diff --git a/lib/ast.ml b/lib/ast.ml
index 0da9f10b..6976466f 100644
--- a/lib/ast.ml
+++ b/lib/ast.ml
@@ -1,9 +1,10 @@
 open Import
 
-type ('a, _) ast =
+type ('a, _) ast : immutable_data with 'a =
   | Alternative : 'a list -> ('a, [> `Uncased ]) ast
   | No_case : 'a -> ('a, [> `Cased ]) ast
   | Case : 'a -> ('a, [> `Cased ]) ast
+[@@unsafe_allow_any_mode_crossing]
 
 let dyn_of_ast f =
   let open Dyn in
diff --git a/lib/ast.mli b/lib/ast.mli
index 04a89e9b..1f675842 100644
--- a/lib/ast.mli
+++ b/lib/ast.mli
@@ -1,7 +1,10 @@
-type ('a, _) ast = private
+@@ portable
+
+type ('a, _) ast : immutable_data with 'a = private
   | Alternative : 'a list -> ('a, [> `Uncased ]) ast
   | No_case : 'a -> ('a, [> `Cased ]) ast
   | Case : 'a -> ('a, [> `Cased ]) ast
+[@@unsafe_allow_any_mode_crossing]
 
 type cset = private
   | Cset of Cset.t
diff --git a/lib/automata.ml b/lib/automata.ml
index 8623436f..2671cdde 100644
--- a/lib/automata.ml
+++ b/lib/automata.ml
@@ -24,9 +24,9 @@ open Import
 
 let hash_combine h accu = (accu * 65599) + h
 
-module Ids : sig
+module Ids : sig @@ portable
   module Id : sig
-    type t
+    type t : immediate
 
     val equal : t -> t -> bool
     val zero : t
@@ -105,7 +105,7 @@ module Rep_kind = struct
   let pp fmt t = Format.pp_print_string fmt (to_string t)
 end
 
-module Mark : sig
+module Mark : sig @@ portable
   type t = private int
 
   val compare : t -> t -> int
@@ -136,7 +136,7 @@ end = struct
   ;;
 end
 
-module Idx : sig
+module Idx : sig @@ portable
   type t = private int
 
   val pp : t Fmt.t
@@ -292,10 +292,12 @@ type expr = Expr.t
 include Expr
 
 module Marks = struct
-  type t =
+  type t : immutable_data =
     { marks : (Mark.t * Idx.t) list
     ; pmarks : Pmark.Set.t
     }
+  [@@unsafe_allow_any_mode_crossing
+    (* SAFETY: [Pmark.Set.t] is actually immutable, but the stdlib doesn't say that *)]
 
   let to_dyn { marks; pmarks } : Dyn.t =
     let open Dyn in
@@ -376,8 +378,8 @@ module Status = struct
     | Running
 end
 
-module Desc : sig
-  type t
+module Desc : sig @@ portable
+  type t : immutable_data
 
   val pp : t Fmt.t
 
@@ -458,7 +460,7 @@ end = struct
     | TMatch m -> variant "TMarks" [ Marks.to_dyn m ]
   ;;
 
-  let to_dyn = to_dyn None
+  let to_dyn t = to_dyn None t
 
   open E
 
@@ -517,8 +519,8 @@ end = struct
     | _ :: r -> first_match r
   ;;
 
-  let remove_matches =
-    List.filter ~f:(function
+  let remove_matches t =
+    List.filter t ~f:(function
       | TMatch _ -> false
       | _ -> true)
   ;;
@@ -590,13 +592,14 @@ end
 module E = Desc.E
 
 module State = struct
-  type t =
+  type t : immutable_data =
     { idx : Idx.t
     ; category : Category.t
     ; desc : Desc.t
     ; mutable status : Status.t option
     ; hash : int
     }
+  [@@unsafe_allow_any_mode_crossing]
   (* Thread-safety: We use double-checked locking to access field
      [status] in function [status] below. *)
 
@@ -651,7 +654,7 @@ module State = struct
       st
   ;;
 
