CristalCaveGem » LustreC

(********************************************************************)

(*                                                                  *)

(*  The LustreC compiler toolset   /  The LustreC Development Team  *)

(*  Copyright 2012 -    --   ONERA - CNRS - INPT                    *)

(*                                                                  *)

(*  LustreC is free software, distributed WITHOUT ANY WARRANTY      *)

(*  under the terms of the GNU Lesser General Public License        *)

(*  version 2.1.                                                    *)

(*                                                                  *)

(********************************************************************)

open Utils

open Lustre_types

open Corelang

open Causality

type context =

{

  mutable evaluated : Disjunction.CISet.t;

  dep_graph : IdentDepGraph.t;

  disjoint : (ident, Disjunction.CISet.t) Hashtbl.t;

  policy : (ident, var_decl) Hashtbl.t;

}

(* computes the in-degree for each local variable of node [n], according to dep graph [g].

*)

let compute_fanin n g =

  let locals = ISet.diff (ExprDep.node_local_variables n) (ExprDep.node_memory_variables n) in

  let inputs = ExprDep.node_input_variables n in

  let fanin = Hashtbl.create 23 in

  begin

    IdentDepGraph.iter_vertex

      (fun v ->

	if ISet.mem v locals

	then Hashtbl.add fanin v (IdentDepGraph.in_degree g v) else

	if ExprDep.is_read_var v && not (ISet.mem v inputs)

	then Hashtbl.add fanin (ExprDep.undo_read_var v) (IdentDepGraph.in_degree g v)) g;

    fanin

  end

let pp_fanin fmt fanin =

  Format.fprintf fmt "@[<v 0>@[<v 2>{ /* locals fanin: */";

  Hashtbl.iter (fun s t -> Format.fprintf fmt "@ %s -> %d" s t) fanin;

  Format.fprintf fmt "@]@ }@]"

(* computes the cone of influence of a given [var] wrt a dependency graph [g].

*)

let cone_of_influence g var =

  (*Format.printf "DEBUG coi: %s@." var;*)

 let frontier = ref (ISet.add var ISet.empty) in

 let explored = ref ISet.empty in

 let coi = ref ISet.empty in

 while not (ISet.is_empty !frontier)

 do

   let head = ISet.min_elt !frontier in

   (*Format.printf "DEBUG head: %s@." head;*)

   frontier := ISet.remove head !frontier;

   explored := ISet.add head !explored;

   if ExprDep.is_read_var head then coi := ISet.add (ExprDep.undo_read_var head) !coi;

   List.iter (fun s -> if not (ISet.mem s !explored) then frontier := ISet.add s !frontier) (IdentDepGraph.succ g head);

 done;

 !coi

let compute_unused_variables n g =

  let inputs = ExprDep.node_input_variables n in

  let mems = ExprDep.node_memory_variables n in

  let outputs = ExprDep.node_output_variables n in

  ISet.fold

    (fun var unused -> ISet.diff unused (cone_of_influence g var))

    (ISet.union outputs mems)

    (ISet.union inputs mems)

(* computes the set of potentially reusable variables.

   We don't reuse input variables, due to possible aliasing *)

let node_reusable_variables node =

  let mems = ExprDep.node_memory_variables node in

  List.fold_left

    (fun acc l ->

      if ISet.mem l.var_id mems then acc else Disjunction.CISet.add l acc)

    Disjunction.CISet.empty

    node.node_locals

let kill_instance_variables ctx inst =

  IdentDepGraph.remove_vertex ctx.dep_graph inst

let kill_root ctx head =

  IdentDepGraph.iter_succ (IdentDepGraph.remove_edge ctx.dep_graph head.var_id) ctx.dep_graph head.var_id

(* Recursively removes useless variables,

   i.e. [ctx.evaluated] variables that are current roots of the dep graph [ctx.dep_graph]

   - [evaluated] is the set of already evaluated variables,

     wrt the scheduling

   - does only remove edges, not variables themselves

   - yet, instance variables are removed

*)

let remove_roots ctx =

  let rem = ref true in

  let remaining = ref ctx.evaluated in

  while !rem

  do

    rem := false;

    let all_roots = graph_roots ctx.dep_graph in

    let inst_roots, var_roots = List.partition (fun v -> ExprDep.is_instance_var v && v <> Causality.world) all_roots in

    let frontier_roots = Disjunction.CISet.filter (fun v -> List.mem v.var_id var_roots) !remaining in

    if not (Disjunction.CISet.is_empty frontier_roots && inst_roots = []) then

      begin

	rem := true;

