Help with nix flake shell for LLVM

Development
1

For LLVM engineers out there, how do you set up the nix flake shell? I want to be able to build a clang that can run on nixos, and also a clang that can run on Linux systems.

The way I’m setting it up now means that everytime I make a change, the whole tree is built afresh.

        clang_dev = pkgs.wrapCC ( pkgs.stdenv.mkDerivation rec {
          pname = "llvm-project";
          version = "dev";

          src = builtins.path { path = "...../src/llvm-project/build"; };
          dontStrip = true;

          installPhase = ''
            mkdir -p $out/bin
            cp bin/clang $out/bin
            cp bin/opt $out/bin
            cp bin/llc $out/bin
            cp -r lib $out/lib
          '';

          passthru.isClang = true;
        });

and later

        devShell = with pkgs; mkShell {
          nativeBuildInputs = [
            clang_dev
          ];
}
2

I’ve found nix develop nixpkgs#llvmPackages.llvm does a good job. Maybe we’ll come up with a better approach in the future that covers all of LLVM but right now, this has been a setup which has worked for me.

3

Thanks for your prompt reply. Using this devshell I am able to build clang, but when I’m using it to compile any C files, it couldn’t find libc file headers. I know that even after complication there might still be issue with finding dynamic libraries.

I know I probably need to use pkgs.wrapCC, but what’s the best way to instrument it? Or if your setup is different, how is yours done?

4

As to be expected, unwrapped compilers don’t have the injected flags.

I don’t use pkgs.wrapCC for clang testing. I build it inside of Nix and then call clang. Right now, clang requires quite a bit of magic to wrap it properly. You probably could write a shell script which generates a derivation which then produces a wrapped clang but it’ll be messy. The other option is to cat the wrapped clang and pull the shell script out and edit in your built clang.

5

I build it inside of Nix and then call clang.
Can you elaborate on your workflow? I think it will be helpful for the community if you can share your flake file for your LLVM setup.

6

I don’t have a Flake file for LLVM development, like I mentioned before I just do nix develop nixpkgs#llvmPackages.llvm. I don’t really work on Clang, I’ve worked on Bolt but not the other components.