-  module Table = Hashtbl.Make (struct
+  module Table = Hashtbl.MakePortable (struct
       type nonrec t = t
 
       let equal = equal
diff --git a/lib/automata.mli b/lib/automata.mli
index 95b57c57..d014a2a2 100644
--- a/lib/automata.mli
+++ b/lib/automata.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 (*
    RE - A regular expression library
 
@@ -23,7 +25,7 @@
 (* Regular expressions *)
 
 module Mark : sig
-  type t [@@immediate]
+  type t : immediate
 
   val compare : t -> t -> int
   val start : t
diff --git a/lib/bit_vector.mli b/lib/bit_vector.mli
index 8f07063f..b88f920d 100644
--- a/lib/bit_vector.mli
+++ b/lib/bit_vector.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 type t
 
 val length : t -> int
diff --git a/lib/category.mli b/lib/category.mli
index 6be2910a..48f63cca 100644
--- a/lib/category.mli
+++ b/lib/category.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 (** Categories represent the various kinds of characters that can be tested
     by look-ahead and look-behind operations.
 
diff --git a/lib/color_map.mli b/lib/color_map.mli
index 98a41d64..46c25f84 100644
--- a/lib/color_map.mli
+++ b/lib/color_map.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 (* Color maps exists to provide an optimization for the regex engine. The fact
    that some characters are entirely equivalent for some regexes means that we
    can use them interchangeably.
diff --git a/lib/compile.ml b/lib/compile.ml
index 00681af8..057fda66 100644
--- a/lib/compile.ml
+++ b/lib/compile.ml
@@ -2,7 +2,7 @@ open Import
 
 let rec iter n f v = if Int.equal n 0 then v else iter (n - 1) f (f v)
 
-module Idx : sig
+module Idx : sig @@ portable
   type t [@@immediate]
 
   val unknown : t
@@ -54,8 +54,8 @@ type state_info =
    color. For performance reason, to avoid an indirection, we manually
    unbox the transition table: we allocate a single array, with the
    state information at index 0, followed by the transitions. *)
-module State : sig
-  type t
+module State : sig @@ portable
+  type t : mutable_data
 
   val make : ncol:int -> state_info -> t
   val make_break : state_info -> t
@@ -93,7 +93,7 @@ end = struct
   let unknown_state = dummy { idx = Idx.unknown; final = []; desc = Automata.State.dummy }
 
   let make ~ncol state =
-    let st = Table (Array.make (ncol + 1) unknown_state) in
+    let st = Table (Array.make (ncol + 1) (Stdlib.Obj.magic_uncontended unknown_state)) in
     set_info st state;
     st
   ;;
@@ -152,6 +152,9 @@ module Positions = struct
     }
 
   let empty = { positions = [||]; length = 0 }
+  let get_empty () =
+    (* SAFETY: this record is never mutated. *)
+    Stdlib.Obj.magic_uncontended empty
   let length t = t.length
   let unsafe_set t idx pos = Array.unsafe_set t.positions idx pos
 
@@ -181,7 +184,7 @@ module Positions = struct
          always checking whether it is large enough before modifying it. *)
       let length = Automata.Working_area.index_count re.tbl + 1 in
       { positions = Array.make length 0; length })
-    else empty
+    else get_empty ()
   ;;
 end
 
@@ -466,7 +469,7 @@ module Stream = struct
   let finalize t s ~pos ~len =
     (* TODO bound checks? *)
     let last = pos + len in
-    let state = scan_str t.re Positions.empty s t.state ~last ~pos ~groups:false in
+    let state = scan_str t.re (Positions.get_empty ()) s t.state ~last ~pos ~groups:false in
     let info = State.get_info state in
     match
       let _idx, res =
@@ -630,7 +633,7 @@ let match_str_no_bounds ~groups ~partial re s ~pos ~len =
 let match_str_p re s ~pos ~len =
   if pos < 0 || len < -1 || pos + len > String.length s
   then invalid_arg "Re.exec: out of bounds";
-  match make_match_str re Positions.empty ~len ~groups:false ~partial:false s ~pos with
+  match make_match_str re (Positions.get_empty ()) ~len ~groups:false ~partial:false s ~pos with
   | Match _ -> true
   | _ -> false
 ;;
@@ -812,7 +815,7 @@ let compile_1 regexp =
     ; greedy = `Greedy
     ; pos = ref A.Mark.start
     ; names = ref []
-    ; cache = ref Cset.CSetMap.empty
+    ; cache = ref (Stdlib.Obj.magic_uncontended Cset.CSetMap.empty)
     ; colors
     }
   in
diff --git a/lib/compile.mli b/lib/compile.mli
index 5e1bde3c..eda96227 100644
--- a/lib/compile.mli
+++ b/lib/compile.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 type re
 
