Embedded compilation with LLVM/clang

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1

I am trying to cross-compile a embedded micro controller code using clang using the following flake:

{
  inputs = {
    nixpkgs = {
      url = "github:NixOS/nixpkgs/nixos-unstable";
    };
  };

  outputs = { self, nixpkgs, flake-utils, ...}@inputs:
    flake-utils.lib.eachDefaultSystem (system:
      let
        local_overlay = final: prev: {
        };

        pkgs = import nixpkgs {
          overlays = [ local_overlay ];

          localSystem = system;
          crossSystem = system;
        };

        crossPkgs = import nixpkgs {
          overlays = [ local_overlay ];

          localSystem = system;
          crossSystem = {
            config = "arm-none-eabi";
          };
        };

        mve_code = pkgs.writeText "main.c" ''
          int main()
          {
            return 0;
          }
        '';
    in {
    packages = rec {
      default = test;
      test = crossPkgs.stdenv.mkDerivation {
        pname = "test";
        version = "0.0.0";

        src = mve_code;

        nativeBuildInputs = [
          crossPkgs.clang
        ];

        unpackPhase = ''
          cp $src ./main.c
        '';

        buildPhase = ''
          $CC -c main.c -o out
        '';

        installPhase = ''
          mkdir -p $out
          cp out $out/out
        '';
      };
    };
  });
}

This works if I switch crossPkgs.clang for crossPkgs.gcc, in which case it correctly uses arm-none-eabi-gcc as compiler. With clang it fails with the following error message:

error: builder for ‘/nix/store/vgmi9qlq5c4lwwvh4jwcplg9xh7w1hdh-compiler-rt-libc-arm-none-eabi-21.1.2.drv’ failed with exit code 1;
last 25 log lines:
– Looking for unwind.h - found
– Looking for rpc/xdr.h
– Looking for rpc/xdr.h - not found
– Performing Test COMPILER_RT_HAS_NO_DEFAULT_CONFIG_FLAG
– Performing Test COMPILER_RT_HAS_NO_DEFAULT_CONFIG_FLAG - Failed
CMake Warning at cmake/Modules/CompilerRTUtils.cmake:307 (message):
LLVM source tree not found at “/build/compiler-rt-src-21.1.2/llvm”. You
are not using the monorepo layout. This configuration is DEPRECATED.
Call Stack (most recent call first):
CMakeLists.txt:110 (load_llvm_config)

– Could NOT find FFI (missing: FFI_LIBRARIES HAVE_FFI_CALL)
– Could NOT find ZLIB (missing: ZLIB_LIBRARY ZLIB_INCLUDE_DIR)
– Could NOT find LibXml2 (missing: LIBXML2_LIBRARY LIBXML2_INCLUDE_DIR)
CMake Error at /nix/store/slaal54bs9c74mf0h3fr45hyvbcfs9mb-llvm-21.1.2-dev/lib/cmake/llvm/LLVMExports.cmake:1455 (add_library):
ADD_LIBRARY called with SHARED option but the target platform does not
support dynamic linking.
Call Stack (most recent call first):
/nix/store/slaal54bs9c74mf0h3fr45hyvbcfs9mb-llvm-21.1.2-dev/lib/cmake/llvm/LLVMConfig.cmake:357 (include)
cmake/Modules/CompilerRTUtils.cmake:312 (find_package)
CMakeLists.txt:110 (load_llvm_config)

– Configuring incomplete, errors occurred!
For full logs, run:
nix log /nix/store/vgmi9qlq5c4lwwvh4jwcplg9xh7w1hdh-compiler-rt-libc-arm-none-eabi-21.1.2.drv
error: 1 dependencies of derivation ‘/nix/store/6bhrsv9drglf3f157w95ihxkbmwwkjgj-arm-none-eabi-clang-wrapper-21.1.2.drv’ failed to build
error: 1 dependencies of derivation ‘/nix/store/nb5p70f6c6kyy81xz3k6l31knzys27rk-test-arm-none-eabi-0.0.0-env.drv’ failed to build

As far as I can see this is a problem with LTO still being build as shared library, as
/nix/store/slaal54bs9c74mf0h3fr45hyvbcfs9mb-llvm-21.1.2-dev/lib/cmake/llvm/LLVMExports.cmake:1455 is

# Create imported target LTO
add_library(LTO SHARED IMPORTED)

while I would expect this to be STATIC.

I get the same error when using x86_64-elf instead of arm-none-eabi, which makes me believe this is not arm specific. It also persists when setting crossSystem explicitly to static using isStatic = true;.

Hopefully someone has any ideas or can point me in the right direction to get this working. It feels like this is just missing a cmake parameter somewhere in LLVM. I didn’t see anything missing when I looked at the LLVM-package, but I can’t claim to fully understand it currently.