diff --git a/CHANGES.md b/CHANGES.md
index 56919dd197..4362e7609a 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,3 +1,8 @@
+# dev (2024-??) - ??
+
+## Features/Changes
+* Compiler: optimize sourcemap processing, improving linking performance #1617
+
 # 5.8.2 (2024-05-26) - Luc
 
 ## Bug fixes
diff --git a/compiler/bin-js_of_ocaml/cmd_arg.ml b/compiler/bin-js_of_ocaml/cmd_arg.ml
index 5b9eff75b1..56dcd74491 100644
--- a/compiler/bin-js_of_ocaml/cmd_arg.ml
+++ b/compiler/bin-js_of_ocaml/cmd_arg.ml
@@ -312,9 +312,10 @@ let options =
             ; file
             ; sourceroot = sourcemap_root
             ; sources = []
-            ; sources_content = (if sourcemap_don't_inline_content then None else Some [])
+            ; sources_contents =
+                (if sourcemap_don't_inline_content then None else Some [])
             ; names = []
-            ; mappings = []
+            ; mappings = Source_map.Mappings.empty
             } )
       else None
     in
@@ -551,9 +552,10 @@ let options_runtime_only =
             ; file
             ; sourceroot = sourcemap_root
             ; sources = []
-            ; sources_content = (if sourcemap_don't_inline_content then None else Some [])
+            ; sources_contents =
+                (if sourcemap_don't_inline_content then None else Some [])
             ; names = []
-            ; mappings = []
+            ; mappings = Source_map.Mappings.empty
             } )
       else None
     in
diff --git a/compiler/bin-js_of_ocaml/link.ml b/compiler/bin-js_of_ocaml/link.ml
index 090913d20b..2719e7fb10 100644
--- a/compiler/bin-js_of_ocaml/link.ml
+++ b/compiler/bin-js_of_ocaml/link.ml
@@ -106,9 +106,9 @@ let options =
             ; file
             ; sourceroot = sourcemap_root
             ; sources = []
-            ; sources_content = Some []
+            ; sources_contents = Some []
             ; names = []
-            ; mappings = []
+            ; mappings = Source_map.Mappings.empty
             } )
       else None
     in
diff --git a/compiler/lib/js_output.ml b/compiler/lib/js_output.ml
index 33d49bb7da..af093fb05e 100644
--- a/compiler/lib/js_output.ml
+++ b/compiler/lib/js_output.ml
@@ -1903,10 +1903,10 @@ let program ?(accept_unnamed_var = false) f ?source_map p =
   let temp_mappings = ref [] in
   let files = Hashtbl.create 17 in
   let names = Hashtbl.create 17 in
-  let contents : string option list ref option =
+  let contents : Source_map.Source_text.t list ref option =
     match source_map with
-    | None | Some { Source_map.sources_content = None; _ } -> None
-    | Some { Source_map.sources_content = Some _; _ } -> Some (ref [])
+    | None | Some { Source_map.sources_contents = None; _ } -> None
+    | Some { Source_map.sources_contents = Some _; _ } -> Some (ref [])
   in
   let push_mapping, get_file_index, get_name_index, source_map_enabled =
     let source_map_enabled =
@@ -1918,27 +1918,28 @@ let program ?(accept_unnamed_var = false) f ?source_map p =
             | [], _ -> ()
             | x :: xs, [] ->
                 Hashtbl.add files x (Hashtbl.length files);
-                Option.iter contents ~f:(fun r -> r := None :: !r);
+                Option.iter contents ~f:(fun r -> r := Source_map.Source_text.empty :: !r);
                 loop xs []
             | x :: xs, y :: ys ->
                 Hashtbl.add files x (Hashtbl.length files);
                 Option.iter contents ~f:(fun r -> r := y :: !r);
                 loop xs ys
           in
-          loop sm.sources (Option.value ~default:[] sm.sources_content);
+          loop sm.sources (Option.value ~default:[] sm.sources_contents);
           List.iter sm.Source_map.names ~f:(fun f ->
               Hashtbl.add names f (Hashtbl.length names));
           true
     in
     let find_source file =
-      match Builtins.find file with
-      | Some f -> Some (Builtins.File.content f)
-      | None ->
-          if Sys.file_exists file && not (Sys.is_directory file)
-          then
-            let content = Fs.read_file file in
-            Some content
-          else None
+      Source_map.Source_text.encode
+        (match Builtins.find file with
+        | Some f -> Some (Builtins.File.content f)
+        | None ->
+            if Sys.file_exists file && not (Sys.is_directory file)
+            then
+              let content = Fs.read_file file in
+              Some content
+            else None)
     in
     ( (fun pos m -> temp_mappings := (pos, m) :: !temp_mappings)
     , (fun file ->
@@ -1979,10 +1980,10 @@ let program ?(accept_unnamed_var = false) f ?source_map p =
     | Some sm ->
         let sources = hashtbl_to_list files in
         let names = hashtbl_to_list names in
-        let sources_content =
-          match contents with
-          | None -> None
-          | Some r -> Some (List.rev !r)
+        let sources_contents =
+          let open Option.Syntax in
+          let* r = contents in
+          Option.return (List.rev !r)
         in
         let sources =
           List.map sources ~f:(fun filename ->
@@ -1990,8 +1991,9 @@ let program ?(accept_unnamed_var = false) f ?source_map p =
               | None -> filename
               | Some _ -> Filename.concat "/builtin" filename)
         in
+        let sm_mappings = Source_map.Mappings.decode sm.mappings in
         let mappings =
-          List.rev_append_map !temp_mappings sm.mappings ~f:(fun (pos, m) ->
+          List.rev_append_map !temp_mappings sm_mappings ~f:(fun (pos, m) ->
               let gen_line = pos.PP.p_line + 1 in
               let gen_col = pos.PP.p_col in
               match m with
@@ -2006,7 +2008,8 @@ let program ?(accept_unnamed_var = false) f ?source_map p =
                   Source_map.Gen_Ori_Name
                     { gen_line; gen_col; ori_source; ori_line; ori_col; ori_name })
         in
-        Some { sm with Source_map.sources; names; sources_content; mappings }
+        let mappings = Source_map.Mappings.encode mappings in
+        Some { sm with Source_map.sources; names; sources_contents; mappings }
   in
   PP.check f;
   (if stats ()
diff --git a/compiler/lib/link_js.ml b/compiler/lib/link_js.ml
index 3fb7f14035..a2bdd19d59 100644
--- a/compiler/lib/link_js.ml
+++ b/compiler/lib/link_js.ml
@@ -35,7 +35,9 @@ module Line_reader : sig
 
   val peek : t -> string option
 
-  val drop : t -> unit
+  val drop : t -> drop_action:(unit -> unit) -> unit
+  (** [drop_action] is the function to call if a line was effectively dropped
+      (if EOF is reached, this function may return without dropping a line). *)
 
   val close : t -> unit
 
@@ -78,15 +80,17 @@ end = struct
           Some s
         with End_of_file -> None)
 
-  let drop t =
+  let drop t ~drop_action =
     match t.next with
     | Some _ ->
         t.next <- None;
-        t.lnum <- t.lnum + 1
+        t.lnum <- t.lnum + 1;
+        drop_action ()
     | None -> (
         try
           let (_ : string) = input_line t.ic in
-          t.lnum <- t.lnum + 1
+          t.lnum <- t.lnum + 1;
+          drop_action ()
         with End_of_file -> ())
 
   let lnum t = t.lnum
@@ -99,9 +103,15 @@ module Line_writer : sig
 
   val of_channel : out_channel -> t
 
-  val write : ?source:Line_reader.t -> t -> string -> unit
+  val write : ?source:Line_reader.t -> t -> add:(int -> unit) -> string -> unit
+  (** [write ~source t s ~add] writes [s], followed by a newline, and calls
+      [edit], giving it in argument the number of "line number" pragma lines
+      emitted before writing [s]. *)
 
-  val write_lines : ?source:Line_reader.t -> t -> string -> unit
+  val write_lines : ?source:Line_reader.t -> t -> total:(int -> unit) -> string -> unit
+  (** [write_lines ~source t s ~total] writes all lines in [s], ensures that the last
+      line ends with a newline, and calls [total], giving it in argument the total number
+      of lines written, including "line number" pragma lines. *)
 
   val lnum : t -> int
 end = struct
@@ -113,7 +123,7 @@ end = struct
 
   let of_channel oc = { oc; source = None; lnum = 0 }
 
-  let write ?source t s =
+  let write ?source t ~add s =
     let source =
       match source with
       | None -> None
@@ -130,21 +140,24 @@ end = struct
       | Some (fname1, lnum1), Some (fname2, lnum2) ->
           if String.equal fname1 fname2 && lnum1 + 1 = lnum2 then 0 else emit fname2 lnum2
     in
+    add lnum_off;
     output_string t.oc s;
     output_string t.oc "\n";
     let lnum_off = lnum_off + 1 in
     t.source <- source;
     t.lnum <- t.lnum + lnum_off
 