 module Stream : sig
diff --git a/lib/core.mli b/lib/core.mli
index a3d09297..0b80cc24 100644
--- a/lib/core.mli
+++ b/lib/core.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 (*
    RE - A regular expression library
 
diff --git a/lib/cset.ml b/lib/cset.ml
index 9183761e..071a2642 100644
--- a/lib/cset.ml
+++ b/lib/cset.ml
@@ -41,8 +41,8 @@ let compare_pair (x, y) (x', y') =
 ;;
 
 let equal_pair (x, y) (x', y') = Int.equal x x' && Int.equal y y'
-let equal = List.equal ~eq:equal_pair
-let compare : t -> t -> int = List.compare ~cmp:compare_pair
+let equal x y = List.equal ~eq:equal_pair x y
+let compare x y = List.compare ~cmp:compare_pair x y
 
 let rec union l l' =
   match l, l' with
@@ -87,7 +87,7 @@ let rec diff l l' =
 ;;
 
 let single =
-  let single c = [ c, c ] in
+  let single (c : c) = [ c, c ] in
   Dense_map.make (* an extra color for lnl *) ~size:257 ~f:single
 ;;
 
@@ -101,10 +101,10 @@ let rec offset o l =
   | (c1, c2) :: r -> (c1 + o, c2 + o) :: offset o r
 ;;
 
-let empty = []
+let empty : t = []
 let cany = [ 0, 255 ]
-let union_all : t list -> t = List.fold_left ~init:empty ~f:union
-let intersect_all : t list -> t = List.fold_left ~init:cany ~f:inter
+let union_all ts = List.fold_left ~init:empty ~f:union ts
+let intersect_all ts = List.fold_left ~init:cany ~f:inter ts
 
 let rec mem (c : int) s =
   match s with
@@ -127,7 +127,7 @@ let print_one ch (c1, c2) =
   if Int.equal c1 c2 then Format.fprintf ch "%d" c1 else Format.fprintf ch "%d-%d" c1 c2
 ;;
 
-let pp = Fmt.list ~pp_sep:(Fmt.lit ", ") print_one
+let pp ts = Fmt.list ~pp_sep:(Fmt.lit ", ") print_one ts
 
 let to_dyn t =
   let open Dyn in
@@ -149,7 +149,7 @@ let one_char = function
   | _ -> None
 ;;
 
-module CSetMap = Map.Make (struct
+module CSetMap = Map.MakePortable (struct
     type t = int * (int * int) list
 
     let compare (i, u) (j, v) =
diff --git a/lib/cset.mli b/lib/cset.mli
index e875d072..7f551f4d 100644
--- a/lib/cset.mli
+++ b/lib/cset.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 (*
    RE - A regular expression library
 
@@ -22,7 +24,7 @@
 
 (* Character sets, represented as sorted list of intervals *)
 
-type c [@@immediate]
+type c : immediate
 
 val equal_c : c -> c -> bool
 val to_int : c -> int
@@ -30,7 +32,7 @@ val of_int : int -> c
 val to_char : c -> char
 val of_char : char -> c
 
