[libcxx] cannot build with clang++ --target=x86_64-windows-msvc

[trcrsired]

hello, No known features for the CXX compiler. What's going on?

-- Performing Test CXX_SUPPORTS_WNO_ERROR_FLAG
-- Performing Test CXX_SUPPORTS_WNO_ERROR_FLAG - Failed
-- Performing Test CXX_SUPPORTS_EHSC_FLAG
-- Performing Test CXX_SUPPORTS_EHSC_FLAG - Failed
-- Configuring done (11.3s)
CMake Error in /home/csrg/toolchains_build/llvm-project/libcxx/src/CMakeLists.txt:
  No known features for CXX compiler

  "Clang"

  version 20.0.0.


CMake Generate step failed.  Build files cannot be regenerated correctly.
runtimesconfigure build failure

[trcrsired]

-- Could NOT find LLVM (missing: LLVM_DIR)
-- Could NOT find Clang (missing: Clang_DIR)
-- LLVM host triple: x86_64-unknown-linux-gnu
-- LLVM default target triple: x86_64-unknown-linux-gnu
-- Configuring for clang-cl
-- Using libc++ testing configuration: /home/csrg/toolchains_build/llvm-project/libcxx/test/configs/llvm-libc++-shared-clangcl.cfg.in
-- Configuring done (0.2s)
CMake Error in /home/csrg/toolchains_build/llvm-project/libcxx/src/CMakeLists.txt:
  No known features for CXX compiler

  "Clang"

  version 20.0.0.

clang-cl isn't correct since I am cross compiling on linux with just normal clang

[trcrsired]

@ldionne

[trcrsired]

clang: error: no such file or directory: '/clang:--target=x86_64-unknown-linux-gnu'
clang: error: no such file or directory: '/clang:-print-target-triple'
clang: error: no input files
CMake Warning at CMakeLists.txt:201 (message):
  Failed to execute
  `/home/csrg/toolchains/llvm/x86_64-generic-linux-gnu/llvm/bin/clang
  /clang:--target=x86_64-unknown-linux-gnu /clang:-print-target-triple` to
  normalize target triple.


-- Configuring for clang-cl

[trcrsired]

diff --git a/runtimes/CMakeLists.txt b/runtimes/CMakeLists.txt
index 830165c799c2..28050c03fc3e 100644
--- a/runtimes/CMakeLists.txt
+++ b/runtimes/CMakeLists.txt
@@ -185,7 +185,7 @@ set(LLVM_TARGET_TRIPLE "${LLVM_DEFAULT_TARGET_TRIPLE}")
 
 if(CMAKE_C_COMPILER_ID MATCHES "Clang")
   set(option_prefix "")
-  if (CMAKE_C_SIMULATE_ID MATCHES "MSVC")
+  if (CMAKE_CXX_COMPILER_FRONTEND_VARIANT MATCHES "MSVC")
     set(option_prefix "/clang:")
   endif()
   set(print_target_triple ${CMAKE_C_COMPILER} ${option_prefix}--target=${LLVM_DEFAULT_TARGET_TRIPLE} ${option_prefix}-print-target-triple)

I think this should be CMAKE_CXX_COMPILER_FRONTEND_VARIANT right?

[androm3da]

I can also reproduce this problem. I have bisected it to 6c4267f - cc @michaelrj-google

If I apply this change to main (or a similar one to 6c4267f), my cmake succeeds:

diff --git a/runtimes/cmake/Modules/FindLibcCommonUtils.cmake b/runtimes/cmake/Modules/FindLibcCommonUtils.cmake
index 0e65fdff7c34..562639a619f0 100644
--- a/runtimes/cmake/Modules/FindLibcCommonUtils.cmake
+++ b/runtimes/cmake/Modules/FindLibcCommonUtils.cmake
@@ -14,6 +14,6 @@ if(NOT TARGET llvm-libc-common-utilities)
     # adding the root "libc" directory to the include path.
     target_include_directories(llvm-libc-common-utilities INTERFACE ${libc_path})
     target_compile_definitions(llvm-libc-common-utilities INTERFACE LIBC_NAMESPACE=__llvm_libc_common_utils)
-    target_compile_features(llvm-libc-common-utilities INTERFACE cxx_std_17)
+#   target_compile_features(llvm-libc-common-utilities INTERFACE cxx_std_17)
   endif()
 endif()

Here's how I am invoking cmake and how it fails without the above change:

+ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DLLVM_CMAKE_DIR:PATH=/home/brian/src/toolchain_for_hexagon/clang+llvm-20.0.0-cross-hexagon-unknown-linux-musl/x86_64-linux-gnu/lib -DCMAKE_INSTALL_PREFIX:PATH=/home/brian/src/toolchain_for_hexagon/clang+llvm-20.0.0-cross-hexagon-unknown-linux-musl/x86_64-linux-gnu/target/hexagon-unknown-linux-musl/usr -DCMAKE_CROSSCOMPILING:BOOL=ON -DCMAKE_CXX_COMPILER_FORCED:BOOL=ON -C ../hexagon-linux-cross.cmake -C ../hexagon-linux-runtimes.cmake -C ../hexagon-linux-clangrt.cmake -B ../obj_libs_repro -S ./runtimes
...
CMake Error in /home/brian/src/toolchain_for_hexagon/llvm-project/libcxx/src/CMakeLists.txt:
  No known features for CXX compiler

  "Clang"

  version 20.0.0.

[michaelrj-google]

Hi, sorry about the slow response, was out for the holidays.

Commenting out the cxx_std line shouldn't break anything, so it should be fine as a temporary fix. If you still need this fixed, please send a PR doing that and CC me, then file a separate issue so we can track this on the libc side.

[androm3da]

Hi, sorry about the slow response, was out for the holidays.

It's understood - I did not expect a reply during late December / early Jan. 😉

Commenting out the cxx_std line shouldn't break anything, so it should be fine as a temporary fix. If you still need this fixed, please send a PR doing that and CC me, then file a separate issue so we can track this on the libc side.

Ok I think I've done this.

[androm3da]

@trcrsired can you confirm whether you see the same behavior in your case regarding the patch in #121883 or the commit where the problem starts?

It's suggested that my report is perhaps distinct from this issue you've reported. So if that workaround is not effective for you then I think it would make sense for me to open a separate issue.