-  let write_lines ?source t lines =
+  let write_lines ?source t ~total lines =
     let l = String.split_on_char ~sep:'\n' lines in
+    let lcount = ref 0 in
     let rec w = function
       | [ "" ] | [] -> ()
       | s :: xs ->
-          write ?source t s;
+          let _ = write ?source t s ~add:(fun n -> lcount := !lcount + n + 1) in
           w xs
     in
-    w l
+    w l;
+    total !lcount
 
   let lnum t = t.lnum
 end
@@ -170,6 +183,10 @@ let prefix_kind line =
       | true -> `Json_base64 (String.length sourceMappingURL_base64)
       | false -> `Url (String.length sourceMappingURL))
 
+let rule_out_index_map = function
+  | `Standard sm -> sm
+  | `Index _ -> failwith "unexpected index map at this stage"
+
 let action ~resolve_sourcemap_url ~drop_source_map file line =
   match prefix_kind line, drop_source_map with
   | `Other, (true | false) -> Keep
@@ -177,7 +194,9 @@ let action ~resolve_sourcemap_url ~drop_source_map file line =
   | `Build_info bi, _ -> Build_info bi
   | (`Json_base64 _ | `Url _), true -> Drop
   | `Json_base64 offset, false ->
-      Source_map (Source_map_io.of_string (Base64.decode_exn ~off:offset line))
+      Source_map
+        (rule_out_index_map
+           (Source_map_io.of_string (Base64.decode_exn ~off:offset line)))
   | `Url _, false when not resolve_sourcemap_url -> Drop
   | `Url offset, false ->
       let url = String.sub line ~pos:offset ~len:(String.length line - offset) in
@@ -186,44 +205,44 @@ let action ~resolve_sourcemap_url ~drop_source_map file line =
       let l = in_channel_length ic in
       let content = really_input_string ic l in
       close_in ic;
-      Source_map (Source_map_io.of_string content)
+      Source_map (rule_out_index_map (Source_map_io.of_string content))
 
 module Units : sig
-  val read : Line_reader.t -> Unit_info.t -> Unit_info.t
+  val read : Line_reader.t -> drop_action:(unit -> unit) -> Unit_info.t -> Unit_info.t
 
   val scan_file : string -> Build_info.t option * Unit_info.t list
 end = struct
-  let rec read ic uinfo =
+  let rec read ic ~drop_action uinfo =
     match Line_reader.peek ic with
     | None -> uinfo
     | Some line -> (
         match Unit_info.parse uinfo line with
         | None -> uinfo
         | Some uinfo ->
-            Line_reader.drop ic;
-            read ic uinfo)
+            Line_reader.drop ~drop_action ic;
+            read ic ~drop_action uinfo)
 
-  let find_unit_info ic =
+  let find_unit_info ~drop_action ic =
     let rec find_next ic =
       match Line_reader.peek ic with
       | None -> None
       | Some line -> (
           match prefix_kind line with
           | `Json_base64 _ | `Url _ | `Other | `Build_info _ ->
-              Line_reader.drop ic;
+              Line_reader.drop ~drop_action ic;
               find_next ic
-          | `Unit -> Some (read ic Unit_info.empty))
+          | `Unit -> Some (read ic ~drop_action Unit_info.empty))
     in
     find_next ic
 
-  let find_build_info ic =
+  let find_build_info ~drop_action ic =
     let rec find_next ic =
       match Line_reader.peek ic with
       | None -> None
       | Some line -> (
           match prefix_kind line with
           | `Json_base64 _ | `Url _ | `Other ->
-              Line_reader.drop ic;
+              Line_reader.drop ~drop_action ic;
               find_next ic
           | `Build_info bi -> Some bi
           | `Unit -> None)
@@ -232,12 +251,13 @@ end = struct
 
   let scan_file file =
     let ic = Line_reader.open_ file in
+    let drop_action () = () in
     let rec scan_all ic acc =
-      match find_unit_info ic with
+      match find_unit_info ~drop_action ic with
       | None -> List.rev acc
       | Some x -> scan_all ic (x :: acc)
     in
-    let build_info = find_build_info ic in
+    let build_info = find_build_info ~drop_action ic in
     let units = scan_all ic [] in
     Line_reader.close ic;
     build_info, units
@@ -318,12 +338,28 @@ let link ~output ~linkall ~mklib ~toplevel ~files ~resolve_sourcemap_url ~source
       in
       let sm_for_file = ref None in
       let ic = Line_reader.open_ file in
-      let skip ic = Line_reader.drop ic in
-      let reloc = ref [] in
+      let old_line_count = Line_writer.lnum oc in
+      let edits = ref [] in
+      let emit_drop_action edits () = edits := Source_map.Line_edits.Drop :: !edits in
+      let skip ic = Line_reader.drop ~drop_action:(emit_drop_action edits) ic in
       let copy ic oc =
         let line = Line_reader.next ic in
-        Line_writer.write ~source:ic oc line;
-        reloc := (Line_reader.lnum ic, Line_writer.lnum oc) :: !reloc
+        Line_writer.write
+          ~source:ic
+          ~add:(fun count -> edits := Add { count } :: !edits)
+          oc
+          line;
+        (* Note: line actions are in reverse order compared to the actual generated
+           lines *)
+        edits := Source_map.Line_edits.Keep :: !edits
+      in
+      let write_line oc str =
+        Line_writer.write oc str ~add:(fun count ->
+            edits := Source_map.Line_edits.(Add { count = count + 1 }) :: !edits)
+      in
+      let write_lines oc str =
+        Line_writer.write_lines oc str ~total:(fun count ->
+            edits := Source_map.Line_edits.(Add { count }) :: !edits)
       in
       let rec read () =
         match Line_reader.peek ic with
@@ -342,11 +378,13 @@ let link ~output ~linkall ~mklib ~toplevel ~files ~resolve_sourcemap_url ~source
                 if not !build_info_emitted
                 then (
                   let bi = Build_info.with_kind bi (if mklib then `Cma else `Unknown) in
-                  Line_writer.write_lines oc (Build_info.to_string bi);
+                  write_lines oc (Build_info.to_string bi);
                   build_info_emitted := true)
             | Drop -> skip ic
             | Unit ->
-                let u = Units.read ic Unit_info.empty in
+                let u =
+                  Units.read ic ~drop_action:(emit_drop_action edits) Unit_info.empty
+                in
                 if StringSet.cardinal (StringSet.inter u.Unit_info.provides to_link) > 0
                 then (
                   if u.effects_without_cps && not !warn_effects
@@ -358,7 +396,7 @@ let link ~output ~linkall ~mklib ~toplevel ~files ~resolve_sourcemap_url ~source
                   (if mklib
                    then
                      let u = if linkall then { u with force_link = true } else u in
-                     Line_writer.write_lines oc (Unit_info.to_string u));
+                     write_lines oc (Unit_info.to_string u));
                   let size = ref 0 in
                   while
                     match Line_reader.peek ic with
@@ -402,7 +440,7 @@ let link ~output ~linkall ~mklib ~toplevel ~files ~resolve_sourcemap_url ~source
             read ()
       in
       read ();
-      Line_writer.write oc "";
+      write_line oc "";
       Line_reader.close ic;
       (match is_runtime with
       | None -> ()
@@ -424,10 +462,11 @@ let link ~output ~linkall ~mklib ~toplevel ~files ~resolve_sourcemap_url ~source
             (Parse_bytecode.Debug.create ~include_cmis:false false)
             code;
           let content = Buffer.contents b in
-          Line_writer.write_lines oc content);
+          write_lines oc content);
       (match !sm_for_file with
       | None -> ()
-      | Some x -> sm := (x, !reloc) :: !sm);
+      | Some x ->
+          sm := (file, x, List.rev !edits, Line_writer.lnum oc - old_line_count) :: !sm);
       match !build_info, build_info_for_file with
       | None, None -> ()
       | Some _, None -> ()
@@ -440,32 +479,51 @@ let link ~output ~linkall ~mklib ~toplevel ~files ~resolve_sourcemap_url ~source
   match source_map with
   | None -> ()
   | Some (file, init_sm) ->
-      let sm =
-        List.rev_map !sm ~f:(fun (sm, reloc) ->
-            let tbl = Hashtbl.create 17 in
-            List.iter reloc ~f:(fun (a, b) -> Hashtbl.add tbl a b);
-            Source_map.filter_map sm ~f:(Hashtbl.find_opt tbl))
+      let sourcemaps_and_line_counts =
+        List.rev_map !sm ~f:(fun (file, sm, edits, lcount) ->
+            if debug ()
+            then (
+              Format.eprintf "@[<v>line actions for '%s' (lcount %d)@," file lcount;
+              Format.eprintf "%a@," Source_map.Line_edits.pp edits;
+              Format.eprintf "@]");
+            let mappings = sm.Source_map.mappings in
+            let mappings = Source_map.Mappings.edit ~strict:false mappings edits in
+            { sm with mappings }, lcount)
       in
-      (match Source_map.merge (init_sm :: sm) with
-      | None -> ()
-      | Some sm -> (
-          (* preserve some info from [init_sm] *)
-          let sm =
-            { sm with
-              version = init_sm.version
-            ; file = init_sm.file
-            ; sourceroot = init_sm.sourceroot
-            }
-          in
-          match file with
-          | None ->
-              let data = Source_map_io.to_string sm in
-              let s = sourceMappingURL_base64 ^ Base64.encode_exn data in
-              Line_writer.write oc s
-          | Some file ->
-              Source_map_io.to_file sm file;
-              let s = sourceMappingURL ^ Filename.basename file in
-              Line_writer.write oc s));
+      let merged_sourcemap =
+        let open Source_map in
+        assert (String.equal (Mappings.to_string init_sm.mappings) "");
+        { version = init_sm.version
+        ; file = init_sm.file
+        ; Index.sections =
+            (let _, sections =
+               List.fold_left
+                 sourcemaps_and_line_counts
+                 ~f:(fun (cur_ofs, sections) (sm, generated_line_count) ->
+                   let offset = Index.{ gen_line = cur_ofs; gen_column = 0 } in
+                   cur_ofs + generated_line_count, (offset, `Map sm) :: sections)
+                 ~init:(0, [])
+             in
+             List.rev sections)
+        }
+      in
+      (* preserve some info from [init_sm] *)
+      let merged_sourcemap =
+        { merged_sourcemap with
+          sections =
+            List.map merged_sourcemap.sections ~f:(fun (ofs, `Map sm) ->
+                ofs, `Map { sm with sourceroot = init_sm.sourceroot })
+        }
+      in
+      (match file with
+      | None ->
+          let data = Source_map_io.Index.to_string merged_sourcemap in
+          let s = sourceMappingURL_base64 ^ Base64.encode_exn data in
+          Line_writer.write oc s ~add:(fun _ -> ()) |> ignore
+      | Some file ->
+          Source_map_io.Index.to_file merged_sourcemap file;
+          let s = sourceMappingURL ^ Filename.basename file in
+          Line_writer.write oc s ~add:(fun _ -> ()) |> ignore);
       if times () then Format.eprintf "  sourcemap: %a@." Timer.print t
 
