diff --git a/src/codegen/containers_codegen.ml b/src/codegen/containers_codegen.ml
new file mode 100644
index 00000000..cc390ff3
--- /dev/null
+++ b/src/codegen/containers_codegen.ml
@@ -0,0 +1,138 @@
+
+(** {1 Code generators} *)
+
+module Fmt = CCFormat
+let spf = Printf.sprintf
+let fpf = Fmt.fprintf
+
+type code =
+  | Base of { pp: unit Fmt.printer }
+  | Struct of string * code list
+  | Sig of string * code list
+
+module Code = struct
+  type t = code
+
+  let in_struct m (cs:t list) : t = Struct (m, cs)
+  let in_sig m (cs:t list) : t = Sig (m, cs)
+
+  let rec pp_rec out c =
+    let ppl = Fmt.(list ~sep:(return "@ ") pp_rec) in
+    match c with
+      | Base {pp} -> pp out ()
+      | Struct (m,cs) ->
+        fpf out "@[<hv2>module %s = struct@ %a@;<1 -2>end@]" m ppl cs
+      | Sig (m,cs) ->
+        fpf out "@[<hv2>module %s : sig@ %a@;<1 -2>end@]" m ppl cs
+
+  let pp out c = fpf out "@[<v>%a@]" pp_rec c
+  let to_string c = Fmt.to_string pp c
+
+  let mk_pp pp = Base {pp}
+  let mk_str s = Base {pp=Fmt.const Fmt.string s}
+end
+
+module Bitfield = struct
+  type field = {
+    f_name: string;
+    f_offset: int;
+    f_def: field_def;
+  }
+  and field_def =
+    | F_bit
+    | F_int of {width: int}
+
+  type t = {
+    name: string;
+    mutable fields: field list;
+    mutable width: int;
+    emit_failure_if_too_wide: bool;
+  }
+
+  let make ?(emit_failure_if_too_wide=true) ~name () : t =
+    { name; fields=[]; width=0; emit_failure_if_too_wide; }
+
+  let total_width self = self.width
+
+  let field_bit self f_name =
+    let f_offset = total_width self in
+    let f = {f_name; f_offset; f_def=F_bit} in
+    self.fields <- f :: self.fields;
+    self.width <- 1 + self.width
+
+  let field_int self ~width f_name : unit =
+    let f_offset = total_width self in
+    let f = {f_name; f_offset; f_def=F_int {width}} in
+    self.fields <- f :: self.fields;
+    self.width <- self.width + width
+
+  let empty_name self =
+    if self.name = "t" then "empty" else spf "empty_%s" self.name
+
+  let gen_ml self : code =
+    Code.mk_pp @@ fun out () ->
+    fpf out "@[<v>type %s = int@," self.name;
+    fpf out "@[let %s : %s = 0@]@," (empty_name self) self.name;
+    List.iter
+      (fun f ->
+         let inline = "[@inline]" in (* TODO: option to enable/disable that *)
+         let off = f.f_offset in
+         match f.f_def with
+         | F_bit ->
+           let x_lsr = if off = 0 then "x" else spf "(x lsr %d)" off in
+           fpf out "@[let%s get_%s (x:%s) : bool = (%s land 1) <> 0@]@,"
+             inline f.f_name self.name x_lsr;
+           let mask_shifted = 1 lsl off in
+           fpf out "@[<2>let%s set_%s (v:bool) (x:%s) : %s =@ \
+                    if v then x lor %d else x land (lnot %d)@]@,"
+             inline f.f_name self.name self.name mask_shifted mask_shifted;
+         | F_int {width} ->
+           let mask0 = (1 lsl width) - 1 in
+           fpf out "@[let%s get_%s (x:%s) : int = ((x lsr %d) land %d)@]@,"
+             inline f.f_name self.name off mask0;
+           fpf out "@[<2>let%s set_%s (i:int) (x:%s) : %s =@ \
+                    assert ((i land %d) == i);@ \
+                    ((x land (lnot %d)) lor (i lsl %d))@]@,"
+             inline f.f_name self.name self.name
+             mask0 (mask0 lsl off) off;
+      )
+      (List.rev self.fields);
+    (* check width *)
+    if self.emit_failure_if_too_wide then (
+      fpf out "(* check that int size is big enough *)@,\
+               @[let () = assert (Sys.int_size >= %d);;@]" (total_width self);
+    );
+    fpf out "@]"
+
+  let gen_mli self : code =
+    Code.mk_pp @@ fun out () ->
+    fpf out "@[<v>type %s = private int@," self.name;
+    fpf out "@[<v>val %s : %s@," (empty_name self) self.name;
+    List.iter
+      (fun f ->
+         match f.f_def with
+         | F_bit ->
+           fpf out "@[val get_%s : %s -> bool@]@," f.f_name self.name;
+           fpf out "@[val set_%s : bool -> %s -> %s@]@," f.f_name self.name self.name;
+         | F_int {width} ->
+           fpf out "@[val get_%s : %s -> int@]@,"
+             f.f_name self.name;
+           fpf out "@,@[(** %d bits integer *)@]@,\
+                    @[val set_%s : int -> %s -> %s@]@,"
+             width f.f_name self.name self.name;
+      )
+      (List.rev self.fields);
+    fpf out "@]"
+end
+
+let emit_chan oc cs =
+  let fmt = Fmt.formatter_of_out_channel oc in
+  List.iter (fun c -> Fmt.fprintf fmt "@[%a@]@." Code.pp c) cs;
+  Fmt.fprintf fmt "@?"
