chore: add nix flake for xs-env

.gitignore

@@ -17,3 +17,6 @@ doc/html
 *.txt
 emu/obj*
 playground/
+
+# nix build .
+result

flake.nix

@@ -0,0 +1,64 @@
+{
+  description = "GSIM: A Fast RTL Simulator for Large-Scale Designs";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11";
+  };
+
+  outputs = { self, nixpkgs, ... }:
+    let
+      systems = nixpkgs.lib.systems.flakeExposed;
+      perSystem = nixpkgs.lib.genAttrs systems (system:
+        let
+          pkgs = import nixpkgs { inherit system; };
+          llvm = pkgs.llvmPackages_19;
+          gsim = llvm.stdenv.mkDerivation {
+            pname = "gsim";
+            version = "1.0.0-dev"; # FIXME?
+            src = self;
+
+            strictDeps = true;
+            enableParallelBuilding = true;
+            dontConfigure = true;
+
+            nativeBuildInputs = [
+              pkgs.flex
+              pkgs.bison
+            ];
+            buildInputs = [
+              pkgs.flex
+              pkgs.gmp
+            ];
+
+            makeFlags = [
+              # gsim defaults to /bin/bash, which is not guaranteed in NixOS
+              "SHELL=${llvm.stdenv.shell}"
+              # override default build target to avoid running tests
+              "build-gsim"
+            ];
+
+            installPhase = ''
+              runHook preInstall
+              install -Dm755 build/gsim/gsim $out/bin/gsim
+              runHook postInstall
+            '';
+
+            meta = {
+              description = "Fast RTL Simulator for Large-Scale Designs";
+              homepage = "https://github.com/OpenXiangShan/gsim";
+              platforms = pkgs.lib.platforms.linux;
+            };
+          };
+        in
+        {
+          packages = {
+            gsim = gsim;
+            default = gsim;
+          };
+        }
+      );
+    in
+    {
+      packages = nixpkgs.lib.mapAttrs (_: v: v.packages) perSystem;
+    };
+}