Project

General

Profile

Statistics
| Branch: | Tag: | Revision:

lustrec / src / liveness.ml @ df39e35a

History | View | Annotate | Download (11.1 KB)

1
(********************************************************************)
2
(*                                                                  *)
3
(*  The LustreC compiler toolset   /  The LustreC Development Team  *)
4
(*  Copyright 2012 -    --   ONERA - CNRS - INPT                    *)
5
(*                                                                  *)
6
(*  LustreC is free software, distributed WITHOUT ANY WARRANTY      *)
7
(*  under the terms of the GNU Lesser General Public License        *)
8
(*  version 2.1.                                                    *)
9
(*                                                                  *)
10
(********************************************************************)
11

    
12
open Utils
13
open LustreSpec
14
open Corelang
15
open Graph
16
open Causality
17

    
18
type context =
19
{
20
  mutable evaluated : Disjunction.CISet.t;
21
  dep_graph : IdentDepGraph.t;
22
  disjoint : (ident, Disjunction.CISet.t) Hashtbl.t;
23
  policy : (ident, var_decl) Hashtbl.t;
24
}
25

    
26
(* computes the in-degree for each local variable of node [n], according to dep graph [g].
27
*)
28
let compute_fanin n g =
29
  let locals = ISet.diff (ExprDep.node_local_variables n) (ExprDep.node_memory_variables n) in
30
  let fanin = Hashtbl.create 23 in
31
  begin
32
    IdentDepGraph.iter_vertex (fun v -> if ISet.mem v locals then Hashtbl.add fanin v (IdentDepGraph.in_degree g v)) g;
33
    fanin
34
  end
35
 
36
let pp_fanin fmt fanin =
37
  begin
38
    Format.fprintf fmt "{ /* locals fanin: */@.";
39
    Hashtbl.iter (fun s t -> Format.fprintf fmt "%s -> %d@." s t) fanin;
40
    Format.fprintf fmt "}@."
41
  end
42

    
43
(* computes the cone of influence of a given [var] wrt a dependency graph [g].
44
*)
45
let cone_of_influence g var =
46
 (*Format.printf "coi: %s@." var;*)
47
 let frontier = ref (ISet.add var ISet.empty) in
48
 let coi = ref ISet.empty in
49
 while not (ISet.is_empty !frontier)
50
 do
51
   let head = ISet.min_elt !frontier in
52
   (*Format.printf "head: %s@." head;*)
53
   frontier := ISet.remove head !frontier;
54
   if ExprDep.is_read_var head then coi := ISet.add (ExprDep.undo_read_var head) !coi;
55
   List.iter (fun s -> frontier := ISet.add s !frontier) (IdentDepGraph.succ g head);
56
 done;
57
 !coi
58

    
59
let compute_unused_variables n g =
60
  let inputs = ExprDep.node_input_variables n in
61
  let mems = ExprDep.node_memory_variables n in
62
  let outputs = ExprDep.node_output_variables n in
63
  ISet.fold
64
    (fun var unused -> ISet.diff unused (cone_of_influence g var))
65
    (ISet.union outputs mems)
66
    (ISet.union inputs mems)
67

    
68
(* computes the set of potentially reusable variables.
69
   We don't reuse input variables, due to possible aliasing *)
70
let node_reusable_variables node =
71
  let mems = ExprDep.node_memory_variables node in
72
  List.fold_left
73
    (fun acc l ->
74
      if ISet.mem l.var_id mems then acc else Disjunction.CISet.add l acc)
75
    Disjunction.CISet.empty
76
    node.node_locals
77

    
78
let kill_root ctx head =
79
  IdentDepGraph.iter_succ (IdentDepGraph.remove_edge ctx.dep_graph head.var_id) ctx.dep_graph head.var_id
80

    
81
(* Recursively removes useless variables,
82
   i.e. [ctx.evaluated] variables that are current roots of the dep graph [ctx.dep_graph]
83
   - [evaluated] is the set of already evaluated variables,
84
     wrt the scheduling
85
   - does only remove edges, not variables themselves
86
*)
87
let remove_roots ctx =
88
  let rem = ref true in
89
  let remaining = ref ctx.evaluated in
90
  while !rem
91
  do
92
    rem := false;
93
    let all_roots = graph_roots ctx.dep_graph in
94
    let frontier_roots = Disjunction.CISet.filter (fun v -> List.mem v.var_id all_roots) !remaining in
95
    if not (Disjunction.CISet.is_empty frontier_roots) then
96
      begin
97
	rem := true;
98
	Disjunction.CISet.iter (kill_root ctx) frontier_roots;
99
	remaining := Disjunction.CISet.diff !remaining frontier_roots
100
      end
101
  done
102
 