+
+let emit_file file cs =
+  CCIO.with_out file (fun oc -> emit_chan oc cs)
+
+let emit_string cs : string =
+  Fmt.asprintf "@[<v>%a@]" (Fmt.list ~sep:(Fmt.return "@ ") Code.pp) cs
+
diff --git a/src/codegen/containers_codegen.mli b/src/codegen/containers_codegen.mli
new file mode 100644
index 00000000..a5680c50
--- /dev/null
+++ b/src/codegen/containers_codegen.mli
@@ -0,0 +1,46 @@
+
+(** {1 Code generators} *)
+
+module Fmt = CCFormat
+
+type code
+
+(** {2 Representation of OCaml code} *)
+module Code : sig
+  type t = code
+
+  val pp : t Fmt.printer
+  val to_string : t -> string
+
+  val mk_pp : unit Fmt.printer -> t
+  val mk_str : string -> t
+  val in_struct : string -> t list -> t
+  val in_sig : string -> t list -> t
+end
+
+(** {2 Generate efficient bitfields that fit in an integer} *)
+module Bitfield : sig
+  type t
+
+  val make :
+    ?emit_failure_if_too_wide:bool ->
+    name:string ->
+    unit -> t
+  (** Make a new bitfield with the given name.
+      @param name the name of the generated type
+      @param emit_failure_if_too_wide if true, generated code includes a runtime
+      assertion that {!Sys.int_size} is wide enough to support this type *)
+
+  val field_bit : t -> string -> unit
+  val field_int : t -> width:int -> string -> unit
+
+  val total_width : t -> int
+
+  val gen_mli : t -> code
+  val gen_ml : t -> code
+end
+
+val emit_file : string -> code list -> unit
+val emit_chan : out_channel -> code list -> unit
+val emit_string : code list -> string
+
diff --git a/src/codegen/dune b/src/codegen/dune
new file mode 100644
index 00000000..61068fc3
--- /dev/null
+++ b/src/codegen/dune
@@ -0,0 +1,7 @@
+
+(library
+  (name containers_codegen)
+  (public_name containers.codegen)
+  (synopsis "code generators for Containers")
+  (libraries containers)
+  (flags :standard -warn-error -a+8))
diff --git a/src/codegen/tests/dune b/src/codegen/tests/dune
new file mode 100644
index 00000000..320af55e
--- /dev/null
+++ b/src/codegen/tests/dune
@@ -0,0 +1,24 @@
+
+; emit tests
+
+(executable
+  (name emit_tests)
+  (modules emit_tests)
+  (flags :standard -warn-error -a+8)
+  (libraries containers containers.codegen))
+
+(rule
+  (targets test_bitfield.ml test_bitfield.mli)
+  (action (run ./emit_tests.exe)))
+
+; run tests
+
+(executables
+  (names test_bitfield)
+  (modules test_bitfield)
+  (flags :standard -warn-error -a+8)
+  (libraries containers))
+
+(alias
+  (name runtest)
+  (action (run ./test_bitfield.exe)))
diff --git a/src/codegen/tests/emit_tests.ml b/src/codegen/tests/emit_tests.ml
new file mode 100644
index 00000000..038d67ca
--- /dev/null
+++ b/src/codegen/tests/emit_tests.ml
@@ -0,0 +1,56 @@
+module CG = Containers_codegen
+module Vec = CCVector
+
+let spf = Printf.sprintf
+
+let emit_bitfields () =
+  let module B = CG.Bitfield in
+  let ml = Vec.create() in
+  let mli = Vec.create() in
+  begin
+    let b = B.make ~name:"t" () in
+    B.field_bit b "x";
+    B.field_bit b "y";
+    B.field_bit b "z";
+    B.field_int b ~width:5 "foo";
+
+    Vec.push ml (CG.Code.in_struct "T1" [B.gen_ml b]);
+    Vec.push mli (CG.Code.in_sig "T1" [B.gen_mli b]);
+    (* check width *)
+    Vec.push ml
+      (CG.Code.mk_str (spf "let() = assert (%d = 8);;" (B.total_width b)));
+    ()
+  end;
+
+  Vec.push ml @@ CG.Code.mk_str {|
+    let n_fails = ref 0;;
+    at_exit (fun () -> if !n_fails > 0 then exit 1);;
+    let assert_true line s =
+      if not s then ( incr n_fails; Printf.eprintf "test failure at %d\n%!" line);;
+
+    |};
+
+  let test1 = {|
+    assert_true __LINE__ T1.(get_y (empty |> set_x true |> set_y true |> set_foo 10));;
+    assert_true __LINE__ T1.(get_x (empty |> set_x true |> set_y true |> set_foo 10));;
+    assert_true __LINE__ T1.(get_y (empty |> set_x true |> set_z true
+                            |> set_y false |> set_x false |> set_y true));;
+    assert_true __LINE__ T1.(get_z (empty |> set_z true));;
+    assert_true __LINE__ T1.(not @@ get_x (empty |> set_z true));;
+    assert_true __LINE__ T1.(not @@ get_y (empty |> set_z true |> set_x true));;
+    assert_true __LINE__ T1.(not @@ get_y (empty |> set_z true |> set_foo 18));;
+    (* check width of foo *)
+    assert_true __LINE__ T1.(try ignore (empty |> set_foo (1 lsl 6)); false with _ -> true);;
+    assert_true __LINE__ T1.(12 = get_foo (empty |> set_x true |> set_foo 12 |> set_x false));;
+    assert_true __LINE__ T1.(24 = get_foo (empty |> set_y true |> set_foo 24 |> set_z true));;
+  |} |> CG.Code.mk_str in
+  Vec.push ml test1;
+
+  CG.emit_file "test_bitfield.ml" (Vec.to_list ml);
+  CG.emit_file "test_bitfield.mli" (Vec.to_list mli);
+  ()
+
+let () =
+  emit_bitfields();
+  ()
+