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annotate lib/CodeGen/UnreachableBlockElim.cpp @ 95:afa8332a0e37 LLVM3.8

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LLVM 3.8
author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Tue, 13 Oct 2015 17:48:58 +0900
parents 60c9769439b8
children 1172e4bd9c6f
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1 //===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===//
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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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10 // This pass is an extremely simple version of the SimplifyCFG pass. Its sole
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11 // job is to delete LLVM basic blocks that are not reachable from the entry
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12 // node. To do this, it performs a simple depth first traversal of the CFG,
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13 // then deletes any unvisited nodes.
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14 //
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15 // Note that this pass is really a hack. In particular, the instruction
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16 // selectors for various targets should just not generate code for unreachable
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17 // blocks. Until LLVM has a more systematic way of defining instruction
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18 // selectors, however, we cannot really expect them to handle additional
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19 // complexity.
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20 //
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21 //===----------------------------------------------------------------------===//
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22
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23 #include "llvm/CodeGen/Passes.h"
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24 #include "llvm/ADT/DepthFirstIterator.h"
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25 #include "llvm/ADT/SmallPtrSet.h"
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26 #include "llvm/CodeGen/MachineDominators.h"
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27 #include "llvm/CodeGen/MachineFunctionPass.h"
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28 #include "llvm/CodeGen/MachineLoopInfo.h"
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29 #include "llvm/CodeGen/MachineModuleInfo.h"
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30 #include "llvm/CodeGen/MachineRegisterInfo.h"
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31 #include "llvm/IR/CFG.h"
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32 #include "llvm/IR/Constant.h"
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33 #include "llvm/IR/Dominators.h"
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34 #include "llvm/IR/Function.h"
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35 #include "llvm/IR/Instructions.h"
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36 #include "llvm/IR/Type.h"
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37 #include "llvm/Pass.h"
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38 #include "llvm/Target/TargetInstrInfo.h"
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39 using namespace llvm;
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40
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41 namespace {
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42 class UnreachableBlockElim : public FunctionPass {
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43 bool runOnFunction(Function &F) override;
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44 public:
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45 static char ID; // Pass identification, replacement for typeid
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46 UnreachableBlockElim() : FunctionPass(ID) {
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47 initializeUnreachableBlockElimPass(*PassRegistry::getPassRegistry());
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48 }
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49
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50 void getAnalysisUsage(AnalysisUsage &AU) const override {
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51 AU.addPreserved<DominatorTreeWrapperPass>();
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52 }
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53 };
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54 }
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55 char UnreachableBlockElim::ID = 0;
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56 INITIALIZE_PASS(UnreachableBlockElim, "unreachableblockelim",
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57 "Remove unreachable blocks from the CFG", false, false)
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58
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59 FunctionPass *llvm::createUnreachableBlockEliminationPass() {
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60 return new UnreachableBlockElim();
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61 }
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62
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63 bool UnreachableBlockElim::runOnFunction(Function &F) {
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64 SmallPtrSet<BasicBlock*, 8> Reachable;
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65
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66 // Mark all reachable blocks.
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67 for (BasicBlock *BB : depth_first_ext(&F, Reachable))
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68 (void)BB/* Mark all reachable blocks */;
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69
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70 // Loop over all dead blocks, remembering them and deleting all instructions
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71 // in them.
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72 std::vector<BasicBlock*> DeadBlocks;
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73 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
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74 if (!Reachable.count(&*I)) {
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75 BasicBlock *BB = &*I;
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76 DeadBlocks.push_back(BB);
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77 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
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78 PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
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79 BB->getInstList().pop_front();
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80 }
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81 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
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82 (*SI)->removePredecessor(BB);
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83 BB->dropAllReferences();
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84 }
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85
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86 // Actually remove the blocks now.
