How to modify cmakeFlags in a library in llvmPackages in a cross-compile flake

RossComputerGuy · September 21, 2025, 11:57pm

Hi, I actually have to do this to get a buildable LLVM kernel on aarch64-linux. If you look here, you can see that overriding libraries as well as the toplevel attribute of what you want is necessary.

github.com/RossComputerGuy/dotfiles

flake.nix

fd373d3ef


      
          llvmPackages =
            prev.llvmPackages
            // (
              let
                libraries = prev.llvmPackages.libraries.extend (
                  f: p: {
                    compiler-rt-no-libc = p.compiler-rt-no-libc.overrideAttrs (
                      f: p: {
                        cmakeFlags =
                          p.cmakeFlags
                          ++ lib.optional (final.stdenv.hostPlatform.isAarch64 && final.stdenv.hostPlatform.useLLVM) (
                            lib.cmakeBool "COMPILER_RT_DISABLE_AARCH64_FMV" true
                          );
                      }
                    );
                  }
                );
              in
              libraries // { inherit libraries; }
            );

However, this isn’t an ideal thing and the LLVM team (includes myself) are looking to make LLVM more override friendly. This PR would greatly make what you’re doing much easier. pkgs/development/compilers/llvm: add replacement entrypoint using mak… by RossComputerGuy · Pull Request #436350 · NixOS/nixpkgs · GitHub