-type t
+type t : immutable_data
 
 (** special characters which isn't present in any set (not even in [cany]) *)
 val null_char : c
diff --git a/lib/dense_map.ml b/lib/dense_map.ml
index 052469c8..1552d726 100644
--- a/lib/dense_map.ml
+++ b/lib/dense_map.ml
@@ -1,4 +1,9 @@
-let make ~size ~f =
-  let cache = Array.init size f in
-  fun i -> cache.(i)
+let make : size:int -> f:(int -> 'a @ portable) -> (int -> 'a @ contended) @ portable =
+  fun ~size ~f ->
+  let cache =
+    (* SAFETY: Interface requires that we never put anything nonportable in here *)
+    Obj.magic_portable (Array.init size f)
+  in
+  fun i ->
+    (((* SAFETY: we never mutate this array. *) Obj.magic_uncontended cache).(i))
 ;;
diff --git a/lib/dense_map.mli b/lib/dense_map.mli
index f83f663d..5e11e123 100644
--- a/lib/dense_map.mli
+++ b/lib/dense_map.mli
@@ -1 +1,3 @@
-val make : size:int -> f:(int -> 'a) -> int -> 'a
+@@ portable
+
+val make : size:int -> f:(int -> 'a @ portable) -> (int -> 'a @ contended) @ portable
diff --git a/lib/emacs.mli b/lib/emacs.mli
index 5092a200..7da1d526 100644
--- a/lib/emacs.mli
+++ b/lib/emacs.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 (*
    RE - A regular expression library
 
diff --git a/lib/fmt.mli b/lib/fmt.mli
index 3dbadf46..44edf810 100644
--- a/lib/fmt.mli
+++ b/lib/fmt.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 type formatter := Format.formatter
 type 'a t = formatter -> 'a -> unit
 
diff --git a/lib/glob.mli b/lib/glob.mli
index 04968066..fcadbc9d 100644
--- a/lib/glob.mli
+++ b/lib/glob.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 (*
    RE - A regular expression library
 
diff --git a/lib/group.mli b/lib/group.mli
index 2e0263a9..e16efbdc 100644
--- a/lib/group.mli
+++ b/lib/group.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 (** Information about groups in a match. *)
 
 (** Result of a successful match. *)
diff --git a/lib/hash_set.ml b/lib/hash_set.ml
index c49ca15c..f249e483 100644
--- a/lib/hash_set.ml
+++ b/lib/hash_set.ml
@@ -60,7 +60,7 @@ let () =
   assert (Array.unsafe_get x 0 = absent)
 ;;
 
-let create () = ref Option.none
+let create () = ref None
 
 let[@inline] index_of_offset slots index i =
   let i = index + !i in
diff --git a/lib/hash_set.mli b/lib/hash_set.mli
index f2f82215..11518f1d 100644
--- a/lib/hash_set.mli
+++ b/lib/hash_set.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 type t
 
 val create : unit -> t
diff --git a/lib/mark_infos.ml b/lib/mark_infos.ml
index f4a4251e..69dd6d08 100644
--- a/lib/mark_infos.ml
+++ b/lib/mark_infos.ml
@@ -5,7 +5,8 @@ type t = int array
 let make marks =
   let len = 1 + List.fold_left ~f:(fun ma (i, _) -> max ma i) ~init:(-1) marks in
   let t = Array.make len (-1) in
-  List.iter ~f:(fun (i, v) -> t.(i) <- v) marks;
+  let set (i, v) = t.(i) <- v in
+  List.iter ~f:set marks;
   t
 ;;
 
diff --git a/lib/mark_infos.mli b/lib/mark_infos.mli
index f729058f..a6783a45 100644
--- a/lib/mark_infos.mli
+++ b/lib/mark_infos.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 (** store mark information for groups in an array *)
 type t
 
diff --git a/lib/parse_buffer.mli b/lib/parse_buffer.mli
index 202b54f7..cedebc49 100644
--- a/lib/parse_buffer.mli
+++ b/lib/parse_buffer.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 type t
 
 exception Parse_error
diff --git a/lib/pcre.mli b/lib/pcre.mli
index 0591211c..34d7d9f5 100644
--- a/lib/pcre.mli
+++ b/lib/pcre.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 (** NOTE: Only a subset of the PCRE spec is supported *)
 
 exception Parse_error
diff --git a/lib/perl.mli b/lib/perl.mli
index 1044812e..e8af45e7 100644
--- a/lib/perl.mli
+++ b/lib/perl.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 (*
    RE - A regular expression library
 
