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annotate lib/CodeGen/UnreachableBlockElim.cpp @ 0:95c75e76d11b LLVM3.4

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LLVM 3.4
author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Thu, 12 Dec 2013 13:56:28 +0900
parents
children 54457678186b
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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/Analysis/Dominators.h"
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27 #include "llvm/CodeGen/MachineDominators.h"
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28 #include "llvm/CodeGen/MachineFunctionPass.h"
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29 #include "llvm/CodeGen/MachineLoopInfo.h"
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30 #include "llvm/CodeGen/MachineModuleInfo.h"
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31 #include "llvm/CodeGen/MachineRegisterInfo.h"
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32 #include "llvm/IR/Constant.h"
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33 #include "llvm/IR/Function.h"
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34 #include "llvm/IR/Instructions.h"
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35 #include "llvm/IR/Type.h"
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36 #include "llvm/Pass.h"
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37 #include "llvm/Support/CFG.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 virtual bool runOnFunction(Function &F);
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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 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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51 AU.addPreserved<DominatorTree>();
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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 (df_ext_iterator<Function*, SmallPtrSet<BasicBlock*, 8> > I =
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68 df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable); I != E; ++I)
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69 /* Mark all reachable blocks */;
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70
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71 // Loop over all dead blocks, remembering them and deleting all instructions
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72 // in them.
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73 std::vector<BasicBlock*> DeadBlocks;
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74 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
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75 if (!Reachable.count(I)) {
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76 BasicBlock *BB = I;
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77 DeadBlocks.push_back(BB);
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78 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
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79 PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
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80 BB->getInstList().pop_front();
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81 }
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82 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
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83 (*SI)->removePredecessor(BB);
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84 BB->dropAllReferences();
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85 }
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86
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87 // Actually remove the blocks now.
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88 for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
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89 DeadBlocks[i]->eraseFromParent();
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90 }
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91
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92 return DeadBlocks.size();
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93 }
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94
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95
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96 namespace {
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97 class UnreachableMachineBlockElim : public MachineFunctionPass {
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98 virtual bool runOnMachineFunction(MachineFunction &F);
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99 virtual void getAnalysisUsage(AnalysisUsage &AU) const;
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100 MachineModuleInfo *MMI;
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101 public:
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102 static char ID; // Pass identification, replacement for typeid
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103 UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
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104 };
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105 }
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106 char UnreachableMachineBlockElim::ID = 0;
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107
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108 INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
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109 "Remove unreachable machine basic blocks", false, false)
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110
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111 char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
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112
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113 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
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114 AU.addPreserved<MachineLoopInfo>();
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115 AU.addPreserved<MachineDominatorTree>();
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116 MachineFunctionPass::getAnalysisUsage(AU);
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117 }
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118
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119 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
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120 SmallPtrSet<MachineBasicBlock*, 8> Reachable;
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121 bool ModifiedPHI = false;
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122
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123 MMI = getAnalysisIfAvailable<MachineModuleInfo>();
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124 MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
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125 MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
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126
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127 // Mark all reachable blocks.
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128 for (df_ext_iterator<MachineFunction*, SmallPtrSet<MachineBasicBlock*, 8> >
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parents:
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129 I = df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable);
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130 I != E; ++I)
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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131 /* Mark all reachable blocks */;
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132
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133 // Loop over all dead blocks, remembering them and deleting all instructions
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diff changeset
134 // in them.
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135 std::vector<MachineBasicBlock*> DeadBlocks;
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diff changeset
136 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
95c75e76d11b LLVM 3.4
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137 MachineBasicBlock *BB = I;
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138
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139 // Test for deadness.
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diff changeset
140 if (!Reachable.count(BB)) {
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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141 DeadBlocks.push_back(BB);
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142
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143 // Update dominator and loop info.
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144 if (MLI) MLI->removeBlock(BB);
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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diff changeset
145 if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
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146
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diff changeset
147 while (BB->succ_begin() != BB->succ_end()) {
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diff changeset
148 MachineBasicBlock* succ = *BB->succ_begin();
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diff changeset
149
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150 MachineBasicBlock::iterator start = succ->begin();
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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diff changeset
151 while (start != succ->end() && start->isPHI()) {
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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diff changeset
152 for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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diff changeset
153 if (start->getOperand(i).isMBB() &&
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diff changeset
154 start->getOperand(i).getMBB() == BB) {
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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diff changeset
155 start->RemoveOperand(i);
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diff changeset
156 start->RemoveOperand(i-1);
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diff changeset
157 }
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diff changeset
158
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159 start++;
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160 }
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diff changeset
161
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diff changeset
162 BB->removeSuccessor(BB->succ_begin());
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163 }
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diff changeset
164 }
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
165 }
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parents:
diff changeset
166
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diff changeset
167 // Actually remove the blocks now.
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parents:
diff changeset
168 for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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diff changeset
169 DeadBlocks[i]->eraseFromParent();
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diff changeset
170
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diff changeset
171 // Cleanup PHI nodes.
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parents:
diff changeset
172 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
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parents:
diff changeset
173 MachineBasicBlock *BB = I;
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diff changeset
174 // Prune unneeded PHI entries.
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parents:
diff changeset
175 SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
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parents:
diff changeset
176 BB->pred_end());
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diff changeset
177 MachineBasicBlock::iterator phi = BB->begin();
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diff changeset
178 while (phi != BB->end() && phi->isPHI()) {
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diff changeset
179 for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
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diff changeset
180 if (!preds.count(phi->getOperand(i).getMBB())) {
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diff changeset
181 phi->RemoveOperand(i);
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diff changeset
182 phi->RemoveOperand(i-1);
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parents:
diff changeset
183 ModifiedPHI = true;
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parents:
diff changeset
184 }
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parents:
diff changeset
185
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diff changeset
186 if (phi->getNumOperands() == 3) {
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
187 unsigned Input = phi->getOperand(1).getReg();
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parents:
diff changeset
188 unsigned Output = phi->getOperand(0).getReg();
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diff changeset
189
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diff changeset
190 MachineInstr* temp = phi;
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
191 ++phi;
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
192 temp->eraseFromParent();
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
193 ModifiedPHI = true;
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
194
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diff changeset
195 if (Input != Output) {
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parents:
diff changeset
196 MachineRegisterInfo &MRI = F.getRegInfo();
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parents:
diff changeset
197 MRI.constrainRegClass(Input, MRI.getRegClass(Output));
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
198 MRI.replaceRegWith(Output, Input);
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
199 }
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
200
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
201 continue;
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
202 }
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
203
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
204 ++phi;
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
205 }
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
206 }
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
207
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
208 F.RenumberBlocks();
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
209
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
210 return (DeadBlocks.size() || ModifiedPHI);
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
211 }