Source file js_assign.ml

(* Js_of_ocaml compiler
 * http://www.ocsigen.org/js_of_ocaml/
 * Copyright (C) 2013 Jérôme Vouillon
 * Copyright (C) 2013 Hugo Heuzard
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation, with linking exception;
 * either version 2.1 of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 *)
open! Stdlib
open Javascript

let debug = Debug.find "shortvar"

module S = Code.Var.Set
module Var = Code.Var

module type Strategy = sig
  type t

  val create : int -> t

  val record_block : t -> Js_traverse.t -> Js_traverse.block -> unit

  val allocate_variables : t -> count:int Javascript.IdentMap.t -> string array
end

module Min : Strategy = struct
  (*
We are trying to achieve the following goals:
(1) variable names should be as short as possible
(2) one should reuse as much as possible a small subsets of variable
    names
(3) function parameters should be: function(a,b,...){...}
(4) for longer variable names, variable which are closed from one
    another should share a same prefix

Point (1) minimizes the size of uncompressed files, while point (2) to
(4) improve compression.

We use the following strategy. We maintain the constraint that
variables occurring in a function should keep different names.
We first assign names a, b, ... (in order) to function parameters,
starting from inner functions, skipping variables which have a
conflict with a previously names variable (goal 3). Then, we order
the remaining variables by their number of occurrences, then by
their index (goal 4), and greedily assigned name to them. For that,
we use for each variable the smallest possible name still available
(goal 1/2).

This algorithm seems effective. Here are some statistics gathered
while compiling the OCaml toplevel:
(1) We get 132025 occurrences of one-char variables out of 169728
    occurrences while the optimal number (determined using a mixed
    integer linear programming solver) is 132105 occurrences (80 more
    occurrences).
(2) Variable names are heavily biased toward character a: among
    variables, we have about 34000 occurrences of character a, less
    than 5000 occurrences of character i (9th character, out of the 54
    characters that can start an identifier), and about 1500
    occurrences of character A.
(3) About 6% of the function parameters are not assigned as wanted;
    it is not clear we can do any better: there are a lot of nested
    functions.
(4) We save 8181 bytes on the compressed file (1.8%) by sorting
    variables using their index as a secondary key rather that just
    based on their weights (the size of the uncompressed file remains
    unchanged)
*)

  type alloc =
    { mutable first_free : int
    ; used : BitSet.t
    }

  let make_alloc_table () = { first_free = 0; used = BitSet.create () }

  let next_available a i = BitSet.next_free a.used (max i a.first_free)

  let allocate a i =
    BitSet.set a.used i;
    if a.first_free = i then a.first_free <- BitSet.next_free a.used a.first_free

  let is_available l i = List.for_all l ~f:(fun a -> not (BitSet.mem a.used i))

  let first_available l =
    let rec find_rec n l =
      let n' = List.fold_left l ~init:n ~f:(fun n a -> next_available a n) in
      if n = n' then n else find_rec n' l
    in
    find_rec 0 l

  let mark_allocated l i = List.iter l ~f:(fun a -> allocate a i)

  type t =
    { constr : alloc list array
    ; (* Constraints on variables *)
      mutable parameters : Var.t list array
    ; (* Function parameters *)
      mutable constraints : S.t list
    }

  (* For debugging *)

  let create nv = { constr = Array.make nv []; parameters = [| [] |]; constraints = [] }

  let allocate_variables t ~count =
    let weight v = try IdentMap.find (V (Var.of_idx v)) count with Not_found -> 0 in
    let constr = t.constr in
    let len = Array.length constr in
    let idx = Array.make len 0 in
    for i = 0 to len - 1 do
      idx.(i) <- i
    done;
    Array.stable_sort idx ~cmp:(fun i j -> compare (weight j) (weight i));
    let name = Array.make len "" in
    let n0 = ref 0 in
    let n1 = ref 0 in
    let n2 = ref 0 in
    let n3 = ref 0 in
    let stats i n =
      incr n0;
      if n < 54
      then (
        incr n1;
        n2 := !n2 + weight i);
      n3 := !n3 + weight i
    in
    let nm ~origin n =
      let n = Var.to_string ~origin:(Var.of_idx origin) (Var.of_idx n) in
      name.(origin) <- n
    in
    let total = ref 0 in
    let bad = ref 0 in
    for i = 0 to Array.length t.parameters - 1 do
      List.iter
        (List.rev t.parameters.(i))
        ~f:(fun x ->
          incr total;
          let idx = Var.idx x in
          let l = constr.(idx) in
          if is_available l i
          then (
            nm ~origin:idx i;
            mark_allocated l i;
            stats idx i)
          else incr bad)
    done;
    if debug ()
    then
      Format.eprintf
        "Function parameter properly assigned: %d/%d@."
        (!total - !bad)
        !total;
    for i = 0 to len - 1 do
      let l = constr.(idx.(i)) in
      if (not (List.is_empty l)) && String.length name.(idx.(i)) = 0
      then (
        let n = first_available l in
        let idx = idx.(i) in
        nm ~origin:idx n;
        mark_allocated l n;
        stats idx n);
      if List.is_empty l then assert (weight idx.(i) = 0)
    done;
    if debug ()
    then (
      Format.eprintf "short variable count: %d/%d@." !n1 !n0;
      Format.eprintf "short variable occurrences: %d/%d@." !n2 !n3);
    name