 let link ~output ~linkall ~mklib ~toplevel ~files ~resolve_sourcemap_url ~source_map =
diff --git a/compiler/lib/source_map.ml b/compiler/lib/source_map.ml
index 3d88b0d1bd..8da81c5fe6 100644
--- a/compiler/lib/source_map.ml
+++ b/compiler/lib/source_map.ml
@@ -40,16 +40,565 @@ type map =
       ; ori_name : int
       }
 
-type mapping = map list
+module Line_edits = struct
+  type action =
+    | Keep
+    | Drop
+    | Add of { count : int }
+
+  let pp_action fmt =
+    let open Format in
+    function
+    | Keep -> pp_print_string fmt "Keep"
+    | Drop -> pp_print_string fmt "Drop"
+    | Add { count } -> fprintf fmt "@[<hov>Add@ {@ count =@ %d@ }@]" count
+
+  type t = action list
+
+  let pp fmt = Format.(pp_print_list pp_action fmt)
+end
+
+module Mappings = struct
+  type t = Uninterpreted of string [@@unboxed]
+
+  let empty = Uninterpreted ""
+
+  external of_string : string -> t = "%identity"
+
+  external to_string : t -> string = "%identity"
+
+  let update_carries_from_segment
+      ~carry_source
+      ~carry_line
+      ~carry_col
+      ~carry_name
+      ~pos
+      ~len
+      str =
+    (* Note: we don't care about the first field since we do linewise editing,
+       and it is reset for every line. *)
+    match Vlq64.decode_l str ~pos ~len with
+    | [ _gen_col ] -> carry_source, carry_line, carry_col, carry_name
+    | [ _gen_col; source; line; col ] ->
+        carry_source + source, carry_line + line, carry_col + col, carry_name
+    | [ _gen_col; source; line; col; name ] ->
+        carry_source + source, carry_line + line, carry_col + col, carry_name + name
+    | _ -> invalid_arg "Mapping.update_carries_from_segment"
+
+  let update_carries_and_write_segment
+      ~carry_source
+      ~carry_line
+      ~carry_col
+      ~carry_name
+      ~pos
+      ~len
+      str
+      ~buf =
+    match Vlq64.decode_l str ~pos ~len with
+    | [ gen_col ] ->
+        Vlq64.encode_l buf [ gen_col ];
+        carry_source, carry_line, carry_col, carry_name
+    | [ gen_col; source; line; col ] ->
+        Vlq64.encode_l
+          buf
+          [ gen_col; source + carry_source; line + carry_line; col + carry_col ];
+        0, 0, 0, carry_name
+    | [ gen_col; source; line; col; name ] ->
+        Vlq64.encode_l
+          buf
+          [ gen_col
+          ; source + carry_source
+          ; line + carry_line
+          ; col + carry_col
+          ; name + carry_name
+          ];
+        0, 0, 0, 0
+    | _ ->
+        invalid_arg
+          (Format.sprintf
+             "Mapping.update_carries_from_segment %s"
+             (String.sub ~pos ~len str))
+
+  (* Fold [f] over the segments in string [str.(pos..len - 1)]. *)
+  let fold_on_segments ~str ~pos ~len f ~init =
+    let rec loop acc pos end_ =
+      if pos >= end_
+      then acc
+      else
+        let next_delim =
+          try min (String.index_from str pos ',') end_ with Not_found -> end_
+        in
+        let len = next_delim - pos in
+        if len <= 0 then acc else loop (f acc str ~pos ~len) (next_delim + 1) end_
+    in
+    loop init pos (pos + len)
+
+  type carries =
+    { carry_source : int
+    ; carry_line : int
+    ; carry_col : int
+    ; carry_name : int
+    }
+
+  let update_carries_from_line
+      ~carry_source
+      ~carry_line
+      ~carry_col
+      ~carry_name
+      ~pos
+      ~len
+      str =
+    fold_on_segments
+      ~str
+      ~pos
+      ~len
+      ~init:{ carry_source; carry_line; carry_col; carry_name }
+      (fun acc str ~pos ~len ->
+        let { carry_source; carry_line; carry_col; carry_name } = acc in
+        let carry_source, carry_line, carry_col, carry_name =
+          update_carries_from_segment
+            ~carry_source
+            ~carry_line
+            ~carry_col
+            ~carry_name
+            ~pos
+            ~len
+            str
+        in
+        { carry_source; carry_line; carry_col; carry_name })
+
+  let update_carries_and_write_line
+      ~carry_source
+      ~carry_line
+      ~carry_col
+      ~carry_name
+      ~pos
+      ~len
+      str
+      ~buf =
+    let _, carries =
+      fold_on_segments
+        ~str
+        ~pos
+        ~len
+        ~init:(true, { carry_source; carry_line; carry_col; carry_name })
+        (fun (is_first, acc) str ~pos ~len ->
+          let { carry_source; carry_line; carry_col; carry_name } = acc in
+          if not is_first then Buffer.add_char buf ',';
+          let carry_source, carry_line, carry_col, carry_name =
+            update_carries_and_write_segment
+              ~carry_source
+              ~carry_line
+              ~carry_col
+              ~carry_name
+              ~pos
+              ~len
+              str
+              ~buf
+          in
+          false, { carry_source; carry_line; carry_col; carry_name })
+    in
+    Buffer.add_char buf ';';
+    carries
+
+  (* If [strict], then the number of [Keep] and [Drop] elementes in [edits]
+     should be the same as the number of generated lines covered by the
+     mappings [orig]. Otherwise, there may be more edit actions, in which case
+     [Keep] just adds a line without mappings and [Drop] does nothing. *)
+  let rec edit_loop
+      ~orig
+      ~carry_source
+      ~carry_line
+      ~carry_col
+      ~carry_name
+      ~strict
+      buf
+      offset_in_orig
+      edits =
+    let open Line_edits in
+    if offset_in_orig >= String.length orig
+    then (
+      (* No more lines in the mappings string *)
+      match edits with
+      | [] -> { carry_source; carry_line; carry_col; carry_name }
+      | _ :: _ ->
+          List.iter edits ~f:(function
+              | Add { count } -> Buffer.add_string buf (String.make count ';')
+              | Keep ->
+                  if strict
+                  then
+                    invalid_arg
+                      "Mapping.edit: more Keep or Drop edits than lines in mappings";
+                  Buffer.add_char buf ';'
+              | Drop ->
+                  if strict
+                  then
+                    invalid_arg
+                      "Mapping.edit: more Keep or Drop edits than lines in mappings");
+          { carry_source; carry_line; carry_col; carry_name })
+    else
+      match edits with
+      | [] -> { carry_source; carry_line; carry_col; carry_name }
+      | Keep :: rem ->
+          let next_group_delim =
+            try String.index_from orig offset_in_orig ';'
+            with Not_found -> String.length orig
+          in
+          let { carry_source; carry_line; carry_col; carry_name } =
+            update_carries_and_write_line
+              ~carry_source
+              ~carry_line
+              ~carry_col
+              ~carry_name
+              ~pos:offset_in_orig
+              ~len:(next_group_delim - offset_in_orig)
+              orig
+              ~buf
+          in
+          edit_loop
+            ~orig
+            ~carry_source
+            ~carry_line
+            ~carry_col
+            ~carry_name
+            ~strict
+            buf
+            (* Skip the ';' *)
+            (next_group_delim + 1)
+            rem
+      | Drop :: rem ->
+          let next_group_delim =
+            try String.index_from orig offset_in_orig ';'
+            with Not_found -> String.length orig
+          in
+          let { carry_source; carry_line; carry_col; carry_name } =
+            update_carries_from_line
+              ~carry_source
+              ~carry_line
+              ~carry_col
+              ~carry_name
+              ~pos:offset_in_orig
+              ~len:(next_group_delim - offset_in_orig)
+              orig
+          in
+          edit_loop
+            ~orig
+            ~carry_source
+            ~carry_line
+            ~carry_col
+            ~carry_name
+            ~strict
+            buf
+            (next_group_delim + 1)
+            rem
+      | Add { count } :: rem ->
+          Buffer.add_string buf (String.make count ';');
+          edit_loop
+            ~orig
+            ~carry_source
+            ~carry_line
+            ~carry_col
+            ~carry_name
+            ~strict
+            buf
+            offset_in_orig
+            rem
+
+  let edit ~strict (Uninterpreted orig) edits =
+    let buf = Buffer.create 8_192 in
+    let _ =
+      edit_loop
+        ~strict
+        ~orig
+        ~carry_source:0
+        ~carry_line:0
+        ~carry_col:0
+        ~carry_name:0
+        buf
+        0
+        edits
+    in
+    Uninterpreted (Buffer.contents buf)
+
+  let num_gen_lines m =
+    let rec loop count pos =
+      if pos >= String.length m
+      then count
+      else
+        let next_delim =
+          try String.index_from m pos ';' with Not_found -> String.length m
+        in
+        if next_delim >= String.length m - 1
+        then (* This was the last line *)
+          count + 1
+        else loop (count + 1) (next_delim + 1)
+    in
+    loop 0 0
+
+  let sum_offsets_segment ~carry_source ~carry_line ~carry_col ~carry_name ~pos ~len str =
+    match Vlq64.decode_l str ~pos ~len with
+    | [ _gen_col ] -> { carry_source; carry_line; carry_col; carry_name }
