## lustrec / src / liveness.ml @ df39e35a

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1 | df39e35a | xthirioux | (********************************************************************) |
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2 | (* *) |
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3 | (* The LustreC compiler toolset / The LustreC Development Team *) |
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4 | (* Copyright 2012 - -- ONERA - CNRS - INPT *) |
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5 | (* *) |
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6 | (* LustreC is free software, distributed WITHOUT ANY WARRANTY *) |
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7 | (* under the terms of the GNU Lesser General Public License *) |
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8 | (* version 2.1. *) |
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9 | (* *) |
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10 | (********************************************************************) |
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11 | |||

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

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

26 | (* computes the in-degree for each local variable of node [n], according to dep graph [g]. |
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27 | *) |
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28 | let compute_fanin n g = |
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29 | let locals = ISet.diff (ExprDep.node_local_variables n) (ExprDep.node_memory_variables n) in |
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30 | let fanin = Hashtbl.create 23 in |
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31 | begin |
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32 | IdentDepGraph.iter_vertex (fun v -> if ISet.mem v locals then Hashtbl.add fanin v (IdentDepGraph.in_degree g v)) g; |
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33 | fanin |
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34 | end |
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35 | |||

36 | let pp_fanin fmt fanin = |
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37 | begin |
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38 | Format.fprintf fmt "{ /* locals fanin: */@."; |
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39 | Hashtbl.iter (fun s t -> Format.fprintf fmt "%s -> %d@." s t) fanin; |
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40 | Format.fprintf fmt "}@." |
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41 | end |
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42 | |||

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

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

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

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

81 | (* Recursively removes useless variables, |
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82 | i.e. [ctx.evaluated] variables that are current roots of the dep graph [ctx.dep_graph] |
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83 | - [evaluated] is the set of already evaluated variables, |
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84 | wrt the scheduling |
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85 | - does only remove edges, not variables themselves |
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86 | *) |
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87 | let remove_roots ctx = |
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88 | let rem = ref true in |
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89 | let remaining = ref ctx.evaluated in |
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90 | while !rem |
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91 | do |
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92 | rem := false; |
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93 | let all_roots = graph_roots ctx.dep_graph in |
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94 | let frontier_roots = Disjunction.CISet.filter (fun v -> List.mem v.var_id all_roots) !remaining in |
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95 | if not (Disjunction.CISet.is_empty frontier_roots) then |
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96 | begin |
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97 | rem := true; |
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98 | Disjunction.CISet.iter (kill_root ctx) frontier_roots; |
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99 | remaining := Disjunction.CISet.diff !remaining frontier_roots |
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100 | end |
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101 | done |
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102 | |||

103 | (* checks whether a variable is aliasable, |
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104 | depending on its (address) type *) |
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105 | let is_aliasable var = |
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106 | Types.is_address_type var.var_type |
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107 | |||

108 | (* checks whether a variable [v] is an input of the [var] equation, with an address type. |
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109 | if so, [var] could not safely reuse/alias [v], should [v] be dead in the caller node, |
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110 | because [v] may not be dead in the callee node when [var] is assigned *) |
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111 | let is_aliasable_input node var = |
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112 | let eq_var = get_node_eq var node in |
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113 | let inputs_var = |
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114 | match NodeDep.get_callee eq_var.eq_rhs with |
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115 | | None -> [] |
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116 | | Some (_, args) -> List.fold_right (fun e r -> match e.expr_desc with Expr_ident id -> id::r | _ -> r) args [] in |
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117 | fun v -> is_aliasable v && List.mem v.var_id inputs_var |
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118 | |||

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

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

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

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

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

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

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

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

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

253 | |||

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

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