diff --git a/src/dune b/src/dune
index cebbd305..d2d9e3db 100644
--- a/src/dune
+++ b/src/dune
@@ -18,3 +18,12 @@
   (with-stdout-to
    %{targets}
    (run %{bin}))))
+
+(rule
+ (targets Tiny_httpd_atomic_.ml)
+ (deps
+  (:bin ./gen/mkshims.exe))
+ (action
+  (with-stdout-to
+   %{targets}
+   (run %{bin}))))
diff --git a/src/gen/dune b/src/gen/dune
index c741e9af..6cd2fd4a 100644
--- a/src/gen/dune
+++ b/src/gen/dune
@@ -1,2 +1,2 @@
-(executable
- (name gentags))
+(executables
+ (names gentags mkshims))
diff --git a/src/gen/mkshims.ml b/src/gen/mkshims.ml
new file mode 100644
index 00000000..a49f1ab7
--- /dev/null
+++ b/src/gen/mkshims.ml
@@ -0,0 +1,41 @@
+let atomic_before_412 =
+  {|
+  type 'a t = {mutable x: 'a}
+  let[@inline] make x = {x}
+  let[@inline] get {x} = x
+  let[@inline] set r x = r.x <- x
+  let[@inline] exchange r x =
+    let y = r.x in
+    r.x <- x;
+    y
+
+  let[@inline] compare_and_set r seen v =
+    if r.x == seen then (
+      r.x <- v;
+      true
+    ) else false
+
+  let[@inline] fetch_and_add r x =
+    let v = r.x in
+    r.x <- x + r.x;
+    v
+
+  let[@inline] incr r = r.x <- 1 + r.x
+  let[@inline] decr r = r.x <- r.x - 1
+  |}
+
+let atomic_after_412 = {|include Atomic|}
+
+let write_file file s =
+  let oc = open_out file in
+  output_string oc s;
+  close_out oc
+
+let () =
+  let version = Scanf.sscanf Sys.ocaml_version "%d.%d.%s" (fun x y _ -> x, y) in
+  print_endline
+    (if version >= (4, 12) then
+      atomic_after_412
+    else
+      atomic_before_412);
+  ()