+    | [ _gen_col; source; line; col ] ->
+        { carry_source = carry_source + source
+        ; carry_line = carry_line + line
+        ; carry_col = carry_col + col
+        ; carry_name
+        }
+    | [ _gen_col; source; line; col; name ] ->
+        { carry_source = carry_source + source
+        ; carry_line = carry_line + line
+        ; carry_col = carry_col + col
+        ; carry_name = carry_name + name
+        }
+    | _ -> invalid_arg "Mapping.sum_offsets_segment: invalid segment"
+
+  let sum_offsets_line ~carry_source ~carry_line ~carry_col ~carry_name ~pos ~len str =
+    fold_on_segments
+      ~str
+      ~pos
+      ~len
+      ~init:{ carry_source; carry_line; carry_col; carry_name }
+      (fun { carry_source; carry_line; carry_col; carry_name } str ~pos ~len ->
+        sum_offsets_segment ~carry_source ~carry_line ~carry_col ~carry_name ~pos ~len str)
+
+  (* Fold [f] over the ';'-separated groups in string [str.(pos..len - 1)]. *)
+  let fold_on_lines ~str f ~init =
+    let rec loop acc pos =
+      if pos >= String.length str
+      then acc
+      else
+        let next_delim =
+          try min (String.index_from str pos ';') (String.length str)
+          with Not_found -> String.length str
+        in
+        let len = next_delim - pos in
+        loop (f acc str ~pos ~len) (next_delim + 1)
+    in
+    loop init 0
+
+  let sum_offsets mapping =
+    fold_on_lines
+      ~str:mapping
+      ~init:{ carry_source = 0; carry_line = 0; carry_col = 0; carry_name = 0 }
+      (fun { carry_source; carry_line; carry_col; carry_name } str ~pos ~len ->
+        sum_offsets_line ~carry_source ~carry_line ~carry_col ~carry_name ~pos ~len str)
+
+  let concat ~source_count1 ~name_count1 (Uninterpreted m1) (Uninterpreted m2) =
+    match m1, m2 with
+    | "", m2 -> Uninterpreted m2
+    | m1, "" -> Uninterpreted m1
+    | _, _ ->
+        let buf = Buffer.create 8_192 in
+        (* First do a pass on [m1] to accumulate its carries. *)
+        let { carry_source; carry_line; carry_col; carry_name } = sum_offsets m1 in
+        Buffer.add_string buf m1;
+        if not (Char.equal m1.[String.length m1 - 1] ';') then Buffer.add_char buf ';';
+        let _ =
+          edit_loop
+            ~orig:m2
+              (* Make the initial absolute offsets in [m2] relative. Also account
+                 for the fact that fields [sources] and [names] of [m2] will be
+                 concatenated to those of [m1]. *)
+            ~carry_source:(source_count1 - carry_source)
+            ~carry_line:~-carry_line
+            ~carry_col:~-carry_col
+            ~carry_name:(name_count1 - carry_name)
+            ~strict:true
+            buf
+            0
+            (List.init ~len:(num_gen_lines m2) ~f:(Fun.const Line_edits.Keep))
+        in
+        Uninterpreted (Buffer.contents buf)
+
+  let gen_line = function
+    | Gen { gen_line; _ } | Gen_Ori { gen_line; _ } | Gen_Ori_Name { gen_line; _ } ->
+        gen_line
+
+  let gen_col = function
+    | Gen { gen_col; _ } | Gen_Ori { gen_col; _ } | Gen_Ori_Name { gen_col; _ } -> gen_col
+
+  let encode mapping =
+    let a = Array.of_list mapping in
+    let len = Array.length a in
+    Array.stable_sort
+      ~cmp:(fun t1 t2 ->
+        match compare (gen_line t1) (gen_line t2) with
+        | 0 -> compare (gen_col t1) (gen_col t2)
+        | n -> n)
+      a;
+    let buf = Buffer.create 1024 in
+    (* The binary format encodes lines starting at zero, but
+       [ori_line] and [gen_line] are 1 based. *)
+    let gen_line_r = ref 1 in
+    let gen_col_r = ref 0 in
+    let ori_source_r = ref 0 in
+    let ori_line_r = ref 1 in
+    let ori_col_r = ref 0 in
+    let ori_name_r = ref 0 in
+    let rec loop prev i =
+      if i < len
+      then
+        let c = a.(i) in
+        if i + 1 < len && gen_line c = gen_line a.(i + 1) && gen_col c = gen_col a.(i + 1)
+        then (* Only keep one source location per generated location *)
+          loop prev (i + 1)
+        else (
+          if !gen_line_r <> gen_line c
+          then (
+            assert (!gen_line_r < gen_line c);
+            for _i = !gen_line_r to gen_line c - 1 do
+              Buffer.add_char buf ';'
+            done;
+            gen_col_r := 0;
+            gen_line_r := gen_line c)
+          else if i > 0
+          then Buffer.add_char buf ',';
+          let l =
+            match c with
+            | Gen { gen_line = _; gen_col } ->
+                let res = [ gen_col - !gen_col_r ] in
+                gen_col_r := gen_col;
+                res
+            | Gen_Ori { gen_line = _; gen_col; ori_source; ori_line; ori_col } ->
+                let res =
+                  [ gen_col - !gen_col_r
+                  ; ori_source - !ori_source_r
+                  ; ori_line - !ori_line_r
+                  ; ori_col - !ori_col_r
+                  ]
+                in
+                gen_col_r := gen_col;
+                ori_col_r := ori_col;
+                ori_line_r := ori_line;
+                ori_source_r := ori_source;
+                res
+            | Gen_Ori_Name
+                { gen_line = _; gen_col; ori_source; ori_line; ori_col; ori_name } ->
+                let res =
+                  [ gen_col - !gen_col_r
+                  ; ori_source - !ori_source_r
+                  ; ori_line - !ori_line_r
+                  ; ori_col - !ori_col_r
+                  ; ori_name - !ori_name_r
+                  ]
+                in
+                gen_col_r := gen_col;
+                ori_col_r := ori_col;
+                ori_line_r := ori_line;
+                ori_source_r := ori_source;
+                ori_name_r := ori_name;
+                res
+          in
+          Vlq64.encode_l buf l;
+          loop i (i + 1))
+    in
+    loop (-1) 0;
+    Uninterpreted (Buffer.contents buf)
+
+  let decode (Uninterpreted str) =
+    let total_len = String.length str in
+    let gen_col = ref 0 in
+    let ori_source = ref 0 in
+    let ori_line = ref 1 in
+    let ori_col = ref 0 in
+    let ori_name = ref 0 in
+    let rec readline line pos acc =
+      if pos >= total_len
+      then List.rev acc
+      else
+        let last = try String.index_from str pos ';' with Not_found -> total_len in
+        gen_col := 0;
+        let pos, acc =
+          if pos = last then pos + 1, acc else read_tokens line pos last acc
+        in
+        readline (succ line) pos acc
+    and read_tokens line start stop acc =
+      let last =
+        try min (String.index_from str start ',') stop with Not_found -> stop
+      in
+      let v = Vlq64.decode_l str ~pos:start ~len:(last - start) in
+      match v with
+      | [] -> last + 1, acc
+      | v ->
+          let v =
+            match v with
+            | [ g ] ->
+                gen_col := !gen_col + g;
+                Gen { gen_line = line; gen_col = !gen_col }
+            | [ g; os; ol; oc ] ->
+                gen_col := !gen_col + g;
+                ori_source := !ori_source + os;
+                ori_line := !ori_line + ol;
+                ori_col := !ori_col + oc;
+                Gen_Ori
+                  { gen_line = line
+                  ; gen_col = !gen_col
+                  ; ori_source = !ori_source
+                  ; ori_line = !ori_line
+                  ; ori_col = !ori_col
+                  }
+            | [ g; os; ol; oc; on ] ->
+                gen_col := !gen_col + g;
+                ori_source := !ori_source + os;
+                ori_line := !ori_line + ol;
+                ori_col := !ori_col + oc;
+                ori_name := !ori_name + on;
+                Gen_Ori_Name
+                  { gen_line = line
+                  ; gen_col = !gen_col
+                  ; ori_source = !ori_source
+                  ; ori_line = !ori_line
+                  ; ori_col = !ori_col
+                  ; ori_name = !ori_name
+                  }
+            | _ -> invalid_arg "Source_map.mapping_of_string"
+          in
+          let acc = v :: acc in
+          if last = stop then last + 1, acc else read_tokens line (last + 1) stop acc
+    in
+    (* The binary format encodes lines starting at zero, but
+       [ori_line] and [gen_line] are 1 based. *)
+    readline 1 0 []
+end
+
+module Source_text = struct
+  type t = Uninterpreted of string [@@unboxed]
+
+  external of_json_string : string -> t = "%identity"
+
+  external to_json_string : t -> string = "%identity"
+
+  let empty = Uninterpreted ""
+
+  let to_json = function
+    | None -> `Null
+    | Some text -> `String text
+
+  let encode t =
+    let json = Yojson.Basic.to_string (to_json t) in
+    Uninterpreted json
+
+  let of_json = function
+    | `String s -> Some s
+    | `Null -> None
+    | _ -> invalid_arg "Source_map.Sources_contents.of_json: expected string or null"
+
+  let decode (Uninterpreted s) : string option =
+    (* The two stages of the encoding, in reverse. *)
+    try of_json (Yojson.Basic.from_string s)
+    with Yojson.Json_error s ->
+      invalid_arg
+        ("Source_map.Sources_contents.decode: This is not a valid JSON object: " ^ s)
+end
 
