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1 //===-- SpillPlacement.h - Optimal Spill Code Placement --------*- C++ -*--===//
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2 //
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3 // The LLVM Compiler Infrastructure
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4 //
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5 // This file is distributed under the University of Illinois Open Source
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6 // License. See LICENSE.TXT for details.
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7 //
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8 //===----------------------------------------------------------------------===//
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9 //
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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10 // This analysis computes the optimal spill code placement between basic blocks.
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11 //
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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12 // The runOnMachineFunction() method only precomputes some profiling information
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13 // about the CFG. The real work is done by prepare(), addConstraints(), and
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14 // finish() which are called by the register allocator.
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15 //
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16 // Given a variable that is live across multiple basic blocks, and given
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17 // constraints on the basic blocks where the variable is live, determine which
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18 // edge bundles should have the variable in a register and which edge bundles
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19 // should have the variable in a stack slot.
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20 //
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21 // The returned bit vector can be used to place optimal spill code at basic
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22 // block entries and exits. Spill code placement inside a basic block is not
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23 // considered.
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24 //
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25 //===----------------------------------------------------------------------===//
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26
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27 #ifndef LLVM_LIB_CODEGEN_SPILLPLACEMENT_H
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28 #define LLVM_LIB_CODEGEN_SPILLPLACEMENT_H
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29
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30 #include "llvm/ADT/ArrayRef.h"
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31 #include "llvm/ADT/SmallVector.h"
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32 #include "llvm/CodeGen/MachineFunctionPass.h"
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33 #include "llvm/Support/BlockFrequency.h"
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34
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35 namespace llvm {
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36
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37 class BitVector;
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38 class EdgeBundles;
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39 class MachineBasicBlock;
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40 class MachineLoopInfo;
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41 class MachineBlockFrequencyInfo;
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42
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43 class SpillPlacement : public MachineFunctionPass {
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44 struct Node;
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45 const MachineFunction *MF;
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46 const EdgeBundles *bundles;
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47 const MachineLoopInfo *loops;
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48 const MachineBlockFrequencyInfo *MBFI;
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49 Node *nodes;
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50
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51 // Nodes that are active in the current computation. Owned by the prepare()
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52 // caller.
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53 BitVector *ActiveNodes;
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54
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55 // Nodes with active links. Populated by scanActiveBundles.
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56 SmallVector<unsigned, 8> Linked;
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57
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58 // Nodes that went positive during the last call to scanActiveBundles or
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59 // iterate.
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60 SmallVector<unsigned, 8> RecentPositive;
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61
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62 // Block frequencies are computed once. Indexed by block number.
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63 SmallVector<BlockFrequency, 8> BlockFrequencies;
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64
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83
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65 /// Decision threshold. A node gets the output value 0 if the weighted sum of
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66 /// its inputs falls in the open interval (-Threshold;Threshold).
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67 BlockFrequency Threshold;
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68
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69 public:
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70 static char ID; // Pass identification, replacement for typeid.
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71
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72 SpillPlacement() : MachineFunctionPass(ID), nodes(nullptr) {}
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73 ~SpillPlacement() override { releaseMemory(); }
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74
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75 /// BorderConstraint - A basic block has separate constraints for entry and
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76 /// exit.
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77 enum BorderConstraint {
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78 DontCare, ///< Block doesn't care / variable not live.
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79 PrefReg, ///< Block entry/exit prefers a register.
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80 PrefSpill, ///< Block entry/exit prefers a stack slot.
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81 PrefBoth, ///< Block entry prefers both register and stack.
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82 MustSpill ///< A register is impossible, variable must be spilled.
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83 };
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84
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85 /// BlockConstraint - Entry and exit constraints for a basic block.
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86 struct BlockConstraint {
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87 unsigned Number; ///< Basic block number (from MBB::getNumber()).
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88 BorderConstraint Entry : 8; ///< Constraint on block entry.
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89 BorderConstraint Exit : 8; ///< Constraint on block exit.
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90
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91 /// True when this block changes the value of the live range. This means
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92 /// the block has a non-PHI def. When this is false, a live-in value on
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93 /// the stack can be live-out on the stack without inserting a spill.
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94 bool ChangesValue;
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95 };
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96
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97 /// prepare - Reset state and prepare for a new spill placement computation.
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98 /// @param RegBundles Bit vector to receive the edge bundles where the
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99 /// variable should be kept in a register. Each bit
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100 /// corresponds to an edge bundle, a set bit means the
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101 /// variable should be kept in a register through the
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102 /// bundle. A clear bit means the variable should be
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103 /// spilled. This vector is retained.
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104 void prepare(BitVector &RegBundles);
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105
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106 /// addConstraints - Add constraints and biases. This method may be called
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107 /// more than once to accumulate constraints.
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108 /// @param LiveBlocks Constraints for blocks that have the variable live in or
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109 /// live out.
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110 void addConstraints(ArrayRef<BlockConstraint> LiveBlocks);
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111
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112 /// addPrefSpill - Add PrefSpill constraints to all blocks listed. This is
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113 /// equivalent to calling addConstraint with identical BlockConstraints with
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114 /// Entry = Exit = PrefSpill, and ChangesValue = false.
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115 ///
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116 /// @param Blocks Array of block numbers that prefer to spill in and out.
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117 /// @param Strong When true, double the negative bias for these blocks.
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118 void addPrefSpill(ArrayRef<unsigned> Blocks, bool Strong);
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119
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120 /// addLinks - Add transparent blocks with the given numbers.
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121 void addLinks(ArrayRef<unsigned> Links);
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122
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123 /// scanActiveBundles - Perform an initial scan of all bundles activated by
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124 /// addConstraints and addLinks, updating their state. Add all the bundles
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125 /// that now prefer a register to RecentPositive.
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126 /// Prepare internal data structures for iterate.
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127 /// Return true is there are any positive nodes.
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128 bool scanActiveBundles();
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129
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130 /// iterate - Update the network iteratively until convergence, or new bundles
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131 /// are found.
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132 void iterate();
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133
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134 /// getRecentPositive - Return an array of bundles that became positive during
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135 /// the previous call to scanActiveBundles or iterate.
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136 ArrayRef<unsigned> getRecentPositive() { return RecentPositive; }
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137
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138 /// finish - Compute the optimal spill code placement given the
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139 /// constraints. No MustSpill constraints will be violated, and the smallest
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140 /// possible number of PrefX constraints will be violated, weighted by
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141 /// expected execution frequencies.
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142 /// The selected bundles are returned in the bitvector passed to prepare().
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143 /// @return True if a perfect solution was found, allowing the variable to be
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144 /// in a register through all relevant bundles.
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145 bool finish();
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146
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147 /// getBlockFrequency - Return the estimated block execution frequency per
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148 /// function invocation.
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149 BlockFrequency getBlockFrequency(unsigned Number) const {
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150 return BlockFrequencies[Number];
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151 }
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152
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153 private:
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154 bool runOnMachineFunction(MachineFunction&) override;
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155 void getAnalysisUsage(AnalysisUsage&) const override;
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156 void releaseMemory() override;
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157
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158 void activate(unsigned);
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83
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159 void setThreshold(const BlockFrequency &Entry);
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160 };
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161
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162 } // end namespace llvm
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163
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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164 #endif
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