package js_of_ocaml-compiler
Compiler from OCaml bytecode to JavaScript
Install
Dune Dependency
Authors
Maintainers
Sources
js_of_ocaml-5.1.1.tbz
sha256=0feb4418d2e491d50f3cffd0f7bc94c8ac02531149a467d8966f89581f7e5ede
sha512=8e031336678dd0ebb53bc5460c123538e4a968153f8b724dab1520f98fce1e105f9dc287a6576644d527bfafc181d4d071f25cc73240ff1b512c3cccdc864fe2
doc/src/js_of_ocaml-compiler/js_assign.ml.html
Source file js_assign.ml
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(* 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
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