 type t =
   { version : int
   ; file : string
   ; sourceroot : string option
   ; sources : string list
-  ; sources_content : string option list option
+  ; sources_contents : Source_text.t list option
   ; names : string list
-  ; mappings : mapping
+  ; mappings : Mappings.t
   }
 
 let empty ~filename =
@@ -57,244 +606,40 @@ let empty ~filename =
   ; file = filename
   ; sourceroot = None
   ; sources = []
-  ; sources_content = None
+  ; sources_contents = None
   ; names = []
-  ; mappings = []
+  ; mappings = Mappings.empty
   }
 
-let gen_line = function
-  | Gen { gen_line; _ } | Gen_Ori { gen_line; _ } | Gen_Ori_Name { gen_line; _ } ->
-      gen_line
-
-let gen_col = function
-  | Gen { gen_col; _ } | Gen_Ori { gen_col; _ } | Gen_Ori_Name { gen_col; _ } -> gen_col
-
-let string_of_mapping mapping =
-  let a = Array.of_list mapping in
-  let len = Array.length a in
-  Array.stable_sort
-    ~cmp:(fun t1 t2 ->
-      match compare (gen_line t1) (gen_line t2) with
-      | 0 -> compare (gen_col t1) (gen_col t2)
-      | n -> n)
-    a;
-  let buf = Buffer.create 1024 in
-  (* The binary format encodes lines starting at zero, but
-     [ori_line] and [gen_line] are 1 based. *)
-  let gen_line_r = ref 1 in
-  let gen_col_r = ref 0 in
-  let ori_source_r = ref 0 in
-  let ori_line_r = ref 1 in
-  let ori_col_r = ref 0 in
-  let ori_name_r = ref 0 in
-  let rec loop prev i =
-    if i < len
-    then
-      let c = a.(i) in
-      if i + 1 < len && gen_line c = gen_line a.(i + 1) && gen_col c = gen_col a.(i + 1)
-      then (* Only keep one source location per generated location *)
-        loop prev (i + 1)
-      else (
-        if !gen_line_r <> gen_line c
-        then (
-          assert (!gen_line_r < gen_line c);
-          for _i = !gen_line_r to gen_line c - 1 do
-            Buffer.add_char buf ';'
-          done;
-          gen_col_r := 0;
-          gen_line_r := gen_line c)
-        else if i > 0
-        then Buffer.add_char buf ',';
-        let l =
-          match c with
-          | Gen { gen_line = _; gen_col } ->
-              let res = [ gen_col - !gen_col_r ] in
-              gen_col_r := gen_col;
-              res
-          | Gen_Ori { gen_line = _; gen_col; ori_source; ori_line; ori_col } ->
-              let res =
-                [ gen_col - !gen_col_r
-                ; ori_source - !ori_source_r
-                ; ori_line - !ori_line_r
-                ; ori_col - !ori_col_r
-                ]
-              in
-              gen_col_r := gen_col;
-              ori_col_r := ori_col;
-              ori_line_r := ori_line;
-              ori_source_r := ori_source;
-              res
-          | Gen_Ori_Name
-              { gen_line = _; gen_col; ori_source; ori_line; ori_col; ori_name } ->
-              let res =
-                [ gen_col - !gen_col_r
-                ; ori_source - !ori_source_r
-                ; ori_line - !ori_line_r
-                ; ori_col - !ori_col_r
-                ; ori_name - !ori_name_r
-                ]
-              in
-              gen_col_r := gen_col;
-              ori_col_r := ori_col;
-              ori_line_r := ori_line;
-              ori_source_r := ori_source;
-              ori_name_r := ori_name;
-              res
-        in
-        Vlq64.encode_l buf l;
-        loop i (i + 1))
-  in
-  loop (-1) 0;
-  Buffer.contents buf
-
-let mapping_of_string str =
-  let total_len = String.length str in
-  let gen_col = ref 0 in
-  let ori_source = ref 0 in
-  let ori_line = ref 1 in
-  let ori_col = ref 0 in
-  let ori_name = ref 0 in
-  let rec readline line pos acc =
-    if pos >= total_len
-    then List.rev acc
-    else
-      let last = try String.index_from str pos ';' with Not_found -> total_len in
-      gen_col := 0;
-      let pos, acc = if pos = last then pos + 1, acc else read_tokens line pos last acc in
-      readline (succ line) pos acc
-  and read_tokens line start stop acc =
-    let last = try min (String.index_from str start ',') stop with Not_found -> stop in
-    let v = Vlq64.decode_l str ~pos:start ~len:(last - start) in
-    match v with
-    | [] -> last + 1, acc
-    | v ->
-        let v =
-          match v with
-          | [ g ] ->
-              gen_col := !gen_col + g;
-              Gen { gen_line = line; gen_col = !gen_col }
-          | [ g; os; ol; oc ] ->
-              gen_col := !gen_col + g;
-              ori_source := !ori_source + os;
-              ori_line := !ori_line + ol;
-              ori_col := !ori_col + oc;
-              Gen_Ori
-                { gen_line = line
-                ; gen_col = !gen_col
-                ; ori_source = !ori_source
-                ; ori_line = !ori_line
-                ; ori_col = !ori_col
-                }
-          | [ g; os; ol; oc; on ] ->
-              gen_col := !gen_col + g;
-              ori_source := !ori_source + os;
-              ori_line := !ori_line + ol;
-              ori_col := !ori_col + oc;
-              ori_name := !ori_name + on;
-              Gen_Ori_Name
-                { gen_line = line
-                ; gen_col = !gen_col
-                ; ori_source = !ori_source
-                ; ori_line = !ori_line
-                ; ori_col = !ori_col
-                ; ori_name = !ori_name
-                }
-          | _ -> invalid_arg "Source_map.mapping_of_string"
-        in
-        let acc = v :: acc in
-        if last = stop then last + 1, acc else read_tokens line (last + 1) stop acc
-  in
-  (* The binary format encodes lines starting at zero, but
-     [ori_line] and [gen_line] are 1 based. *)
-  readline 1 0 []
-
-let maps ~sources_offset ~names_offset x =
-  match x with
-  | Gen _ -> x
-  | Gen_Ori { gen_line; gen_col; ori_source; ori_line; ori_col } ->
-      let ori_source = ori_source + sources_offset in
-      Gen_Ori { gen_line; gen_col; ori_source; ori_line; ori_col }
-  | Gen_Ori_Name { gen_line; gen_col; ori_source; ori_line; ori_col; ori_name } ->
-      let ori_source = ori_source + sources_offset in
-      let ori_name = ori_name + names_offset in
-      Gen_Ori_Name { gen_line; gen_col; ori_source; ori_line; ori_col; ori_name }
-
-let filter_map sm ~f =
-  let a = Array.of_list sm.mappings in
-  Array.stable_sort
-    ~cmp:(fun t1 t2 ->
-      match compare (gen_line t1) (gen_line t2) with
-      | 0 -> compare (gen_col t1) (gen_col t2)
-      | n -> n)
-    a;
-  let l = Array.to_list a |> List.group ~f:(fun a b -> gen_line a = gen_line b) in
-
-  let rec loop acc mapping =
-    match mapping with
-    | [] -> List.rev acc
-    | x :: xs ->
-        let gen_line = gen_line (List.hd x) in
-        let acc =
-          match f gen_line with
-          | None -> acc
-          | Some gen_line ->
-              List.rev_append_map
-                x
-                ~f:(function
-                  | Gen { gen_line = _; gen_col } -> Gen { gen_line; gen_col }
-                  | Gen_Ori { gen_line = _; gen_col; ori_source; ori_line; ori_col } ->
-                      Gen_Ori { gen_line; gen_col; ori_source; ori_line; ori_col }
-                  | Gen_Ori_Name
-                      { gen_line = _; gen_col; ori_source; ori_line; ori_col; ori_name }
-                    ->
-                      Gen_Ori_Name
-                        { gen_line; gen_col; ori_source; ori_line; ori_col; ori_name })
-                acc
-        in
-        loop acc xs
-  in
-  let mappings = loop [] l in
-  { sm with mappings }
-
-let merge = function
-  | [] -> None
-  | _ :: _ as l ->
-      let rec loop acc_rev ~sources_offset ~names_offset l =
-        match l with
-        | [] -> acc_rev
-        | sm :: rest ->
-            let acc_rev =
-              { acc_rev with
-                sources = List.rev_append sm.sources acc_rev.sources
-              ; names = List.rev_append sm.names acc_rev.names
-              ; sources_content =
-                  (match sm.sources_content, acc_rev.sources_content with
-                  | Some x, Some acc_rev -> Some (List.rev_append x acc_rev)
-                  | None, _ | _, None -> None)
-              ; mappings =
-                  List.rev_append_map
-                    ~f:(maps ~sources_offset ~names_offset)
-                    sm.mappings
-                    acc_rev.mappings
-              }
-            in
-            loop
-              acc_rev
-              ~sources_offset:(sources_offset + List.length sm.sources)
-              ~names_offset:(names_offset + List.length sm.names)
-              rest
-      in
-      let acc_rev =
-        loop
-          { (empty ~filename:"") with sources_content = Some [] }
-          ~sources_offset:0
-          ~names_offset:0
-          l
-      in
-      Some
-        { acc_rev with
-          mappings = List.rev acc_rev.mappings
-        ; sources = List.rev acc_rev.sources
-        ; names = List.rev acc_rev.names
-        ; sources_content = Option.map ~f:List.rev acc_rev.sources_content
-        }
+let concat ~file ~sourceroot s1 s2 =
+  if not (Int.equal s1.version s2.version)
+  then invalid_arg "Source_map.concat: different versions";
+  { version = s1.version
+  ; file
+  ; sourceroot
+  ; sources = s1.sources @ s2.sources
+  ; sources_contents =
+      (match s1.sources_contents, s2.sources_contents with
+      | None, contents | contents, None -> contents
+      | Some c1, Some c2 -> Some (c1 @ c2))
+  ; names = s1.names @ s2.names
+  ; mappings =
+      Mappings.concat
+        ~source_count1:(List.length s1.sources)
+        ~name_count1:(List.length s1.names)
+        s1.mappings
+        s2.mappings
+  }
+
+module Index = struct
+  type offset =
+    { gen_line : int
+    ; gen_column : int
+    }
+
+  type nonrec t =
+    { version : int
+    ; file : string
+    ; sections : (offset * [ `Map of t ]) list
+    }
+end
diff --git a/compiler/lib/source_map.mli b/compiler/lib/source_map.mli
index b394fe8970..c40a0e8dc5 100644
--- a/compiler/lib/source_map.mli
+++ b/compiler/lib/source_map.mli
@@ -38,24 +38,99 @@ type map =
       ; ori_name : int
       }
 
