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1 //===-- GCRootLowering.cpp - Garbage collection infrastructure ------------===//
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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 file implements the lowering for the gc.root mechanism.
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11 //
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12 //===----------------------------------------------------------------------===//
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13
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14 #include "llvm/CodeGen/GCMetadata.h"
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15 #include "llvm/CodeGen/GCStrategy.h"
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16 #include "llvm/CodeGen/MachineFrameInfo.h"
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17 #include "llvm/CodeGen/MachineFunctionPass.h"
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18 #include "llvm/CodeGen/MachineInstrBuilder.h"
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19 #include "llvm/CodeGen/MachineModuleInfo.h"
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20 #include "llvm/CodeGen/Passes.h"
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134
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21 #include "llvm/CodeGen/TargetFrameLowering.h"
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22 #include "llvm/CodeGen/TargetInstrInfo.h"
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23 #include "llvm/CodeGen/TargetRegisterInfo.h"
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24 #include "llvm/CodeGen/TargetSubtargetInfo.h"
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83
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25 #include "llvm/IR/Dominators.h"
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26 #include "llvm/IR/IntrinsicInst.h"
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27 #include "llvm/IR/Module.h"
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28 #include "llvm/Support/Debug.h"
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29 #include "llvm/Support/ErrorHandling.h"
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30 #include "llvm/Support/raw_ostream.h"
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31
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32 using namespace llvm;
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33
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34 namespace {
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35
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36 /// LowerIntrinsics - This pass rewrites calls to the llvm.gcread or
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37 /// llvm.gcwrite intrinsics, replacing them with simple loads and stores as
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38 /// directed by the GCStrategy. It also performs automatic root initialization
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39 /// and custom intrinsic lowering.
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40 class LowerIntrinsics : public FunctionPass {
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41 bool PerformDefaultLowering(Function &F, GCStrategy &Coll);
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42
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43 public:
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44 static char ID;
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45
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46 LowerIntrinsics();
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120
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47 StringRef getPassName() const override;
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83
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48 void getAnalysisUsage(AnalysisUsage &AU) const override;
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49
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50 bool doInitialization(Module &M) override;
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51 bool runOnFunction(Function &F) override;
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52 };
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53
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54 /// GCMachineCodeAnalysis - This is a target-independent pass over the machine
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55 /// function representation to identify safe points for the garbage collector
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56 /// in the machine code. It inserts labels at safe points and populates a
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57 /// GCMetadata record for each function.
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58 class GCMachineCodeAnalysis : public MachineFunctionPass {
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59 GCFunctionInfo *FI;
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60 MachineModuleInfo *MMI;
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61 const TargetInstrInfo *TII;
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62
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63 void FindSafePoints(MachineFunction &MF);
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64 void VisitCallPoint(MachineBasicBlock::iterator MI);
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65 MCSymbol *InsertLabel(MachineBasicBlock &MBB, MachineBasicBlock::iterator MI,
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120
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66 const DebugLoc &DL) const;
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67
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68 void FindStackOffsets(MachineFunction &MF);
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69
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70 public:
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71 static char ID;
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72
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73 GCMachineCodeAnalysis();
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74 void getAnalysisUsage(AnalysisUsage &AU) const override;
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75
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76 bool runOnMachineFunction(MachineFunction &MF) override;
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77 };
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78 }
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79
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80 // -----------------------------------------------------------------------------
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81
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82 INITIALIZE_PASS_BEGIN(LowerIntrinsics, "gc-lowering", "GC Lowering", false,
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83 false)
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84 INITIALIZE_PASS_DEPENDENCY(GCModuleInfo)
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85 INITIALIZE_PASS_END(LowerIntrinsics, "gc-lowering", "GC Lowering", false, false)
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86
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87 FunctionPass *llvm::createGCLoweringPass() { return new LowerIntrinsics(); }
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88
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89 char LowerIntrinsics::ID = 0;
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90
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91 LowerIntrinsics::LowerIntrinsics() : FunctionPass(ID) {
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92 initializeLowerIntrinsicsPass(*PassRegistry::getPassRegistry());
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93 }
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94
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120
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95 StringRef LowerIntrinsics::getPassName() const {
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96 return "Lower Garbage Collection Instructions";
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97 }
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98
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99 void LowerIntrinsics::getAnalysisUsage(AnalysisUsage &AU) const {
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100 FunctionPass::getAnalysisUsage(AU);
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101 AU.addRequired<GCModuleInfo>();
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102 AU.addPreserved<DominatorTreeWrapperPass>();
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103 }
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104
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105 static bool NeedsDefaultLoweringPass(const GCStrategy &C) {
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106 // Default lowering is necessary only if read or write barriers have a default
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107 // action. The default for roots is no action.
