Add Nix CI

.github/workflows/ci.yml

@@ -38,3 +38,12 @@ jobs:
       run: |
         export PATH="/usr/lib/ccache:/usr/local/opt/ccache/libexec:$PATH"
         make test -j$(nproc)
+  build-with-nix:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+      - uses: cachix/install-nix-action@v13
+        with:
+          nix_path: nixpkgs=channel:nixos-stable
+      - name: Build the CI package set
+        run: nix-build default.nix -A ciPackages

README.md

@@ -122,6 +122,31 @@ String dump of section '.comment':
 
 If `mold` is in `.comment`, the file is created by mold.
 
+### Use with Nix
+
+The `default.nix` of this repository provides an easy method to build
+packages with mold as linker. General usage is `nix-build -A $attribute`,
+the following attributes are provided:
+
+- `pkgs`: A baseline package set for comparison. Sometimes packages are simply
+  broken and mold has nothing to do with it. Use `pkgs.packageName` to build specific
+  packages, and `nix search` or https://search.nixos.org/packages to find the
+  attribute name.
+- `pkgsMold`: Equivalent to `pkgs`, except that `bfd` is replaced by mold as linker.
+  This has a few implications that are important to know:
+  - This will bootstrap the entire toolchain and build all transitive dependencies every
+    time mold changes.
+  - Packages that require a custom linker (like lld or some special GCC) won't be built with
+    mold.
+- `pkgsMoldCI`: Equivalent to `pkgsMold`, except that mold is disabled for known failing
+  packages until they are resolved.
+- `ciPackages`: A set of working packages that are built as part of the CI in order to catch
+  regressions.
+
+After a successful build, `./result` will be a symlink to the output.
+You can install Nix alongside your usual package manager, see https://nixos.org/download.html.
+There are also Docker images providing a local Nix installation.
+
 # Design and implementation of mold
 
 For the rest of this documentation, I'll explain the design and the

default.nix

@@ -0,0 +1,69 @@
+let
+  # Pin some fairly new nixpkgs
+  sources = builtins.fetchTarball {
+    name = "nixpkgs-unstable-2021-07-06";
+    url = "https://github.com/nixos/nixpkgs/archive/291b3ff5af268eb7a656bb11c73f2fe535ea2170.tar.gz";
+    sha256 = "1z2l7q4cmiaqb99cd8yfisdr1n6xbwcczr9020ss47y2z1cn1x7x";
+  };
+
+  # For bootstrapping
+  pkgs = import sources {
+    overlays = [
+      (pkgs: super: {
+        # TODO replace with proper packaging once https://github.com/NixOS/nixpkgs/pull/128889 is merged
+        mold = pkgs.stdenv.mkDerivation {
+          pname = "mold";
+          version = "0.9.1";
+          src = ./.;
+          nativeBuildInputs = with pkgs; [ clang_12 cmake lld_12 tbb xxHash zlib openssl git ];
+          dontUseCmakeConfigure = "true";
+          buildPhase = "make -j $NIX_BUILD_CORES";
+          installPhase = "mkdir -p $out $out/bin $out/share/man/man1 && PREFIX=$out make install";
+        };
+
+        binutils_mold = pkgs.wrapBintoolsWith {
+          bintools = pkgs.binutils-unwrapped.overrideAttrs (old: {
+            postInstall = ''
+              rm $out/bin/ld.gold
+              rm $out/bin/ld.bfd
+              ln -sf ${pkgs.mold}/bin/mold $out/bin/ld.bfd
+            '';
+          });
+        };
+
+        stdenv_mold = super.overrideCC super.stdenv (super.wrapCCWith rec {
+          cc = super.gcc-unwrapped;
+          bintools = pkgs.binutils_mold;
+        });
+      })
+    ];
+  };
+
+  # Actual nixpkgs with patched linker in all packages
+  pkgsMold = import sources {
+    overlays = [
+      (self: super: {
+        stdenv = pkgs.stdenv_mold;
+        mold = pkgs.mold;
+      })
+    ];
+  };
+
+  # Like pkgsMold, but we disable mold individually for known failing packages until their issues are resolved
+  pkgsMoldCI = pkgsMold.appendOverlays [
+    (self: super: {
+      inherit (pkgs)
+        valgrind # https://github.com/rui314/mold/issues/81#issuecomment-876425070
+      ;
+    })
+  ];
+in {
+  inherit pkgs pkgsMold pkgsMoldCI;
+
+  # Packages we already know that work in order to catch regressions
+  ciPackages = with pkgsMoldCI; linkFarmFromDrvs "packages-with-mold" [
+    binutils
+    stdenv
+    # TODO
+  ];
+}