1 | (* Pasted from Pottier's PP compiler *) |
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2 | |
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3 | open Interference |
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4 | (* open Integer *) |
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5 | open Printf |
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6 | |
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7 | (* ------------------------------------------------------------------------- *) |
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8 | (* Colorings. *) |
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9 | |
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10 | (* This module performs graph coloring with an unlimited number of |
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11 | colors and aggressive coalescing. It is used for assigning stack |
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12 | slots to the pseudo-registers that have been spilled by register |
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13 | allocation. *) |
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14 | |
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15 | (* A coloring is a partial function of graph vertices to stack |
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16 | slots. Vertices that are not in the domain of the coloring are |
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17 | waiting for a decision to be made. *) |
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18 | |
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19 | type decision = |
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20 | AST.immediate |
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21 | |
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22 | type coloring = |
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23 | decision Vertex.Map.t |
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24 | |
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25 | (* ------------------------------------------------------------------------- *) |
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26 | (* Here is the coloring algorithm. *) |
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27 | |
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28 | module Color (G : sig |
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29 | |
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30 | val graph: graph |
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31 | val verbose: bool |
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32 | |
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33 | end) = struct |
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34 | |
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35 | module SlotSet = |
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36 | Set.Make(struct type t = int let compare = Pervasives.compare end) |
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37 | |
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38 | (* [forbidden_slots graph coloring v] is the set of stack slots that |
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39 | cannot be assigned to [v] considering the (partial) coloring |
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40 | [coloring]. This takes into account [v]'s possible interferences |
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41 | with other spilled vertices. *) |
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42 | |
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43 | let add_slot coloring r slots = |
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44 | SlotSet.add (Vertex.Map.find r coloring) slots |
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45 | |
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46 | let forbidden_slots graph coloring v = |
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47 | Vertex.Set.fold (add_slot coloring) (ipp graph v) SlotSet.empty |
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48 | |
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49 | (* [allocate_slot forbidden] returns a stack slot that is not a |
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50 | member of the set [forbidden]. Unlike hardware registers, stack |
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51 | slots are infinitely many, so it is always possible to allocate a |
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52 | new one. The reference [locals] holds the space that must be |
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53 | reserved on the stack for locals. *) |
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54 | |
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55 | let locals = |
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56 | ref 0 |
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57 | |
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58 | let allocate_slot forbidden = |
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59 | let rec loop slot = |
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60 | if SlotSet.mem slot forbidden then |
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61 | loop (slot + I8051.int_size) |
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62 | else |
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63 | slot |
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64 | in |
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65 | let slot = loop 0 in |
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66 | locals := max (slot + I8051.int_size) !locals; |
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67 | slot |
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68 | |
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69 | (* Allocation is in two phases, implemented by [coalescing] and |
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70 | [simplification]. Each of these functions produces a coloring of its |
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71 | graph argument. *) |
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72 | |
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73 | (* [simplification] expects a graph that does not contain any preference |
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74 | edges. It picks a vertex [v], removes it, colors the remaining graph, |
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75 | then colors [v] using a color that is still available. Such a color must |
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76 | exist, since there is an unlimited number of colors. *) |
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77 | |
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78 | (* Following Appel, [v] is chosen with lowest degree: this will make this |
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79 | vertex easier to color and might (?) help use fewer colors. *) |
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80 | |
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81 | let rec simplification graph : coloring = |
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82 | |
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83 | match lowest graph with |
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84 | | Some (v, _) -> |
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85 | |
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86 | if G.verbose then |
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87 | printf "SPILL: Picking vertex: %s.\n" (print_vertex graph v); |
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88 | |
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89 | (* Remove [v] from the graph and color what remains. *) |
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90 | |
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91 | let coloring = simplification (Interference.remove graph v) in |
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92 | |
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93 | (* Choose a color for [v]. *) |
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94 | |
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95 | let decision = |
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96 | allocate_slot (forbidden_slots graph coloring v) |
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97 | in |
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98 | |
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99 | if G.verbose then |
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100 | printf "SPILL: Decision concerning %s: offset %d.\n" (print_vertex graph v) decision; |
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101 | |
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102 | (* Record our decision and return. *) |
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103 | |
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104 | Vertex.Map.add v decision coloring |
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105 | |
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106 | | None -> |
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107 | |
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108 | (* The graph is empty. Return an empty coloring. *) |
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109 | |
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110 | Vertex.Map.empty |
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111 | |
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112 | (* [coalescing] looks for a preference edge, that is, for two vertices |
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113 | [x] and [y] such that [x] and [y] are move-related. In that case, |
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114 | [x] and [y] cannot interfere, because the [Interference] module |
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115 | does not allow two vertices to be related by both an interference |
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116 | edge and a preference edge. If [coalescing] finds such an edge, it |
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117 | coalesces [x] and [y] and continues coalescing. Otherwise, it |
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118 | invokes the next phase, [simplification]. |
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119 | |
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120 | This is aggressive coalescing: we coalesce all preference edges, |
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121 | without fear of creating high-degree nodes. This is good because |
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122 | a move between two pseudo-registers that have been spilled in |
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123 | distinct stack slots is very expensive: one load followed by one |
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124 | store. *) |
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125 | |
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126 | let rec coalescing graph : coloring = |
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127 | |
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128 | match pppick graph (fun _ -> true) with |
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129 | | Some (x, y) -> |
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130 | |
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131 | if G.verbose then |
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132 | printf "SPILL: Coalescing %s and %s.\n" (print_vertex graph x) (print_vertex graph y); |
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133 | |
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134 | let graph = Interference.coalesce graph x y in |
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135 | let coloring = coalescing graph in |
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136 | Vertex.Map.add x (Vertex.Map.find y coloring) coloring |
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137 | |
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138 | | None -> |
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139 | |
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140 | simplification graph |
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141 | |
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142 | (* Run the algorithm. [coalescing] runs first and calls [simplification] |
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143 | when it is done. *) |
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144 | |
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145 | let coloring = |
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146 | coalescing G.graph |
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147 | |
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148 | (* Report how much stack space was used. *) |
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149 | |
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150 | let locals = |
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151 | !locals |
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152 | |
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153 | end |
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