-type mapping = map list
+module Line_edits : sig
+  type action =
+    | Keep
+    | Drop
+    | Add of { count : int }
+
+  val pp_action : Format.formatter -> action -> unit
+
+  type t = action list
+
+  val pp : Format.formatter -> t -> unit
+end
+
+module Mappings : sig
+  type t
+
+  val empty : t
+  (** Represents the empty mapping. *)
+
+  val of_string : string -> t
+  (** By default, mappings are left uninterpreted, since many operations can be
+      performed efficiently directly on the encoded form. It is guaranteed that
+      {!val:of_string} and {!val:to_string} are inverse functions. *)
+
+  val decode : t -> map list
+
+  val encode : map list -> t
+
+  val to_string : t -> string
+  (** Returns the mappings as a string in the Source map v3 format. *)
+
+  val edit : strict:bool -> t -> Line_edits.t -> t
+  (** Apply line edits in order. If the number of {!const:Line_edits.Keep} and
+      {!const:Line_edits.Drop} actions does not match the number of lines in
+      the domain of the input mapping, only the lines affected by an edit are
+      included in the result. *)
+end
+
+module Source_text : sig
+  type t
+
+  val empty : t
+
+  val of_json_string : string -> t
+  (** By default, sources contents are left uninterpreted as decoding this field can be
+      costly if the amount of code is large, and is seldom required. It is guaranteed that
+      {!val:of_json_string} and {!val:to_json_string} are inverse functions.  *)
+
+  val decode : t -> string option
+
+  val encode : string option -> t
+
+  val to_json_string : t -> string
+  (** Returns a valid JSON object (in this instance, a string literal, double quotes
+      included) representing the source text. *)
+end
 
 type t =
   { version : int
   ; file : string
   ; sourceroot : string option
   ; sources : string list
-  ; sources_content : string option list option
+  ; sources_contents : Source_text.t list option
+        (** Left uninterpreted by default, since decoding it requires to handle special
+          characters, which can be costly for huge codebases. *)
   ; names : string list
-  ; mappings : mapping
+  ; mappings : Mappings.t
+        (** Left uninterpreted, since most useful operations can be performed efficiently
+          directly on the encoded form, and a full decoding can be costly for big
+          sourcemaps. *)
   }
 
-val filter_map : t -> f:(int -> int option) -> t
-
-val merge : t list -> t option
+val empty : filename:string -> t
 
-val mapping_of_string : string -> mapping
+val concat : file:string -> sourceroot:string option -> t -> t -> t
+(** If [s1] encodes a mapping for a generated file [f1], and [s2] for a
+    generated file [f2], then [concat ~file ~sourceroot s1 s2] encodes the
+    union of these mappings for the concatenation of [f1] and [f2], with name
+    [file] and source root [sourceroot). *)
 
-val string_of_mapping : mapping -> string
+module Index : sig
+  type offset =
+    { gen_line : int
+    ; gen_column : int
+    }
 
-val empty : filename:string -> t
+  type nonrec t =
+    { version : int
+    ; file : string
+    ; sections : (offset * [ `Map of t ]) list
+          (** List of [(offset, map)] pairs. The sourcemap spec allows for [map] to be
+            either a sourcemap object or a URL, but we don't need to generate
+            composite sourcemaps with URLs for now, and it is therefore not
+            implemented. *)
+    }
+end
diff --git a/compiler/lib/source_map_io.mli b/compiler/lib/source_map_io.mli
index 65c6b905b2..41c41cba1b 100644
--- a/compiler/lib/source_map_io.mli
+++ b/compiler/lib/source_map_io.mli
@@ -25,4 +25,10 @@ val to_string : t -> string
 
 val to_file : t -> string -> unit
 
-val of_string : string -> t
+module Index : sig
+  val to_string : Index.t -> string
+
+  val to_file : Index.t -> string -> unit
+end
+
+val of_string : string -> [ `Standard of Source_map.t | `Index of Source_map.Index.t ]
diff --git a/compiler/lib/source_map_io.unsupported.ml b/compiler/lib/source_map_io.unsupported.ml
index dbdb35816c..9199c0fa45 100644
--- a/compiler/lib/source_map_io.unsupported.ml
+++ b/compiler/lib/source_map_io.unsupported.ml
@@ -25,4 +25,10 @@ let of_string _ = fail ()
 
 let to_file _ _ = fail ()
 
+module Composite = struct
+  let to_string _ _ = fail ()
+
+  let to_file _ _ = fail ()
+end
+
 let enabled = false
diff --git a/compiler/lib/source_map_io.yojson.ml b/compiler/lib/source_map_io.yojson.ml
index 05f0975e63..ae0267de0a 100644
--- a/compiler/lib/source_map_io.yojson.ml
+++ b/compiler/lib/source_map_io.yojson.ml
@@ -19,47 +19,58 @@
 
 open Source_map
 
+let rewrite_path path =
+  if Filename.is_relative path
+  then path
+  else
+    match Build_path_prefix_map.get_build_path_prefix_map () with
+    | Some map -> Build_path_prefix_map.rewrite map path
+    | None -> path
+
+(* Escapes special characters and wrap in double quotes *)
+let stringlit_of_string s = `Stringlit (Yojson.Basic.to_string (`String s))
+
 let json t =
-  let rewrite_path path =
-    if Filename.is_relative path
-    then path
-    else
-      match Build_path_prefix_map.get_build_path_prefix_map () with
-      | Some map -> Build_path_prefix_map.rewrite map path
-      | None -> path
+  let mappings =
+    `Stringlit ("\"" ^ Mappings.to_string t.mappings ^ "\"")
+    (* Nothing to escape *)
   in
-  `Assoc
-    [ "version", `Float (float_of_int t.version)
-    ; "file", `String (rewrite_path t.file)
+  let fields =
+    [ "version", `Intlit (Int.to_string t.version)
+    ; "file", stringlit_of_string (rewrite_path t.file)
     ; ( "sourceRoot"
-      , `String
+      , stringlit_of_string
           (match t.sourceroot with
           | None -> ""
           | Some s -> rewrite_path s) )
-    ; "names", `List (List.map (fun s -> `String s) t.names)
-    ; "sources", `List (List.map (fun s -> `String (rewrite_path s)) t.sources)
-    ; "mappings", `String (string_of_mapping t.mappings)
-    ; ( "sourcesContent"
-      , `List
-          (match t.sources_content with
-          | None -> []
-          | Some l ->
-              List.map
-                (function
-                  | None -> `Null
-                  | Some s -> `String s)
-                l) )
+    ; "names", `List (List.map (fun s -> stringlit_of_string s) t.names)
+    ; ( "sources"
+      , `List (List.map (fun s -> stringlit_of_string (rewrite_path s)) t.sources) )
+    ; "mappings", mappings
     ]
+  in
+  match t.sources_contents with
+  | None -> `Assoc fields
+  | Some cs ->
+      `Assoc
+        (fields
+        @ [ ( "sourcesContent"
+            , `List (List.map (fun t -> `Stringlit (Source_text.to_json_string t)) cs) )
+          ])
 
 let invalid () = invalid_arg "Source_map.of_json"
 