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108 return !C.customWriteBarrier() || !C.customReadBarrier() ||
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109 C.initializeRoots();
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110 }
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111
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112 /// doInitialization - If this module uses the GC intrinsics, find them now.
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113 bool LowerIntrinsics::doInitialization(Module &M) {
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114 GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
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115 assert(MI && "LowerIntrinsics didn't require GCModuleInfo!?");
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116 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
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117 if (!I->isDeclaration() && I->hasGC())
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118 MI->getFunctionInfo(*I); // Instantiate the GC strategy.
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119
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120 return false;
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121 }
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122
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123 /// CouldBecomeSafePoint - Predicate to conservatively determine whether the
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124 /// instruction could introduce a safe point.
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125 static bool CouldBecomeSafePoint(Instruction *I) {
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126 // The natural definition of instructions which could introduce safe points
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127 // are:
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128 //
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129 // - call, invoke (AfterCall, BeforeCall)
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130 // - phis (Loops)
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131 // - invoke, ret, unwind (Exit)
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132 //
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133 // However, instructions as seemingly inoccuous as arithmetic can become
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134 // libcalls upon lowering (e.g., div i64 on a 32-bit platform), so instead
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135 // it is necessary to take a conservative approach.
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136
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137 if (isa<AllocaInst>(I) || isa<GetElementPtrInst>(I) || isa<StoreInst>(I) ||
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138 isa<LoadInst>(I))
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139 return false;
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140
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141 // llvm.gcroot is safe because it doesn't do anything at runtime.
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142 if (CallInst *CI = dyn_cast<CallInst>(I))
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143 if (Function *F = CI->getCalledFunction())
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144 if (Intrinsic::ID IID = F->getIntrinsicID())
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145 if (IID == Intrinsic::gcroot)
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146 return false;
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147
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148 return true;
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149 }
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150
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151 static bool InsertRootInitializers(Function &F, AllocaInst **Roots,
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152 unsigned Count) {
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153 // Scroll past alloca instructions.
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154 BasicBlock::iterator IP = F.getEntryBlock().begin();
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155 while (isa<AllocaInst>(IP))
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156 ++IP;
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157
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158 // Search for initializers in the initial BB.
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159 SmallPtrSet<AllocaInst *, 16> InitedRoots;
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160 for (; !CouldBecomeSafePoint(&*IP); ++IP)
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161 if (StoreInst *SI = dyn_cast<StoreInst>(IP))
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162 if (AllocaInst *AI =
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163 dyn_cast<AllocaInst>(SI->getOperand(1)->stripPointerCasts()))
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164 InitedRoots.insert(AI);
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165
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166 // Add root initializers.
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167 bool MadeChange = false;
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168
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169 for (AllocaInst **I = Roots, **E = Roots + Count; I != E; ++I)
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170 if (!InitedRoots.count(*I)) {
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171 StoreInst *SI = new StoreInst(
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172 ConstantPointerNull::get(cast<PointerType>((*I)->getAllocatedType())),
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173 *I);
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174 SI->insertAfter(*I);
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175 MadeChange = true;
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176 }
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177
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178 return MadeChange;
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179 }
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180
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181 /// runOnFunction - Replace gcread/gcwrite intrinsics with loads and stores.
