1 //===- DCE.cpp - Code to perform dead code elimination --------------------===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // This file implements dead inst elimination and dead code elimination. 11 // 12 // Dead Inst Elimination performs a single pass over the function removing 13 // instructions that are obviously dead. Dead Code Elimination is similar, but 14 // it rechecks instructions that were used by removed instructions to see if 15 // they are newly dead. 16 // 17 //===----------------------------------------------------------------------===// 18 19 #define DEBUG_TYPE "dce" 20 #include "llvm/Transforms/Scalar.h" 21 #include "llvm/Transforms/Utils/Local.h" 22 #include "llvm/Instruction.h" 23 #include "llvm/Pass.h" 24 #include "llvm/Support/InstIterator.h" 25 #include "llvm/Target/TargetLibraryInfo.h" 26 #include "llvm/ADT/Statistic.h" 27 using namespace llvm; 28 29 STATISTIC(DIEEliminated, "Number of insts removed by DIE pass"); 30 STATISTIC(DCEEliminated, "Number of insts removed"); 31 32 namespace { 33 //===--------------------------------------------------------------------===// 34 // DeadInstElimination pass implementation 35 // 36 struct DeadInstElimination : public BasicBlockPass { 37 static char ID; // Pass identification, replacement for typeid 38 DeadInstElimination() : BasicBlockPass(ID) { 39 initializeDeadInstEliminationPass(*PassRegistry::getPassRegistry()); 40 } 41 virtual bool runOnBasicBlock(BasicBlock &BB) { 42 TargetLibraryInfo *TLI = getAnalysisIfAvailable<TargetLibraryInfo>(); 43 bool Changed = false; 44 for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) { 45 Instruction *Inst = DI++; 46 if (isInstructionTriviallyDead(Inst, TLI)) { 47 Inst->eraseFromParent(); 48 Changed = true; 49 ++DIEEliminated; 50 } 51 } 52 return Changed; 53 } 54 55 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 56 AU.setPreservesCFG(); 57 } 58 }; 59 } 60 61 char DeadInstElimination::ID = 0; 62 INITIALIZE_PASS(DeadInstElimination, "die", 63 "Dead Instruction Elimination", false, false) 64 65 Pass *llvm::createDeadInstEliminationPass() { 66 return new DeadInstElimination(); 67 } 68 69 70 namespace { 71 //===--------------------------------------------------------------------===// 72 // DeadCodeElimination pass implementation 73 // 74 struct DCE : public FunctionPass { 75 static char ID; // Pass identification, replacement for typeid 76 DCE() : FunctionPass(ID) { 77 initializeDCEPass(*PassRegistry::getPassRegistry()); 78 } 79 80 virtual bool runOnFunction(Function &F); 81 82 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 83 AU.setPreservesCFG(); 84 } 85 }; 86 } 87 88 char DCE::ID = 0; 89 INITIALIZE_PASS(DCE, "dce", "Dead Code Elimination", false, false) 90 91 bool DCE::runOnFunction(Function &F) { 92 TargetLibraryInfo *TLI = getAnalysisIfAvailable<TargetLibraryInfo>(); 93 94 // Start out with all of the instructions in the worklist... 95 std::vector<Instruction*> WorkList; 96 for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) 97 WorkList.push_back(&*i); 98 99 // Loop over the worklist finding instructions that are dead. If they are 100 // dead make them drop all of their uses, making other instructions 101 // potentially dead, and work until the worklist is empty. 102 // 103 bool MadeChange = false; 104 while (!WorkList.empty()) { 105 Instruction *I = WorkList.back(); 106 WorkList.pop_back(); 107 108 if (isInstructionTriviallyDead(I, TLI)) { // If the instruction is dead. 109 // Loop over all of the values that the instruction uses, if there are 110 // instructions being used, add them to the worklist, because they might 111 // go dead after this one is removed. 112 // 113 for (User::op_iterator OI = I->op_begin(), E = I->op_end(); OI != E; ++OI) 114 if (Instruction *Used = dyn_cast<Instruction>(*OI)) 115 WorkList.push_back(Used); 116 117 // Remove the instruction. 118 I->eraseFromParent(); 119 120 // Remove the instruction from the worklist if it still exists in it. 121 for (std::vector<Instruction*>::iterator WI = WorkList.begin(); 122 WI != WorkList.end(); ) { 123 if (*WI == I) 124 WI = WorkList.erase(WI); 125 else 126 ++WI; 127 } 128 129 MadeChange = true; 130 ++DCEEliminated; 131 } 132 } 133 return MadeChange; 134 } 135 136 FunctionPass *llvm::createDeadCodeEliminationPass() { 137 return new DCE(); 138 } 139 140