+let string_of_stringlit (`Stringlit s) =
+  match Yojson.Basic.from_string s with
+  | `String s -> s
+  | _ -> invalid_arg "Source_map_io.string_of_stringlit: not a JSON string literal"
+
 let string name rest =
-  try
-    match List.assoc name rest with
-    | `String s -> Some s
-    | `Null -> None
-    | _ -> invalid ()
-  with Not_found -> None
+  match List.assoc name rest with
+  | `Stringlit _ as s -> Some (string_of_stringlit s)
+  | `Null -> None
+  | _ -> invalid ()
+  | exception Not_found -> None
 
 let list_string name rest =
   try
@@ -68,54 +79,161 @@ let list_string name rest =
         Some
           (List.map
              (function
-               | `String s -> s
+               | `Stringlit _ as lit -> string_of_stringlit lit
                | _ -> invalid ())
              l)
     | _ -> invalid ()
   with Not_found -> None
 
-let list_string_opt name rest =
-  try
-    match List.assoc name rest with
-    | `List l ->
-        Some
-          (List.map
-             (function
-               | `String s -> Some s
-               | `Null -> None
-               | _ -> invalid ())
-             l)
-    | _ -> invalid ()
-  with Not_found -> None
+let stringlit_opt name assoc =
+  match List.assoc name assoc with
+  | `Stringlit s -> Some s
+  | _ | (exception Not_found) -> None
+
+let stringlit_list_opt name assoc =
+  match List.assoc name assoc with
+  | `List l ->
+      Some
+        (List.map
+           (function
+             | `Stringlit lit -> lit
+             | _ -> invalid ())
+           l)
+  | _ -> invalid ()
+  | exception Not_found -> None
 
-let of_json json =
+let standard_map_of_json json =
   match json with
-  | `Assoc (("version", `Float version) :: rest) when int_of_float version = 3 ->
-      let def v d =
-        match v with
-        | None -> d
-        | Some v -> v
-      in
+  | `Assoc (("version", version) :: rest) ->
+      (match version with
+      | `Floatlit version when Float.equal (Float.of_string version) 3.0 -> ()
+      | `Intlit version when Int.equal (int_of_string version) 3 -> ()
+      | `Floatlit _ | `Intlit _ -> invalid_arg "Source_map_io.of_json: version != 3"
+      | _ -> invalid_arg "Source_map_io.of_json: version field is not a number");
+      (match List.assoc "sections" rest with
+      | _ -> invalid_arg "Source_map_io.standard_map_of_json: not a standard map"
+      | exception Not_found -> ());
       let file = string "file" rest in
       let sourceroot = string "sourceRoot" rest in
       let names = list_string "names" rest in
       let sources = list_string "sources" rest in
-      let sources_content = list_string_opt "sourcesContent" rest in
-      let mappings = string "mappings" rest in
-      { version = int_of_float version
-      ; file = def file ""
+      let sources_contents = stringlit_list_opt "sourcesContent" rest in
+      let mappings = stringlit_opt "mappings" rest in
+      let mappings =
+        Option.map
+          (fun mappings ->
+            assert (
+              String.length mappings >= 2
+              && Char.equal mappings.[0] '"'
+              && Char.equal mappings.[String.length mappings - 1] '"');
+            Mappings.of_string (String.sub mappings 1 (String.length mappings - 2)))
+          mappings
+      in
+      { version = 3
+      ; file = Option.value file ~default:""
       ; sourceroot
-      ; names = def names []
-      ; sources_content
-      ; sources = def sources []
-      ; mappings = mapping_of_string (def mappings "")
+      ; names = Option.value names ~default:[]
+      ; sources_contents =
+          Option.map (List.map Source_text.of_json_string) sources_contents
+      ; sources = Option.value sources ~default:[]
+      ; mappings = Option.value mappings ~default:Mappings.empty
       }
   | _ -> invalid ()
 
-let of_string s = of_json (Yojson.Basic.from_string s)
-
-let to_string m = Yojson.Basic.to_string (json m)
+let to_string m = Yojson.Raw.to_string (json m)
 
-let to_file m file = Yojson.Basic.to_file file (json m)
+let to_file m file = Yojson.Raw.to_file file (json m)
 
 let enabled = true
+
+module Index = struct
+  let json t =
+    `Assoc
+      [ "version", `Intlit (Int.to_string t.Index.version)
+      ; "file", stringlit_of_string (rewrite_path t.file)
+      ; ( "sections"
+        , `List
+            (List.map
+               (fun ({ Index.gen_line; gen_column }, `Map sm) ->
+                 `Assoc
+                   [ ( "offset"
+                     , `Assoc
+                         [ "line", `Intlit (Int.to_string gen_line)
+                         ; "column", `Intlit (Int.to_string gen_column)
+                         ] )
+                   ; "map", json sm
+                   ])
+               t.sections) )
+      ]
+
+  let intlit ~errmsg name json =
+    match List.assoc name json with
+    | `Intlit i -> int_of_string i
+    | _ -> invalid_arg errmsg
+    | exception Not_found -> invalid_arg errmsg
+
+  let section_of_json : Yojson.Raw.t -> Index.offset * [ `Map of t ] = function
+    | `Assoc json ->
+        let offset =
+          match List.assoc "offset" json with
+          | `Assoc fields ->
+              let gen_line =
+                intlit
+                  "line"
+                  fields
+                  ~errmsg:
+                    "Source_map_io.Index.of_json: field 'line' absent or invalid from \
+                     section"
+              in
+              let gen_column =
+                intlit
+                  "column"
+                  fields
+                  ~errmsg:
+                    "Source_map_io.Index.of_json: field 'column' absent or invalid from \
+                     section"
+              in
+              Index.{ gen_line; gen_column }
+          | _ ->
+              invalid_arg "Source_map_io.Index.of_json: 'offset' field of unexpected type"
+        in
+        (match List.assoc "url" json with
+        | _ ->
+            invalid_arg
+              "Source_map_io.Index.of_json: URLs in index maps are not currently \
+               supported"
+        | exception Not_found -> ());
+        let map =
+          try standard_map_of_json (List.assoc "map" json) with
+          | Not_found -> invalid_arg "Source_map_io.Index.of_json: field 'map' absent"
+          | Invalid_argument _ ->
+              invalid_arg "Source_map_io.Index.of_json: invalid sub-map object"
+        in
+        offset, `Map map
+    | _ -> invalid_arg "Source_map_io.Index.of_json: section of unexpected type"
+
+  let of_json = function
+    | `Assoc fields -> (
+        let file = string "file" fields in
+        match List.assoc "sections" fields with
+        | `List sections ->
+            let sections = List.map section_of_json sections in
+            { Index.version = 3; file = Option.value file ~default:""; sections }
+        | _ -> invalid_arg "Source_map_io.Index.of_json: `sections` is not an array"
+        | exception Not_found ->
+            invalid_arg "Source_map_io.Index.of_json: no `sections` field")
+    | _ -> invalid_arg "Source_map_io.of_json: map is not an object"
+
+  let to_string m = Yojson.Raw.to_string (json m)
+
+  let to_file m file = Yojson.Raw.to_file file (json m)
+end
+
+let of_json = function
+  | `Assoc fields as json -> (
+      match List.assoc "sections" fields with
+      | _ -> `Index (Index.of_json json)
+      | exception Not_found -> `Standard (standard_map_of_json json))
+  | _ -> invalid_arg "Source_map_io.of_json: map is not an object"
+
+let of_string s = of_json (Yojson.Raw.from_string s)
diff --git a/compiler/lib/stdlib.ml b/compiler/lib/stdlib.ml
index e1a108e6a6..58ae474597 100644
--- a/compiler/lib/stdlib.ml
+++ b/compiler/lib/stdlib.ml
@@ -390,6 +390,12 @@ module Option = struct
   let value ~default = function
     | None -> default
     | Some s -> s
+
+  let return v = Some v
+
+  module Syntax = struct
+    let ( let* ) x f = bind ~f x
+  end
 end
 