	List.iter (kill_instance_variables ctx) inst_roots;

	Disjunction.CISet.iter (kill_root ctx) frontier_roots;

	remaining := Disjunction.CISet.diff !remaining frontier_roots

      end

  done

(* checks whether a variable is aliasable,

   depending on its (address) type *)

let is_aliasable var =

  (not (!Options.mpfr && Types.is_real_type var.var_type)) && Types.is_address_type var.var_type

(* checks whether a variable [v] is an input of the [var] equation, with an address type.

   if so, [var] could not safely reuse/alias [v], should [v] be dead in the caller node,

   because [v] may not be dead in the callee node when [var] is assigned *)

let is_aliasable_input node var =

  let eq_var = get_node_eq var node in

  let inputs_var =

    match NodeDep.get_callee eq_var.eq_rhs with

    | None           -> []

    | Some (_, args) -> List.fold_right (fun e r -> match e.expr_desc with Expr_ident id -> id::r | _ -> r) args [] in

  fun v -> is_aliasable v && List.mem v.var_id inputs_var

(* replace variable [v] by [v'] in graph [g].

   [v'] is a dead variable

*)

let replace_in_dep_graph v v' g =

  begin

    IdentDepGraph.add_vertex g v';

    IdentDepGraph.iter_succ (fun s -> IdentDepGraph.add_edge g v' s) g v;

    IdentDepGraph.iter_pred (fun p -> IdentDepGraph.add_edge g p v') g v;

    IdentDepGraph.remove_vertex g v

  end

let pp_reuse_policy fmt policy =

  Format.(fprintf fmt "@[<v 2>{ /* reuse policy */%t@] }"

    (fun fmt -> Hashtbl.iter (fun s t -> fprintf fmt "@,%s -> %s" s t.var_id) policy))

let pp_context fmt ctx =

  Format.fprintf fmt

    "@[<v 2>{ /*BEGIN context */@,\

     eval     = %a;@,\

     graph    = %a;@,\

     disjoint = %a;@,\

     policy   = %a;@,\

     /* END context */ }@]"

    Disjunction.pp_ciset ctx.evaluated

    pp_dep_graph ctx.dep_graph

    Disjunction.pp_disjoint_map ctx.disjoint

    pp_reuse_policy ctx.policy

(* computes the reusable dependencies of variable [var] in graph [g],

   once [var] has been evaluated

   - [locals] is the set of potentially reusable variables

   - [evaluated] is the set of evaluated variables

   - [quasi] is the set of quasi-reusable variables

   - [reusable] is the set of dead/reusable dependencies of [var] in graph [g]

   - [policy] is the reuse map (which domain is [evaluated])

*)

let compute_dependencies heads ctx =

  begin

    (*Log.report ~level:6 (fun fmt -> Format.fprintf fmt "compute_reusable_dependencies %a %a %a@." Disjunction.pp_ciset locals Printers.pp_var_name var pp_context ctx);*)

    List.iter (kill_root ctx) heads;

    remove_roots ctx;

  end

let compute_evaluated heads ctx =

  begin

    List.iter (fun head -> ctx.evaluated <- Disjunction.CISet.add head ctx.evaluated) heads;

  end

(* tests whether a variable [v] may be (re)used instead of [var]. The conditions are:

   - [v] has been really used ([v] is its own representative)

   - same type

   - [v] is not an aliasable input of the equation defining [var]

   - [v] is not one of the current heads (which contain [var])

   - [v] is not currently in use

 *)

let eligible node ctx heads var v =

     Hashtbl.find ctx.policy v.var_id == v

  && Typing.eq_ground (Types.unclock_type var.var_type) (Types.unclock_type v.var_type)

  && not (is_aliasable_input node var.var_id v)

  && not (List.exists (fun h -> h.var_id = v.var_id) heads)

