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 LustreSpec

open Corelang

open Graph

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 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)) g;

    fanin

  end

let pp_fanin fmt fanin =

  begin

    Format.fprintf fmt "{ /* locals fanin: */@.";

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

    Format.fprintf fmt "}@."

  end

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

*)

let cone_of_influence g var =

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

 let frontier = ref (ISet.add var 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 "head: %s@." head;*)

   frontier := ISet.remove head !frontier;

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

   List.iter (fun s -> 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_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

*)

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

    if not (Disjunction.CISet.is_empty frontier_roots) then

      begin

	rem := true;

	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 =

 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 =

  begin

    Format.fprintf fmt "{ /* reuse policy */@.";

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

    Format.fprintf fmt "}@."

  end

let pp_context fmt ctx =

  begin

    Format.fprintf fmt "{ /*BEGIN context */@.";

    Format.fprintf fmt "eval=%a;@." Disjunction.pp_ciset ctx.evaluated;

    Format.fprintf fmt "graph=%a;@." pp_dep_graph ctx.dep_graph;

    Format.fprintf fmt "disjoint=%a;@." Disjunction.pp_disjoint_map ctx.disjoint;

    Format.fprintf fmt "policy=%a;@." pp_reuse_policy ctx.policy;

    Format.fprintf fmt "/* END context */ }@.";

  end

(* 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

let compute_reuse node ctx var =

  let aliasable = is_aliasable_input node var.var_id in

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

  let eligible v =

       Typing.eq_ground var.var_type v.var_type

    && not (aliasable v) 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 = Disjunction.CISet.filter eligible ctx.evaluated in

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

  try

    let disjoint_live = Disjunction.CISet.inter disjoint live in

    let reuse = Disjunction.CISet.max_elt disjoint_live in

    begin

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

      Hashtbl.add ctx.policy var.var_id (Hashtbl.find ctx.policy reuse.var_id);

    end

  with Not_found ->

  try

    let reuse = Disjunction.CISet.choose dead in

    begin

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

      Hashtbl.add ctx.policy var.var_id (Hashtbl.find ctx.policy reuse.var_id);

    end

      with Not_found ->

    begin

      Hashtbl.add ctx.policy var.var_id var;

    end

let compute_reuse_policy node schedule disjoint g =

  let sort = ref schedule in

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

	      dep_graph = g;

	      disjoint  = disjoint;

	      policy    = Hashtbl.create 23; } in

  while !sort <> []

  do

    Log.report ~level:6 (fun fmt -> Format.fprintf fmt "new context:%a@." pp_context ctx);

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

    Log.report ~level:6 (fun fmt -> Format.fprintf fmt "NEW HEADS:");

    List.iter (fun head -> Log.report ~level:6 (fun fmt -> Format.fprintf fmt "%s " head.var_id)) heads;

    Log.report ~level:6 (fun fmt -> Format.fprintf fmt "@.");

    Log.report ~level:6 (fun fmt -> Format.fprintf fmt "COMPUTE_DEPENDENCIES@.");

    compute_dependencies heads ctx;

    Log.report ~level:6 (fun fmt -> Format.fprintf fmt "new context:%a@." pp_context ctx);

    Log.report ~level:6 (fun fmt -> Format.fprintf fmt "COMPUTE_REUSE@.");

    List.iter (compute_reuse node ctx) heads;

    compute_evaluated heads ctx;

    List.iter (fun head -> Log.report ~level:6 (fun fmt -> Format.fprintf fmt "reuse %s instead of %s@." (Hashtbl.find ctx.policy head.var_id).var_id head.var_id)) heads;

    sort := List.tl !sort;

  done;

  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: *)