 module Int64 = struct
diff --git a/compiler/tests-compiler/build_path_prefix_map.ml b/compiler/tests-compiler/build_path_prefix_map.ml
index 6e2ce10fdc..00117ee13e 100644
--- a/compiler/tests-compiler/build_path_prefix_map.ml
+++ b/compiler/tests-compiler/build_path_prefix_map.ml
@@ -29,12 +29,13 @@ let%expect_test _ =
       |> compile_cmo_to_javascript ~sourcemap:true ~pretty:false
       |> extract_sourcemap
       |> function
-      | Some (sm : Js_of_ocaml_compiler.Source_map.t) ->
+      | Some (`Standard (sm : Js_of_ocaml_compiler.Source_map.t)) ->
           Printf.printf "file: %s\n" sm.file;
           Printf.printf "sourceRoot: %s\n" (Option.value ~default:"<none>" sm.sourceroot);
           Printf.printf "sources:\n";
           List.iter sm.sources ~f:(fun source ->
               Printf.printf "- %s\n" (normalize_path source))
+      | Some (`Index _) -> failwith "unexpected index map"
       | None -> failwith "no sourcemap generated!");
   [%expect
     {|
diff --git a/compiler/tests-compiler/sourcemap.ml b/compiler/tests-compiler/sourcemap.ml
index 791cb94d27..65469a029e 100644
--- a/compiler/tests-compiler/sourcemap.ml
+++ b/compiler/tests-compiler/sourcemap.ml
@@ -23,7 +23,8 @@ open Util
 let print_mapping (sm : Source_map.t) =
   let sources = Array.of_list sm.sources in
   let _names = Array.of_list sm.names in
-  List.iter sm.mappings ~f:(fun (m : Source_map.map) ->
+  let mappings = Source_map.Mappings.decode sm.mappings in
+  List.iter mappings ~f:(fun (m : Source_map.map) ->
       match m with
       | Gen_Ori { gen_line; gen_col; ori_line; ori_col; ori_source }
       | Gen_Ori_Name { gen_line; gen_col; ori_line; ori_col; ori_source; ori_name = _ } ->
@@ -54,7 +55,8 @@ let%expect_test _ =
       print_file (Filetype.path_of_js_file js_file);
       match extract_sourcemap js_file with
       | None -> Printf.printf "No sourcemap found\n"
-      | Some sm -> print_mapping sm);
+      | Some (`Standard sm) -> print_mapping sm
+      | Some (`Index _) -> failwith "unexpected index map");
   [%expect
     {|
       $ cat "test.ml"
@@ -110,8 +112,8 @@ function x (a, b) {
 
 let%expect_test _ =
   let map_str = ";;;;EAEE,EAAE,EAAC,CAAE;ECQY,UACC" in
-  let map = Source_map.mapping_of_string map_str in
-  let map_str' = Source_map.string_of_mapping map in
+  let map = Source_map.Mappings.(decode (of_string map_str)) in
+  let map_str' = Source_map.Mappings.(to_string (encode map)) in
   print_endline map_str;
   print_endline map_str';
   [%expect
@@ -128,25 +130,30 @@ let%expect_test _ =
     { (Source_map.empty ~filename:"1.map") with
       names = [ "na"; "nb"; "nc" ]
     ; sources = [ "sa"; "sb" ]
-    ; mappings = [ gen (1, 1) (10, 10) 0; gen (3, 3) (20, 20) 1 ]
+    ; mappings =
+        Source_map.Mappings.encode [ gen (1, 1) (10, 10) 0; gen (3, 3) (20, 20) 1 ]
     }
   in
   let s2 : Source_map.t =
     { (Source_map.empty ~filename:"2.map") with
       names = [ "na2"; "nb2" ]
     ; sources = [ "sa2" ]
-    ; mappings = [ gen (3, 3) (5, 5) 0 ]
+    ; mappings = Source_map.Mappings.encode [ gen (3, 3) (5, 5) 0 ]
     }
   in
-  let m = Source_map.merge [ s1; Source_map.filter_map s2 ~f:(fun x -> Some (x + 20)) ] in
-  (match m with
-  | None -> ()
-  | Some sm ->
-      print_endline (Source_map.string_of_mapping sm.mappings);
-      print_mapping sm);
+  let edits =
+    Source_map.Line_edits.([ Add { count = 17 } ] @ List.init ~len:3 ~f:(Fun.const Keep))
+  in
+  let s2 =
+    { s2 with mappings = Source_map.Mappings.edit ~strict:true s2.mappings edits }
+  in
+  let m = Source_map.concat ~file:"" ~sourceroot:None s1 s2 in
+  let encoded_mappings = m.Source_map.mappings in
+  print_endline (Source_map.Mappings.to_string encoded_mappings);
+  print_mapping m;
   [%expect
     {|
-    CASU;;GCUU;;;;;;;;;;;;;;;;;;;;GCff
+    CASU;;GCUU;;;;;;;;;;;;;;;;;;;;GCff;
     sa:10:10 -> 1:1
     sb:20:20 -> 3:3
     sa2:5:5 -> 23:3 |}]
diff --git a/compiler/tests-compiler/util/util.mli b/compiler/tests-compiler/util/util.mli
index 5788400928..99b0581637 100644
--- a/compiler/tests-compiler/util/util.mli
+++ b/compiler/tests-compiler/util/util.mli
@@ -53,7 +53,12 @@ val compile_bc_to_javascript :
 val jsoo_minify :
   ?flags:string list -> pretty:bool -> Filetype.js_file -> Filetype.js_file
 
-val extract_sourcemap : Filetype.js_file -> Js_of_ocaml_compiler.Source_map.t option
+val extract_sourcemap :
+     Filetype.js_file
+  -> [ `Standard of Js_of_ocaml_compiler.Source_map.t
+     | `Index of Js_of_ocaml_compiler.Source_map.Index.t
+     ]
+     option
 
 val run_javascript : Filetype.js_file -> string
 
diff --git a/compiler/tests-sourcemap/dump_sourcemap.ml b/compiler/tests-sourcemap/dump_sourcemap.ml
index eec55e5a8e..78729c7cf1 100644
--- a/compiler/tests-sourcemap/dump_sourcemap.ml
+++ b/compiler/tests-sourcemap/dump_sourcemap.ml
@@ -36,11 +36,12 @@ let extract_sourcemap lines =
       Some (Source_map_io.of_string content)
   | _ -> None
 
-let print_mapping lines (sm : Source_map.t) =
+let print_mapping lines ?(line_offset = 0) (sm : Source_map.t) =
   let lines = Array.of_list lines in
   let sources = Array.of_list sm.sources in
   let _names = Array.of_list sm.names in
-  List.iter sm.mappings ~f:(fun (m : Source_map.map) ->
+  let mappings = Source_map.Mappings.decode sm.mappings in
+  List.iter mappings ~f:(fun (m : Source_map.map) ->
       let file = function
         | -1 -> "null"
         | n -> normalize_path sources.(n)
@@ -67,9 +68,22 @@ let print_mapping lines (sm : Source_map.t) =
                 ori_line
                 ori_col
                 gen_col
-                (mark gen_col lines.(gen_line - 1))
+                (mark gen_col lines.(gen_line - 1 + line_offset))
           | _ -> ()))
 
+let print_sourcemap lines = function
+  | `Standard sm -> print_mapping lines sm
+  | `Index l ->
+      List.iter
+        l.Source_map.Index.sections
+        ~f:(fun (Source_map.Index.{ gen_line; gen_column }, `Map sm) ->
+          assert (gen_column = 0);
+          (*
+          Format.eprintf "[@SOURCEMAP (line = %d)@," gen_line;
+          Yojson.Raw.pretty_print Format.err_formatter sm;
+          *)
+          print_mapping lines ~line_offset:gen_line sm)
+
 let files = Sys.argv |> Array.to_list |> List.tl
 
 let () =
@@ -79,4 +93,4 @@ let () =
       | None -> Printf.printf "not sourcemap for %s\n" f
       | Some sm ->
           Printf.printf "sourcemap for %s\n" f;
-          print_mapping lines sm)
+          print_sourcemap lines sm)
diff --git a/compiler/tests-sourcemap/dune b/compiler/tests-sourcemap/dune
index e690f5a3ee..efaf86e885 100644
--- a/compiler/tests-sourcemap/dune
+++ b/compiler/tests-sourcemap/dune
@@ -1,6 +1,10 @@
 (env
  (_
   (js_of_ocaml
+   (flags
+    (:standard --source-map))
+   (link_flags
+    (:standard --source-map --resolve-sourcemap-url true))
    (compilation_mode separate))))
 
 (executable
diff --git a/tools/sourcemap/jsoo_sourcemap.ml b/tools/sourcemap/jsoo_sourcemap.ml
index 309358705d..9c359c6587 100644
--- a/tools/sourcemap/jsoo_sourcemap.ml
+++ b/tools/sourcemap/jsoo_sourcemap.ml
@@ -44,5 +44,6 @@ let () =
         | Some base64 -> Js_of_ocaml_compiler.Base64.decode_exn base64)
     | _ -> failwith "unable to find sourcemap"
   in
-  let sm = Js_of_ocaml_compiler.Source_map_io.of_string content in
-  print_endline (Js_of_ocaml_compiler.Source_map_io.to_string sm)
+  match Js_of_ocaml_compiler.Source_map_io.of_string content with
+  | `Standard sm -> print_endline (Js_of_ocaml_compiler.Source_map_io.to_string sm)
+  | `Index im -> print_endline (Js_of_ocaml_compiler.Source_map_io.Index.to_string im)