  let add_constraints global u ~offset (params : ident list) =
    let constr = global.constr in
    let c = make_alloc_table () in
    S.iter
      (fun v ->
        let i = Var.idx v in
        constr.(i) <- c :: constr.(i))
      u;
    let params = Array.of_list params in
    let len = Array.length params in
    let len_max = len + offset in
    if Array.length global.parameters < len_max
    then (
      let a = Array.make (2 * len_max) [] in
      Array.blit
        ~src:global.parameters
        ~src_pos:0
        ~dst:a
        ~dst_pos:0
        ~len:(Array.length global.parameters);
      global.parameters <- a);
    for i = 0 to len - 1 do
      match params.(i) with
      | V x -> global.parameters.(i + offset) <- x :: global.parameters.(i + offset)
      | _ -> ()
    done;
    global.constraints <- u :: global.constraints

  let record_block state scope (block : Js_traverse.block) =
    let all =
      Javascript.IdentSet.union
        (Javascript.IdentSet.union scope.Js_traverse.def_var scope.Js_traverse.def_local)
        scope.Js_traverse.use
    in
    let all =
      match block with
      | Normal -> all
      | Params _ -> all
      | Catch (p, _) ->
          let ids = bound_idents_of_binding p in
          List.fold_left ids ~init:all ~f:(fun all i -> Javascript.IdentSet.add i all)
    in
    let all =
      Javascript.IdentSet.fold
        (fun x acc ->
          match x with
          | V i -> S.add i acc
          | S _ -> acc)
        all
        S.empty
    in
    match block with
    | Normal -> add_constraints state all ~offset:0 []
    | Catch (v, _) -> add_constraints state all ~offset:5 (bound_idents_of_binding v)
    | Params p -> add_constraints state all ~offset:0 (bound_idents_of_params p)
end

module Preserve : Strategy = struct
  (* Try to preserve variable names.
     - Assign the origin name if present: "{original_name}"
     - If present but not available, derive a similar name: "{original_name}${n}" (eg. result$3).
     - If not present, make up a name: "$${n}"

     Color variables one scope/block at a time - outer scope first.
  *)

  type t =
    { size : int
    ; mutable scopes : (S.t * Js_traverse.t) list
    }

  let create size = { size; scopes = [] }

  let record_block t scope (b : Js_traverse.block) =
    let defs =
      match b with
      | Catch (p, _) -> bound_idents_of_binding p
      | Normal -> Javascript.IdentSet.elements scope.Js_traverse.def_local
      | Params _ ->
          Javascript.IdentSet.elements
            (IdentSet.union scope.Js_traverse.def_var scope.Js_traverse.def_local)
    in
    let defs =
      List.fold_left
        ~init:S.empty
        ~f:(fun acc x ->
          match (x : Javascript.ident) with
          | V i -> S.add i acc
          | S _ -> acc)
        defs
    in

    t.scopes <- (defs, scope) :: t.scopes

  let allocate_variables t ~count:_ =
    let names = Array.make t.size "" in
    List.iter t.scopes ~f:(fun (defs, state) ->
        let assigned =
          IdentSet.fold
            (fun var acc ->
              match var with
              | S { name = Utf8 s; _ } -> StringSet.add s acc
              | V v ->
                  let name = names.(Var.idx v) in
                  if not (String.is_empty name) then StringSet.add name acc else acc)
            (IdentSet.union
               state.Js_traverse.use
               (IdentSet.union state.Js_traverse.def_var state.Js_traverse.def_local))
            Reserved.keyword
        in
        let _assigned =
          S.fold
            (fun var assigned ->
              assert (String.is_empty names.(Var.idx var));
              let name =
                match Var.get_name var with
                | Some expected_name ->
                    assert (not (String.is_empty expected_name));
                    if not (StringSet.mem expected_name assigned)
                    then expected_name
                    else
                      let i = ref 0 in
                      while
                        StringSet.mem (Printf.sprintf "%s$%d" expected_name !i) assigned
                      do
                        incr i
                      done;
                      Printf.sprintf "%s$%d" expected_name !i
                | None -> Var.to_string var
              in
              names.(Var.idx var) <- name;
              StringSet.add name assigned)
            defs
            assigned
        in
        ());
    names
end

class traverse record_block =
  object (m)
    inherit Js_traverse.free as super

    method! record_block b =
      record_block m#state b;
      super#record_block b
  end

let program' (module Strategy : Strategy) p =
  let nv = Var.count () in
  let state = Strategy.create nv in
  let mapper = new traverse (Strategy.record_block state) in
  let p = mapper#program p in
  mapper#record_block Normal;
  let free =
    IdentSet.filter
      (function
        | V _ -> true
        | S _ -> false)
      mapper#get_free
  in
  if IdentSet.cardinal free <> 0
  then (
    if not (debug ())
    then failwith_ "Some variables escaped (#%d)" (IdentSet.cardinal free)
    else
      let (_ : Source_map.t option) =
        Js_output.program (Pretty_print.to_out_channel stderr) p
      in
      Format.eprintf "Some variables escaped:";
      IdentSet.iter
        (function
          | S _ -> ()
          | V v -> Format.eprintf " <%s>" (Var.to_string v))
        free;
      Format.eprintf "@.");
  let names = Strategy.allocate_variables state ~count:mapper#get_count in
  let color = function
    | V v ->
        let name = names.(Var.idx v) in
        assert (not (String.is_empty name));
        ident ~var:v (Utf8_string.of_string_exn name)
    | x -> x
  in
  (new Js_traverse.subst color)#program p

let program p =
  if Config.Flag.shortvar ()
  then program' (module Min) p
  else program' (module Preserve) p