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182 /// Leave gcroot intrinsics; the code generator needs to see those.
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183 bool LowerIntrinsics::runOnFunction(Function &F) {
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184 // Quick exit for functions that do not use GC.
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185 if (!F.hasGC())
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186 return false;
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187
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188 GCFunctionInfo &FI = getAnalysis<GCModuleInfo>().getFunctionInfo(F);
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189 GCStrategy &S = FI.getStrategy();
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190
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191 bool MadeChange = false;
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192
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193 if (NeedsDefaultLoweringPass(S))
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194 MadeChange |= PerformDefaultLowering(F, S);
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195
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196 return MadeChange;
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197 }
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198
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199 bool LowerIntrinsics::PerformDefaultLowering(Function &F, GCStrategy &S) {
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200 bool LowerWr = !S.customWriteBarrier();
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201 bool LowerRd = !S.customReadBarrier();
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202 bool InitRoots = S.initializeRoots();
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203
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204 SmallVector<AllocaInst *, 32> Roots;
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205
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206 bool MadeChange = false;
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207 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
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208 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
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209 if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++)) {
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210 Function *F = CI->getCalledFunction();
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211 switch (F->getIntrinsicID()) {
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212 case Intrinsic::gcwrite:
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213 if (LowerWr) {
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214 // Replace a write barrier with a simple store.
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215 Value *St =
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216 new StoreInst(CI->getArgOperand(0), CI->getArgOperand(2), CI);
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217 CI->replaceAllUsesWith(St);
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218 CI->eraseFromParent();
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219 }
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220 break;
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221 case Intrinsic::gcread:
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222 if (LowerRd) {
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223 // Replace a read barrier with a simple load.
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224 Value *Ld = new LoadInst(CI->getArgOperand(1), "", CI);
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225 Ld->takeName(CI);
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226 CI->replaceAllUsesWith(Ld);
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227 CI->eraseFromParent();
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228 }
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229 break;
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230 case Intrinsic::gcroot:
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231 if (InitRoots) {
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232 // Initialize the GC root, but do not delete the intrinsic. The
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233 // backend needs the intrinsic to flag the stack slot.
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234 Roots.push_back(
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235 cast<AllocaInst>(CI->getArgOperand(0)->stripPointerCasts()));
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236 }
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237 break;
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238 default:
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239 continue;
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240 }
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241
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242 MadeChange = true;
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243 }
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244 }
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245 }
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246
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247 if (Roots.size())
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248 MadeChange |= InsertRootInitializers(F, Roots.begin(), Roots.size());
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249
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250 return MadeChange;
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251 }
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252
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253 // -----------------------------------------------------------------------------
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254
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255 char GCMachineCodeAnalysis::ID = 0;
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256 char &llvm::GCMachineCodeAnalysisID = GCMachineCodeAnalysis::ID;
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257
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258 INITIALIZE_PASS(GCMachineCodeAnalysis, "gc-analysis",
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259 "Analyze Machine Code For Garbage Collection", false, false)
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260
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261 GCMachineCodeAnalysis::GCMachineCodeAnalysis() : MachineFunctionPass(ID) {}
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262
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263 void GCMachineCodeAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
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264 MachineFunctionPass::getAnalysisUsage(AU);
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265 AU.setPreservesAll();
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266 AU.addRequired<MachineModuleInfo>();
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267 AU.addRequired<GCModuleInfo>();
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268 }
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269
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270 MCSymbol *GCMachineCodeAnalysis::InsertLabel(MachineBasicBlock &MBB,
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271 MachineBasicBlock::iterator MI,
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272 const DebugLoc &DL) const {
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273 MCSymbol *Label = MBB.getParent()->getContext().createTempSymbol();
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274 BuildMI(MBB, MI, DL, TII->get(TargetOpcode::GC_LABEL)).addSym(Label);
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275 return Label;
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276 }
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277
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278 void GCMachineCodeAnalysis::VisitCallPoint(MachineBasicBlock::iterator CI) {
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279 // Find the return address (next instruction), too, so as to bracket the call
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280 // instruction.