  && (*let repr_v = Hashtbl.find ctx.policy v.var_id*)

     not (Disjunction.CISet.exists (fun p -> IdentDepGraph.mem_edge ctx.dep_graph p.var_id v.var_id) ctx.evaluated)

let compute_reuse node ctx heads var =

  let disjoint = Hashtbl.find ctx.disjoint var.var_id in

  let locally_reusable v =

    IdentDepGraph.fold_pred (fun p r -> r && Disjunction.CISet.exists (fun d -> p = d.var_id) disjoint) ctx.dep_graph v.var_id true in

  let eligibles =

    if ISet.mem var.var_id (ExprDep.node_memory_variables node)

    then Disjunction.CISet.empty

    else Disjunction.CISet.filter (eligible node ctx heads var) ctx.evaluated in

  let quasi_dead, live = Disjunction.CISet.partition locally_reusable eligibles in

  let disjoint_live = Disjunction.CISet.inter disjoint live in

  let dead = Disjunction.CISet.filter (fun v -> is_graph_root v.var_id ctx.dep_graph) quasi_dead in

  Log.report ~level:7 (fun fmt ->

      Format.fprintf fmt

        "@[<v>\

         eligibles    : %a@,\

         live         : %a@,\

         disjoint live: %a@,\

         dead         : %a@,@]"

        Disjunction.pp_ciset eligibles

        Disjunction.pp_ciset live

        Disjunction.pp_ciset disjoint_live

        Disjunction.pp_ciset dead);

  begin try

      let reuse = match Disjunction.CISet.max_elt_opt disjoint_live with

        | Some reuse -> reuse

        | None -> Disjunction.CISet.choose dead in

      IdentDepGraph.add_edge ctx.dep_graph var.var_id reuse.var_id;

      Hashtbl.add ctx.policy var.var_id reuse

    with Not_found -> Hashtbl.add ctx.policy var.var_id var

  end;

  ctx.evaluated <- Disjunction.CISet.add var ctx.evaluated

let compute_reuse_policy node schedule disjoint g =

  let ctx = { evaluated = Disjunction.CISet.empty;

              dep_graph = g;

              disjoint  = disjoint;

              policy    = Hashtbl.create 23; } in

  List.iter (fun heads ->

      let heads = List.map (fun v -> get_node_var v node) heads in

      Log.report ~level:6 (fun fmt ->

          Format.(fprintf fmt

                    "@[<v>@[<v 2>new context:@,%a@]@,NEW HEADS:%a@,COMPUTE_DEPENDENCIES@,@]"

                    pp_context ctx

                    (pp_print_list

                       ~pp_open_box:pp_open_hbox

                       ~pp_sep:pp_print_space

                       (fun fmt head ->

                          fprintf fmt "%s (%a)"

                            head.var_id Printers.pp_node_eq

                            (get_node_eq head.var_id node)))

                    heads));

      compute_dependencies heads ctx;

      Log.report ~level:6 (fun fmt ->

          Format.fprintf fmt "@[<v>@[<v 2>new context:@,%a@]@,COMPUTE_REUSE@,@]" pp_context ctx);

      List.iter (compute_reuse node ctx heads) heads;

      (*compute_evaluated heads ctx;*)

      Log.report ~level:6 (fun fmt ->

          Format.(fprintf fmt "@[<v>%a@,@]"

                    (pp_print_list

                       ~pp_open_box:pp_open_vbox0

                       (fun fmt head -> fprintf fmt "reuse %s instead of %s"

                           (Hashtbl.find ctx.policy head.var_id).var_id head.var_id))

                    heads)))

    schedule;

  IdentDepGraph.clear ctx.dep_graph;

  ctx.policy

(* Reuse policy:

   - could reuse variables with the same type exactly only (simple).

   - reusing variables with different types would involve:

     - either dirty castings

     - or complex inclusion expression (for instance: array <-> array cell, struct <-> struct field) to be able to reuse only some parts of structured data.

     ... it seems too complex and potentially unsafe

   - for node instance calls: output variables could NOT reuse aliasable input variables, 

     even if inputs become dead, because the correctness would depend on the scheduling

     of the callee (so, the compiling strategy could NOT be modular anymore).

   - once a policy is set, we need to:

     - replace each variable by its reuse alias.

     - simplify resulting equations, as we may now have:

        x = x;                     --> ;           for scalar vars

       or:

        x = &{ f1 = x->f1; f2 = t; } --> x->f2 = t;   for struct vars

 *)

(* the reuse policy seeks to use less local variables

   by replacing local variables, applying the rules

   in the following order:

    1) use another clock disjoint still live variable,

       with the greatest possible disjoint clock

    2) reuse a dead variable

   For the sake of safety, we replace variables by others:

    - with the same type

    - not aliasable (i.e. address type)

*)

(* Local Variables: *)

(* compile-command:"make -C .." *)

(* End: *)