103
(* checks whether a variable is aliasable,
104
   depending on its (address) type *)
105
let is_aliasable var =
106
 Types.is_address_type var.var_type
107
 
108
(* checks whether a variable [v] is an input of the [var] equation, with an address type.
109
   if so, [var] could not safely reuse/alias [v], should [v] be dead in the caller node,
110
   because [v] may not be dead in the callee node when [var] is assigned *)
111
let is_aliasable_input node var =
112
  let eq_var = get_node_eq var node in
113
  let inputs_var =
114
    match NodeDep.get_callee eq_var.eq_rhs with
115
    | None           -> []
116
    | Some (_, args) -> List.fold_right (fun e r -> match e.expr_desc with Expr_ident id -> id::r | _ -> r) args [] in
117
  fun v -> is_aliasable v && List.mem v.var_id inputs_var
118

    
119
(* replace variable [v] by [v'] in graph [g].
120
   [v'] is a dead variable
121
*)
122
let replace_in_dep_graph v v' g =
123
  begin
124
    IdentDepGraph.add_vertex g v';
125
    IdentDepGraph.iter_succ (fun s -> IdentDepGraph.add_edge g v' s) g v;
126
    IdentDepGraph.iter_pred (fun p -> IdentDepGraph.add_edge g p v') g v;
127
    IdentDepGraph.remove_vertex g v
128
  end
129

    
130
let pp_reuse_policy fmt policy =
131
  begin
132
    Format.fprintf fmt "{ /* reuse policy */@.";
133
    Hashtbl.iter (fun s t -> Format.fprintf fmt "%s -> %s@." s t.var_id) policy;
134
    Format.fprintf fmt "}@."
135
  end
136

    
137
let pp_context fmt ctx =
138
  begin
139
    Format.fprintf fmt "{ /*BEGIN context */@.";
140
    Format.fprintf fmt "eval=%a;@." Disjunction.pp_ciset ctx.evaluated;
141
    Format.fprintf fmt "graph=%a;@." pp_dep_graph ctx.dep_graph;
142
    Format.fprintf fmt "disjoint=%a;@." Disjunction.pp_disjoint_map ctx.disjoint;
143
    Format.fprintf fmt "policy=%a;@." pp_reuse_policy ctx.policy;
144
    Format.fprintf fmt "/* END context */ }@.";
145
  end
146

    
147
(* computes the reusable dependencies of variable [var] in graph [g],
148
   once [var] has been evaluated
149
   - [locals] is the set of potentially reusable variables
150
   - [evaluated] is the set of evaluated variables
151
   - [quasi] is the set of quasi-reusable variables
152
   - [reusable] is the set of dead/reusable dependencies of [var] in graph [g]
153
   - [policy] is the reuse map (which domain is [evaluated])
154
*)
155
let compute_dependencies heads ctx =
156
  begin
157
    (*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);*)
158
    List.iter (kill_root ctx) heads;
159
    remove_roots ctx;
160
  end
161

    
162
let compute_evaluated heads ctx =
163
  begin
164
    List.iter (fun head -> ctx.evaluated <- Disjunction.CISet.add head ctx.evaluated) heads;
165
  end
166

    
167
(* tests whether a variable [v] may be (re)used instead of [var]. The conditions are:
168
   - same type
169
   - [v] is not an aliasable input of the equation defining [var]
170
   - [v] is not one of the current heads (which contain [var])
171
   - the representative of [v] is not currently in use
172
 *)
173
let eligible node ctx heads var v =
174
     Typing.eq_ground var.var_type v.var_type
175
  && not (is_aliasable_input node var.var_id v)
176
  && not (List.exists (fun h -> h.var_id = v.var_id) heads)
177
  && let repr_v = Hashtbl.find ctx.policy v.var_id
178
     in not (Disjunction.CISet.exists (fun p -> IdentDepGraph.mem_edge ctx.dep_graph p.var_id repr_v.var_id) ctx.evaluated)
179