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281 MachineBasicBlock::iterator RAI = CI;
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282 ++RAI;
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283
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284 if (FI->getStrategy().needsSafePoint(GC::PreCall)) {
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285 MCSymbol *Label = InsertLabel(*CI->getParent(), CI, CI->getDebugLoc());
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286 FI->addSafePoint(GC::PreCall, Label, CI->getDebugLoc());
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287 }
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288
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289 if (FI->getStrategy().needsSafePoint(GC::PostCall)) {
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290 MCSymbol *Label = InsertLabel(*CI->getParent(), RAI, CI->getDebugLoc());
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291 FI->addSafePoint(GC::PostCall, Label, CI->getDebugLoc());
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292 }
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293 }
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294
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295 void GCMachineCodeAnalysis::FindSafePoints(MachineFunction &MF) {
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296 for (MachineFunction::iterator BBI = MF.begin(), BBE = MF.end(); BBI != BBE;
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297 ++BBI)
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298 for (MachineBasicBlock::iterator MI = BBI->begin(), ME = BBI->end();
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299 MI != ME; ++MI)
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300 if (MI->isCall()) {
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301 // Do not treat tail or sibling call sites as safe points. This is
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302 // legal since any arguments passed to the callee which live in the
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303 // remnants of the callers frame will be owned and updated by the
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304 // callee if required.
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305 if (MI->isTerminator())
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306 continue;
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307 VisitCallPoint(MI);
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308 }
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309 }
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310
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311 void GCMachineCodeAnalysis::FindStackOffsets(MachineFunction &MF) {
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312 const TargetFrameLowering *TFI = MF.getSubtarget().getFrameLowering();
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313 assert(TFI && "TargetRegisterInfo not available!");
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314
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315 for (GCFunctionInfo::roots_iterator RI = FI->roots_begin();
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316 RI != FI->roots_end();) {
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317 // If the root references a dead object, no need to keep it.
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318 if (MF.getFrameInfo().isDeadObjectIndex(RI->Num)) {
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319 RI = FI->removeStackRoot(RI);
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320 } else {
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321 unsigned FrameReg; // FIXME: surely GCRoot ought to store the
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322 // register that the offset is from?
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323 RI->StackOffset = TFI->getFrameIndexReference(MF, RI->Num, FrameReg);
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324 ++RI;
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325 }
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326 }
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327 }
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328
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329 bool GCMachineCodeAnalysis::runOnMachineFunction(MachineFunction &MF) {
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330 // Quick exit for functions that do not use GC.
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134
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331 if (!MF.getFunction().hasGC())
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332 return false;
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333
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334 FI = &getAnalysis<GCModuleInfo>().getFunctionInfo(MF.getFunction());
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335 MMI = &getAnalysis<MachineModuleInfo>();
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336 TII = MF.getSubtarget().getInstrInfo();
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337
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338 // Find the size of the stack frame. There may be no correct static frame
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339 // size, we use UINT64_MAX to represent this.
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120
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340 const MachineFrameInfo &MFI = MF.getFrameInfo();
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341 const TargetRegisterInfo *RegInfo = MF.getSubtarget().getRegisterInfo();
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120
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342 const bool DynamicFrameSize = MFI.hasVarSizedObjects() ||
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343 RegInfo->needsStackRealignment(MF);
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344 FI->setFrameSize(DynamicFrameSize ? UINT64_MAX : MFI.getStackSize());
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345
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346 // Find all safe points.
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347 if (FI->getStrategy().needsSafePoints())
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348 FindSafePoints(MF);
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83
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349
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95
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350 // Find the concrete stack offsets for all roots (stack slots)
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83
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351 FindStackOffsets(MF);
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352
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353 return false;
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354 }
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