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/ADT/Statistic.h" 26 #include <set> 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 virtual bool runOnBasicBlock(BasicBlock &BB) { 40 bool Changed = false; 41 for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) { 42 Instruction *Inst = DI++; 43 if (isInstructionTriviallyDead(Inst)) { 44 Inst->eraseFromParent(); 45 Changed = true; 46 ++DIEEliminated; 47 } 48 } 49 return Changed; 50 } 51 52 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 53 AU.setPreservesCFG(); 54 } 55 }; 56 } 57 58 char DeadInstElimination::ID = 0; 59 INITIALIZE_PASS(DeadInstElimination, "die", 60 "Dead Instruction Elimination", false, false) 61 62 Pass *llvm::createDeadInstEliminationPass() { 63 return new DeadInstElimination(); 64 } 65 66 67 namespace { 68 //===--------------------------------------------------------------------===// 69 // DeadCodeElimination pass implementation 70 // 71 struct DCE : public FunctionPass { 72 static char ID; // Pass identification, replacement for typeid 73 DCE() : FunctionPass(ID) {} 74 75 virtual bool runOnFunction(Function &F); 76 77 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 78 AU.setPreservesCFG(); 79 } 80 }; 81 } 82 83 char DCE::ID = 0; 84 INITIALIZE_PASS(DCE, "dce", "Dead Code Elimination", false, false) 85 86 bool DCE::runOnFunction(Function &F) { 87 // Start out with all of the instructions in the worklist... 88 std::vector<Instruction*> WorkList; 89 for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) 90 WorkList.push_back(&*i); 91 92 // Loop over the worklist finding instructions that are dead. If they are 93 // dead make them drop all of their uses, making other instructions 94 // potentially dead, and work until the worklist is empty. 95 // 96 bool MadeChange = false; 97 while (!WorkList.empty()) { 98 Instruction *I = WorkList.back(); 99 WorkList.pop_back(); 100 101 if (isInstructionTriviallyDead(I)) { // If the instruction is dead. 102 // Loop over all of the values that the instruction uses, if there are 103 // instructions being used, add them to the worklist, because they might 104 // go dead after this one is removed. 105 // 106 for (User::op_iterator OI = I->op_begin(), E = I->op_end(); OI != E; ++OI) 107 if (Instruction *Used = dyn_cast<Instruction>(*OI)) 108 WorkList.push_back(Used); 109 110 // Remove the instruction. 111 I->eraseFromParent(); 112 113 // Remove the instruction from the worklist if it still exists in it. 114 for (std::vector<Instruction*>::iterator WI = WorkList.begin(); 115 WI != WorkList.end(); ) { 116 if (*WI == I) 117 WI = WorkList.erase(WI); 118 else 119 ++WI; 120 } 121 122 MadeChange = true; 123 ++DCEEliminated; 124 } 125 } 126 return MadeChange; 127 } 128 129 FunctionPass *llvm::createDeadCodeEliminationPass() { 130 return new DCE(); 131 } 132 133