diff --git a/lib/pmark.ml b/lib/pmark.ml
index 9686b245..eec7c2cb 100644
--- a/lib/pmark.ml
+++ b/lib/pmark.ml
@@ -3,20 +3,15 @@ module Pmark = struct
 
   let equal (x : int) (y : int) = x = y
   let compare (x : int) (y : int) = compare x y
-  let r = ref 0
-
-  let gen () =
-    incr r;
-    !r
-  ;;
-
+  let r = Atomic.make 1
+  let gen () = Atomic.fetch_and_add r 1
   let pp = Format.pp_print_int
 end
 
 include Pmark
 
 module Set = struct
-  module Set = Set.Make (Pmark)
+  module Set = Set.MakePortable (Pmark)
 
   let[@warning "-32"] to_list x =
     let open Set in
diff --git a/lib/pmark.mli b/lib/pmark.mli
index 100ee4ca..3cc498a3 100644
--- a/lib/pmark.mli
+++ b/lib/pmark.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 type t = private int
 
 val equal : t -> t -> bool
diff --git a/lib/posix.mli b/lib/posix.mli
index a2213b0d..ac7463a0 100644
--- a/lib/posix.mli
+++ b/lib/posix.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 (*
    RE - A regular expression library
 
diff --git a/lib/posix_class.mli b/lib/posix_class.mli
index a06c4f7b..ad1cd23e 100644
--- a/lib/posix_class.mli
+++ b/lib/posix_class.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 val names : string list
 val of_name : string -> Core.t
 val parse : Parse_buffer.t -> Core.t option
diff --git a/lib/replace.mli b/lib/replace.mli
index 387ee951..84d38891 100644
--- a/lib/replace.mli
+++ b/lib/replace.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 (** [replace ~all re ~f s] iterates on [s], and replaces every occurrence
     of [re] with [f substring] where [substring] is the current match.
     If [all = false], then only the first occurrence of [re] is replaced. *)
diff --git a/lib/slice.mli b/lib/slice.mli
index 67d9ec20..dc837ff9 100644
--- a/lib/slice.mli
+++ b/lib/slice.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 type t =
   { s : string
   ; pos : int
diff --git a/lib/view.mli b/lib/view.mli
index d069f8c4..26e49bc0 100644
--- a/lib/view.mli
+++ b/lib/view.mli
@@ -1,3 +1,5 @@
+@@ portable
+
 (** A view of the top-level of a regex. This type is unstable and may change *)
 
 module Cset : sig
diff --git a/lib_test/concurrency/suppress.txt b/lib_test/concurrency/suppress.txt
index 513d95e1..8357a0fd 100644
--- a/lib_test/concurrency/suppress.txt
+++ b/lib_test/concurrency/suppress.txt
@@ -16,3 +16,7 @@ race_top:^camlRe__Automata.status
 
 # Race within Compile.final
 race_top:^camlRe__Compile.final
+
+# Spurious data race due to the two-step initialization in Mark_info.make
+# (between Mark_info.make and other functions in module Mark_infos)
+race_top:^camlRe__Mark_infos.set
diff --git a/lib_test/concurrency/test.ml b/lib_test/concurrency/test.ml
index 0ee42c4a..2fffb2b7 100644
--- a/lib_test/concurrency/test.ml
+++ b/lib_test/concurrency/test.ml
@@ -81,12 +81,16 @@ let execute ~short re a =
     (inverse_permutation a)
     (Array.map
        (fun i ->
-         try Some (Re.exec ~pos:(if short then 30 - 7 else 0) re strings.(i)) with
+         try
+           Some
+             (Re.Group.all_offset
+              @@ Re.exec ~pos:(if short then 30 - 7 else 0) re strings.(i))
+         with
          | Not_found -> None)
        a)
 ;;
 
-let compare_groups g g' = Re.Group.(all_offset g = all_offset g')
+let compare_groups g g' = g = g'
 
 let concurrent f f' =
   let barrier = Barrier.create 2 in