diff --git a/check-generated-code.nix b/check-generated-code.nix
new file mode 100644
index 0000000000..a408a44b69
--- /dev/null
+++ b/check-generated-code.nix
@@ -0,0 +1,31 @@
+# Check that generated sources match the repo version.
+{ pkgs, raptorjit }:
+with pkgs; with lib;
+
+# Generated files that are kept in tree.
+let generatedFiles =
+  "lj_bcdef.h lj_ffdef.h lj_libdef.h lj_recdef.h lj_folddef.h host/buildvm_arch.h";
+in
+
+overrideDerivation raptorjit (as:
+  {
+    preBuild = ''
+      pushd src
+      mkdir old
+      for f in ${generatedFiles}; do
+        cp $f old/
+      done
+      popd
+    '' + as.preBuild;
+    checkPhase = ''
+      pushd src
+      mkdir new
+      for f in ${generatedFiles}; do
+        cp $f new/
+      done
+      echo "Checking that in-tree generated VM code is up-to-date..."
+      diff -u old new || (echo "Error: Stale generated code"; exit 1)
+      popd
+    '';
+    doCheck = true;
+  })
diff --git a/default.nix b/default.nix
index 552fa8751d..330a8aa3e5 100644
--- a/default.nix
+++ b/default.nix
@@ -9,12 +9,14 @@
 
 { pkgs ? (import ./pkgs.nix) {}
 , source ? pkgs.lib.cleanSource ./.
-, version ? "dev" }:
+, version ? "dev"
+, check ? false }:
 
 let
-  callPackage = (pkgs.lib.callPackageWith { inherit pkgs; inherit source; inherit version; });
+  callPackage = (pkgs.lib.callPackageWith { inherit pkgs source version; });
   raptorjit = (callPackage ./raptorjit.nix {});
-  test = name: args: (callPackage ./test.nix { inherit raptorjit; inherit name; inherit args; });
+  test = name: args: (callPackage ./test.nix { inherit raptorjit name args; });
+  check-generated-code = (callPackage ./check-generated-code.nix { inherit raptorjit; });
 in
 
 # Build RaptorJIT and run mulitple test suites.
@@ -24,5 +26,6 @@ in
   test-O2    = test "O2"    "-O2";
   test-O1    = test "O1"    "-O1";
   test-nojit = test "nojit" "-joff";
-}
+} //
+(if check then { inherit check-generated-code; } else {})