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/Compiler.h" 25 #include "llvm/Support/InstIterator.h" 26 #include "llvm/ADT/Statistic.h" 27 #include <set> 28 using namespace llvm; 29 30 STATISTIC(DIEEliminated, "Number of insts removed by DIE pass"); 31 STATISTIC(DCEEliminated, "Number of insts removed"); 32 33 namespace { 34 //===--------------------------------------------------------------------===// 35 // DeadInstElimination pass implementation 36 // 37 struct VISIBILITY_HIDDEN DeadInstElimination : public BasicBlockPass { 38 static char ID; // Pass identification, replacement for typeid 39 DeadInstElimination() : BasicBlockPass(&ID) {} 40 virtual bool runOnBasicBlock(BasicBlock &BB) { 41 bool Changed = false; 42 for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) { 43 Instruction *Inst = DI++; 44 if (isInstructionTriviallyDead(Inst)) { 45 Inst->eraseFromParent(); 46 Changed = true; 47 ++DIEEliminated; 48 } 49 } 50 return Changed; 51 } 52 53 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 54 AU.setPreservesCFG(); 55 } 56 }; 57 } 58 59 char DeadInstElimination::ID = 0; 60 static RegisterPass<DeadInstElimination> 61 X("die", "Dead Instruction Elimination"); 62 63 Pass *llvm::createDeadInstEliminationPass() { 64 return new DeadInstElimination(); 65 } 66 67 68 namespace { 69 //===--------------------------------------------------------------------===// 70 // DeadCodeElimination pass implementation 71 // 72 struct DCE : public FunctionPass { 73 static char ID; // Pass identification, replacement for typeid 74 DCE() : FunctionPass(&ID) {} 75 76 virtual bool runOnFunction(Function &F); 77 78 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 79 AU.setPreservesCFG(); 80 } 81 }; 82 } 83 84 char DCE::ID = 0; 85 static RegisterPass<DCE> Y("dce", "Dead Code Elimination"); 86 87 bool DCE::runOnFunction(Function &F) { 88 // Start out with all of the instructions in the worklist... 89 std::vector<Instruction*> WorkList; 90 for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) 91 WorkList.push_back(&*i); 92 93 // Loop over the worklist finding instructions that are dead. If they are 94 // dead make them drop all of their uses, making other instructions 95 // potentially dead, and work until the worklist is empty. 96 // 97 bool MadeChange = false; 98 while (!WorkList.empty()) { 99 Instruction *I = WorkList.back(); 100 WorkList.pop_back(); 101 102 if (isInstructionTriviallyDead(I)) { // If the instruction is dead. 103 // Loop over all of the values that the instruction uses, if there are 104 // instructions being used, add them to the worklist, because they might 105 // go dead after this one is removed. 106 // 107 for (User::op_iterator OI = I->op_begin(), E = I->op_end(); OI != E; ++OI) 108 if (Instruction *Used = dyn_cast<Instruction>(*OI)) 109 WorkList.push_back(Used); 110 111 // Remove the instruction. 112 I->eraseFromParent(); 113 114 // Remove the instruction from the worklist if it still exists in it. 115 for (std::vector<Instruction*>::iterator WI = WorkList.begin(); 116 WI != WorkList.end(); ) { 117 if (*WI == I) 118 WI = WorkList.erase(WI); 119 else 120 ++WI; 121 } 122 123 MadeChange = true; 124 ++DCEEliminated; 125 } 126 } 127 return MadeChange; 128 } 129 130 FunctionPass *llvm::createDeadCodeEliminationPass() { 131 return new DCE(); 132 } 133 134