[clang] [llvm] Skipping host target exports during cross-compilation (PR #209922)

[via cfe-commits]

https://github.com/zeroomega created https://github.com/llvm/llvm-project/pull/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 cross compile runtimes for baremetal targets like armv6m-none-eabi. This patch skips including the host target exports when doing cross-compiling in any LLVM sub builds to solve this issue.

>From 5ac5d847ef9e4f14de68aa381420b27033f5b37d Mon Sep 17 00:00:00 2001
From: Haowei Wu <haowei at google.com>
Date: Wed, 15 Jul 2026 16:41:32 -0700
Subject: [PATCH] Skipping host target exports during cross-compilation

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 cross compile
runtimes for baremetal targets like armv6m-none-eabi. This patch
skips including the host target exports when doing cross-compiling
in any LLVM sub builds to solve this issue.
---
 clang/cmake/modules/ClangConfig.cmake.in | 5 ++++-
 llvm/cmake/modules/LLVMConfig.cmake.in   | 2 +-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/clang/cmake/modules/ClangConfig.cmake.in b/clang/cmake/modules/ClangConfig.cmake.in
index 68f723d050117..e199c7e17b6b7 100644
--- a/clang/cmake/modules/ClangConfig.cmake.in
+++ b/clang/cmake/modules/ClangConfig.cmake.in
@@ -13,7 +13,10 @@ set(CLANG_LINK_CLANG_DYLIB "@CLANG_LINK_CLANG_DYLIB@")
 set(CLANG_DEFAULT_LINKER "@CLANG_DEFAULT_LINKER@")
 
 # Provide all our library targets to users.
- at CLANG_CONFIG_INCLUDE_EXPORTS@
+# Skip when cross-compiling, as host library targets are not usable.
+if(NOT CMAKE_CROSSCOMPILING)
+  @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
diff --git a/llvm/cmake/modules/LLVMConfig.cmake.in b/llvm/cmake/modules/LLVMConfig.cmake.in
index 300c25e7c6101..60e155acbd9ba 100644
--- a/llvm/cmake/modules/LLVMConfig.cmake.in
+++ b/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 CMAKE_CROSSCOMPILING)
   @LLVM_CONFIG_INCLUDE_EXPORTS@
   @llvm_config_include_buildtree_only_exports@
 endif()