depot/third_party/nixpkgs/pkgs/development/compilers/llvm/multi.nix

{ runCommand,
clang,
gcc64,
gcc32,
glibc_multi
}:

let
  combine = basegcc: runCommand "combine-gcc-libc" {} ''
    mkdir -p $out
    cp -r ${basegcc.cc}/lib $out/lib

    chmod u+rw -R $out/lib
    cp -r ${basegcc.libc}/lib/* $(ls -d $out/lib/gcc/*/*)
  '';
  gcc_multi_sysroot = runCommand "gcc-multi-sysroot" {
    passthru = {
      inherit (gcc64) version;
      lib = gcc_multi_sysroot;
    };
  } ''
    mkdir -p $out/lib{,64}/gcc

    ln -s ${combine gcc64}/lib/gcc/* $out/lib64/gcc/
    ln -s ${combine gcc32}/lib/gcc/* $out/lib/gcc/
    # XXX: This shouldn't be needed, clang just doesn't look for "i686-unknown"
    ln -s $out/lib/gcc/i686-unknown-linux-gnu $out/lib/gcc/i686-pc-linux-gnu


    # includes
    mkdir -p $out/include
    ln -s ${glibc_multi.dev}/include/* $out/include
    ln -s ${gcc64.cc}/include/c++ $out/include/c++

    # dynamic linkers
    mkdir -p $out/lib/32
    ln -s ${glibc_multi.out}/lib/ld-linux* $out/lib
    ln -s ${glibc_multi.out}/lib/32/ld-linux* $out/lib/32/
  '';

  clangMulti = clang.override {
    # Only used for providing expected structure re:dynamic linkers, AFAIK Most
    # of the magic is done by setting the --gcc-toolchain option via
    # `gccForLibs`.
    libc = gcc_multi_sysroot;

    bintools = clang.bintools.override {
      libc = gcc_multi_sysroot;
    };

    gccForLibs = gcc_multi_sysroot // {
      inherit (glibc_multi) libgcc;
      langCC =
        assert (gcc64.cc.langCC != gcc32.cc.langCC)
               -> throw "(gcc64.cc.langCC=${gcc64.cc.langCC}) != (gcc32.cc.langCC=${gcc32.cc.langCC})";
        gcc64.cc.langCC;
    };
  };

in clangMulti