    
180
let compute_reuse node ctx heads var =
181
  let disjoint = Hashtbl.find ctx.disjoint var.var_id in
182
  let locally_reusable v =
183
    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
184
  let eligibles = Disjunction.CISet.filter (eligible node ctx heads var) ctx.evaluated in
185
  let quasi_dead, live = Disjunction.CISet.partition locally_reusable eligibles in
186
  try
187
    let disjoint_live = Disjunction.CISet.inter disjoint live in
188
    let reuse = Disjunction.CISet.max_elt disjoint_live in
189
    begin
190
      IdentDepGraph.add_edge ctx.dep_graph var.var_id reuse.var_id;
191
      Hashtbl.add ctx.policy var.var_id (Hashtbl.find ctx.policy reuse.var_id);
192
      ctx.evaluated <- Disjunction.CISet.add var ctx.evaluated;
193
      (*Format.eprintf "%s reused by live@." var.var_id;*)
194
    end
195
  with Not_found ->
196
  try
197
    let dead = Disjunction.CISet.filter (fun v -> is_graph_root v.var_id ctx.dep_graph) quasi_dead in
198
    let reuse = Disjunction.CISet.choose dead in
199
    begin
200
      IdentDepGraph.add_edge ctx.dep_graph var.var_id reuse.var_id;
201
      Hashtbl.add ctx.policy var.var_id (Hashtbl.find ctx.policy reuse.var_id);
202
      ctx.evaluated <- Disjunction.CISet.add var ctx.evaluated;
203
      (*Format.eprintf "%s reused by dead %a@." var.var_id Disjunction.pp_ciset dead;*)
204
    end
205
      with Not_found ->
206
    begin
207
      Hashtbl.add ctx.policy var.var_id var;
208
      ctx.evaluated <- Disjunction.CISet.add var ctx.evaluated;
209
    end
210

    
211
let compute_reuse_policy node schedule disjoint g =
212
  let sort = ref schedule in
213
  let ctx = { evaluated = Disjunction.CISet.empty;
214
	      dep_graph = g;
215
	      disjoint  = disjoint;
216
	      policy    = Hashtbl.create 23; } in
217
  while !sort <> []
218
  do
219
    Log.report ~level:6 (fun fmt -> Format.fprintf fmt "new context:%a@." pp_context ctx);
220
    let heads = List.map (fun v -> get_node_var v node) (List.hd !sort) in
221
    Log.report ~level:6 (fun fmt -> Format.fprintf fmt "NEW HEADS:");
222
    List.iter (fun head -> Log.report ~level:6 (fun fmt -> Format.fprintf fmt "%s " head.var_id)) heads;
223
    Log.report ~level:6 (fun fmt -> Format.fprintf fmt "@.");
224
    Log.report ~level:6 (fun fmt -> Format.fprintf fmt "COMPUTE_DEPENDENCIES@.");
225
    compute_dependencies heads ctx;
226
    Log.report ~level:6 (fun fmt -> Format.fprintf fmt "new context:%a@." pp_context ctx);
227
    Log.report ~level:6 (fun fmt -> Format.fprintf fmt "COMPUTE_REUSE@.");
228
    List.iter (compute_reuse node ctx heads) heads;
229
    (*compute_evaluated heads ctx;*)
230
    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;
231
    sort := List.tl !sort;
232
  done;
233
  IdentDepGraph.clear ctx.dep_graph;
234
  ctx.policy
235

    
236
(* Reuse policy:
237
   - could reuse variables with the same type exactly only (simple).
238
   - reusing variables with different types would involve:
239
     - either dirty castings
240
     - or complex inclusion expression (for instance: array <-> array cell, struct <-> struct field) to be able to reuse only some parts of structured data.
241
     ... it seems too complex and potentially unsafe
242
   - for node instance calls: output variables could NOT reuse aliasable input variables, 
243
     even if inputs become dead, because the correctness would depend on the scheduling
244
     of the callee (so, the compiling strategy could NOT be modular anymore).
245
   - once a policy is set, we need to:
246
     - replace each variable by its reuse alias.
247
     - simplify resulting equations, as we may now have:
248
        x = x;                     --> ;           for scalar vars
249
       or:
250
        x = &{ f1 = x->f1; f2 = t; } --> x->f2 = t;   for struct vars
251
 *)
252

    
253

    
254
(* the reuse policy seeks to use less local variables
255
   by replacing local variables, applying the rules
256
   in the following order:
257
    1) use another clock disjoint still live variable,
258
       with the greatest possible disjoint clock
259
    2) reuse a dead variable
260
   For the sake of safety, we replace variables by others:
261
    - with the same type
262
    - not aliasable (i.e. address type)
263
*)
264

    
265
(* Local Variables: *)
266
(* compile-command:"make -C .." *)
267
(* End: *)