LLVM API Documentation
00001 //===- DCE.cpp - Code to perform dead code elimination --------------------===// 00002 // 00003 // The LLVM Compiler Infrastructure 00004 // 00005 // This file was developed by the LLVM research group and is distributed under 00006 // the University of Illinois Open Source License. See LICENSE.TXT for details. 00007 // 00008 //===----------------------------------------------------------------------===// 00009 // 00010 // This file implements dead inst elimination and dead code elimination. 00011 // 00012 // Dead Inst Elimination performs a single pass over the function removing 00013 // instructions that are obviously dead. Dead Code Elimination is similar, but 00014 // it rechecks instructions that were used by removed instructions to see if 00015 // they are newly dead. 00016 // 00017 //===----------------------------------------------------------------------===// 00018 00019 #include "llvm/Transforms/Scalar.h" 00020 #include "llvm/Transforms/Utils/Local.h" 00021 #include "llvm/Instruction.h" 00022 #include "llvm/Pass.h" 00023 #include "llvm/Support/InstIterator.h" 00024 #include "llvm/ADT/Statistic.h" 00025 #include <set> 00026 using namespace llvm; 00027 00028 namespace { 00029 Statistic<> DIEEliminated("die", "Number of insts removed"); 00030 Statistic<> DCEEliminated("dce", "Number of insts removed"); 00031 00032 //===--------------------------------------------------------------------===// 00033 // DeadInstElimination pass implementation 00034 // 00035 00036 struct DeadInstElimination : public BasicBlockPass { 00037 virtual bool runOnBasicBlock(BasicBlock &BB) { 00038 bool Changed = false; 00039 for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) 00040 if (dceInstruction(DI)) { 00041 Changed = true; 00042 ++DIEEliminated; 00043 } else 00044 ++DI; 00045 return Changed; 00046 } 00047 00048 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 00049 AU.setPreservesCFG(); 00050 } 00051 }; 00052 00053 RegisterOpt<DeadInstElimination> X("die", "Dead Instruction Elimination"); 00054 } 00055 00056 FunctionPass *llvm::createDeadInstEliminationPass() { 00057 return new DeadInstElimination(); 00058 } 00059 00060 00061 //===----------------------------------------------------------------------===// 00062 // DeadCodeElimination pass implementation 00063 // 00064 00065 namespace { 00066 struct DCE : public FunctionPass { 00067 virtual bool runOnFunction(Function &F); 00068 00069 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 00070 AU.setPreservesCFG(); 00071 } 00072 }; 00073 00074 RegisterOpt<DCE> Y("dce", "Dead Code Elimination"); 00075 } 00076 00077 bool DCE::runOnFunction(Function &F) { 00078 // Start out with all of the instructions in the worklist... 00079 std::vector<Instruction*> WorkList; 00080 for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) 00081 WorkList.push_back(&*i); 00082 00083 // Loop over the worklist finding instructions that are dead. If they are 00084 // dead make them drop all of their uses, making other instructions 00085 // potentially dead, and work until the worklist is empty. 00086 // 00087 bool MadeChange = false; 00088 while (!WorkList.empty()) { 00089 Instruction *I = WorkList.back(); 00090 WorkList.pop_back(); 00091 00092 if (isInstructionTriviallyDead(I)) { // If the instruction is dead. 00093 // Loop over all of the values that the instruction uses, if there are 00094 // instructions being used, add them to the worklist, because they might 00095 // go dead after this one is removed. 00096 // 00097 for (User::op_iterator OI = I->op_begin(), E = I->op_end(); OI != E; ++OI) 00098 if (Instruction *Used = dyn_cast<Instruction>(*OI)) 00099 WorkList.push_back(Used); 00100 00101 // Remove the instruction. 00102 I->eraseFromParent(); 00103 00104 // Remove the instruction from the worklist if it still exists in it. 00105 for (std::vector<Instruction*>::iterator WI = WorkList.begin(), 00106 E = WorkList.end(); WI != E; ++WI) 00107 if (*WI == I) { 00108 WorkList.erase(WI); 00109 --E; 00110 --WI; 00111 } 00112 00113 MadeChange = true; 00114 ++DCEEliminated; 00115 } 00116 } 00117 return MadeChange; 00118 } 00119 00120 FunctionPass *llvm::createDeadCodeEliminationPass() { 00121 return new DCE(); 00122 } 00123