[libc][CMake] Fix build issues of libc on Windows

[Lancern]

Due to an issue mentioned in llvm/projects/CMakeLists.txt, libc build is disabled by default when building with clang-cl on Windows. This PR sets LLVM_FORCE_BUILD_RUNTIME to ON to bypass this limit for libc and make libc build with clang-cl on Windows.

[llvmbot]

@llvm/pr-subscribers-libc

Author: Sirui Mu (Lancern)

Changes

Due to an issue mentioned in llvm/projects/CMakeLists.txt, libc build is disabled by default when building with clang-cl on Windows. This PR sets LLVM_FORCE_BUILD_RUNTIME to ON to bypass this limit for libc and make libc build with clang-cl on Windows.

---

Full diff: https://github.com/llvm/llvm-project/pull/102499.diff

1 Files Affected:

• (modified) llvm/cmake/modules/CrossCompile.cmake (+6)

diff --git a/llvm/cmake/modules/CrossCompile.cmake b/llvm/cmake/modules/CrossCompile.cmake
index 39b4abaa0d9313..e36a71f522d82c 100644
--- a/llvm/cmake/modules/CrossCompile.cmake
+++ b/llvm/cmake/modules/CrossCompile.cmake
@@ -71,6 +71,12 @@ function(llvm_create_cross_target project_name target_name toolchain buildtype)
 
   if("libc" IN_LIST LLVM_ENABLE_PROJECTS AND NOT LIBC_HDRGEN_EXE)
     set(libc_flags -DLLVM_LIBC_FULL_BUILD=ON -DLIBC_HDRGEN_ONLY=ON)
+    if(MSVC)
+      # Due to some issues mentioned in llvm/projects/CMakeLists.txt, libc build is disabled by
+      # default in the cross target when building with MSVC compatible compilers on Windows. Add
+      # LLVM_FORCE_BUILD_RUNTIME to bypass this issue and force its building on Windows.
+      list(APPEND libc_flags -DLLVM_FORCE_BUILD_RUNTIME=ON)
+    endif()
   endif()
 
   add_custom_command(OUTPUT ${${project_name}_${target_name}_BUILD}/CMakeCache.txt

[SchrodingerZhu]

Another way as we tested is to add libc to LLVM_ENABLE_RUNTIMES and use runtimes as the source directory instead of llvm.