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87 for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
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88 DeadBlocks[i]->eraseFromParent();
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89 }
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90
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91 return !DeadBlocks.empty();
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92 }
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93
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94
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95 namespace {
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96 class UnreachableMachineBlockElim : public MachineFunctionPass {
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97 bool runOnMachineFunction(MachineFunction &F) override;
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98 void getAnalysisUsage(AnalysisUsage &AU) const override;
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99 MachineModuleInfo *MMI;
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100 public:
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101 static char ID; // Pass identification, replacement for typeid
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102 UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
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103 };
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104 }
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105 char UnreachableMachineBlockElim::ID = 0;
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106
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107 INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
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108 "Remove unreachable machine basic blocks", false, false)
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109
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110 char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
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111
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112 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
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113 AU.addPreserved<MachineLoopInfo>();
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114 AU.addPreserved<MachineDominatorTree>();
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115 MachineFunctionPass::getAnalysisUsage(AU);
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116 }
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117
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118 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
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119 SmallPtrSet<MachineBasicBlock*, 8> Reachable;
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120 bool ModifiedPHI = false;
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121
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122 MMI = getAnalysisIfAvailable<MachineModuleInfo>();
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123 MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
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124 MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
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125
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126 // Mark all reachable blocks.
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127 for (MachineBasicBlock *BB : depth_first_ext(&F, Reachable))
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128 (void)BB/* Mark all reachable blocks */;
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129
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130 // Loop over all dead blocks, remembering them and deleting all instructions
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131 // in them.
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132 std::vector<MachineBasicBlock*> DeadBlocks;
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133 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
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134 MachineBasicBlock *BB = &*I;
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135
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136 // Test for deadness.
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137 if (!Reachable.count(BB)) {
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138 DeadBlocks.push_back(BB);
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139
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140 // Update dominator and loop info.
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141 if (MLI) MLI->removeBlock(BB);
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142 if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
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143
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144 while (BB->succ_begin() != BB->succ_end()) {
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145 MachineBasicBlock* succ = *BB->succ_begin();
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146
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147 MachineBasicBlock::iterator start = succ->begin();
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148 while (start != succ->end() && start->isPHI()) {
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149 for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
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150 if (start->getOperand(i).isMBB() &&
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151 start->getOperand(i).getMBB() == BB) {
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152 start->RemoveOperand(i);
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153 start->RemoveOperand(i-1);
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154 }
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155
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156 start++;
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157 }
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158
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159 BB->removeSuccessor(BB->succ_begin());
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160 }
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161 }
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162 }
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163
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164 // Actually remove the blocks now.
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165 for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
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166 DeadBlocks[i]->eraseFromParent();
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167
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168 // Cleanup PHI nodes.
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169 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
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170 MachineBasicBlock *BB = &*I;
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171 // Prune unneeded PHI entries.
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172 SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
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173 BB->pred_end());
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174 MachineBasicBlock::iterator phi = BB->begin();
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175 while (phi != BB->end() && phi->isPHI()) {
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176 for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
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177 if (!preds.count(phi->getOperand(i).getMBB())) {
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178 phi->RemoveOperand(i);
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179 phi->RemoveOperand(i-1);
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180 ModifiedPHI = true;
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181 }
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182
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183 if (phi->getNumOperands() == 3) {
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184 unsigned Input = phi->getOperand(1).getReg();
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185 unsigned Output = phi->getOperand(0).getReg();
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186
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187 MachineInstr* temp = phi;
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188 ++phi;
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189 temp->eraseFromParent();
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190 ModifiedPHI = true;
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191
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192 if (Input != Output) {
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193 MachineRegisterInfo &MRI = F.getRegInfo();
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194 MRI.constrainRegClass(Input, MRI.getRegClass(Output));
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195 MRI.replaceRegWith(Output, Input);
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196 }
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197
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198 continue;
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199 }
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200
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201 ++phi;
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202 }
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203 }
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204
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205 F.RenumberBlocks();
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206
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diff changeset
207 return (!DeadBlocks.empty() || ModifiedPHI);
0
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208 }