS_FROM_CONFIG" in LLVM. When used in sub builds like
LLVM runtimes, it makes CMake to skip including the LLVM and Clang
exports. This mitigates the CMake 4 errors on baremetal runtimes.

42eb7db188 originally
apply this flag on all runtimes build and causing issues on runtimes
like intel-sycl-gpu, amdgpu-offload-ubuntu-22-cmake-build-only.
This PR only set this flag when the target platform lacks shared library
support.

clang/cmake/modules/ClangConfig.cmake.in

@@ -12,8 +12,11 @@ set(CLANG_INCLUDE_DIRS "@CLANG_CONFIG_INCLUDE_DIRS@")
 set(CLANG_LINK_CLANG_DYLIB "@CLANG_LINK_CLANG_DYLIB@")
 set(CLANG_DEFAULT_LINKER "@CLANG_DEFAULT_LINKER@")
 
-# Provide all our library targets to users.
+# Provide all our library targets to users. Skip when
-@CLANG_CONFIG_INCLUDE_EXPORTS@
+# LLVM_OMIT_EXPORTS_FROM_CONFIG flag is set.
+if(NOT LLVM_OMIT_EXPORTS_FROM_CONFIG)
+  @CLANG_CONFIG_INCLUDE_EXPORTS@
+endif()
 
 # By creating clang-tablegen-targets here, subprojects that depend on Clang's
 # tablegen-generated headers can always depend on this target whether building

compiler-rt/cmake/Modules/CompilerRTUtils.cmake

@@ -313,6 +313,16 @@ macro(load_llvm_config)
       "You are not using the monorepo layout. This configuration is DEPRECATED.")
   endif()
 
+  # Exports from LLVM and Clang may contain shared libraries. When targeting
+  # platforms that lack shared library support, importing such
+  # exports unrestrictedly will trigger an error. Set
+  # LLVM_OMIT_EXPORTS_FROM_CONFIG flag to skip importing these exports
+  # when the target platform does not support shared libraries.
+  get_property(HAS_SHARED_SUPPORT GLOBAL PROPERTY TARGET_SUPPORTS_SHARED_LIBS)
+  message(NOTICE "HAS_SHARED_SUPPORT is ${HAS_SHARED_SUPPORT}")
+  if (NOT HAS_SHARED_SUPPORT)
+    set(LLVM_OMIT_EXPORTS_FROM_CONFIG ON)
+  endif()
   find_package(LLVM HINTS "${LLVM_CMAKE_DIR}")
   if (NOT LLVM_FOUND)
      message(WARNING "UNSUPPORTED COMPILER-RT CONFIGURATION DETECTED: "

llvm/cmake/modules/LLVMConfig.cmake.in

@@ -152,7 +152,7 @@ set(LLVM_ENABLE_SHARED_LIBS @BUILD_SHARED_LIBS@)
 set(LLVM_DEFAULT_EXTERNAL_LIT "@LLVM_CONFIG_DEFAULT_EXTERNAL_LIT@")
 set(LLVM_LIT_ARGS "@LLVM_LIT_ARGS@")
 
-if(NOT TARGET LLVMSupport)
+if(NOT TARGET LLVMSupport AND NOT LLVM_OMIT_EXPORTS_FROM_CONFIG)
   @LLVM_CONFIG_INCLUDE_EXPORTS@
   @llvm_config_include_buildtree_only_exports@
 endif()

runtimes/CMakeLists.txt

@@ -76,6 +76,17 @@ if(LLVM_BINARY_DIR)
   list(APPEND llvm_search_paths "${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
   list(APPEND clang_search_paths "${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/clang")
 endif()
+
+# Exports from LLVM and Clang may contain shared libraries. When targeting
+# platforms that lack shared library support, importing such
+# exports unrestrictedly will trigger an error. Set
+# LLVM_OMIT_EXPORTS_FROM_CONFIG flag to skip importing these exports
+# when the target platform does not support shared libraries.
+get_property(HAS_SHARED_SUPPORT GLOBAL PROPERTY TARGET_SUPPORTS_SHARED_LIBS)
+message(NOTICE "HAS_SHARED_SUPPORT is ${HAS_SHARED_SUPPORT}")
+if (NOT HAS_SHARED_SUPPORT)
+  set(LLVM_OMIT_EXPORTS_FROM_CONFIG ON)
+endif()
 find_package(LLVM PATHS ${llvm_search_paths} NO_DEFAULT_PATH NO_CMAKE_FIND_ROOT_PATH)
 find_package(Clang PATHS ${clang_search_paths} NO_DEFAULT_PATH NO_CMAKE_FIND_ROOT_PATH)