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 the Aggressive Dead Code Elimination pass. This pass 11 // optimistically assumes that all instructions are dead until proven otherwise, 12 // allowing it to eliminate dead computations that other DCE passes do not 13 // catch, particularly involving loop computations. 14 // 15 //===----------------------------------------------------------------------===// 16 17 #define DEBUG_TYPE "adce" 18 #include "llvm/Transforms/Scalar.h" 19 #include "llvm/BasicBlock.h" 20 #include "llvm/Instructions.h" 21 #include "llvm/Pass.h" 22 #include "llvm/Support/CFG.h" 23 #include "llvm/Support/Compiler.h" 24 #include "llvm/Support/InstIterator.h" 25 #include "llvm/ADT/DenseSet.h" 26 #include "llvm/ADT/DepthFirstIterator.h" 27 #include "llvm/ADT/Statistic.h" 28 #include "llvm/ADT/SmallVector.h" 29 30 using namespace llvm; 31 32 STATISTIC(NumRemoved, "Number of instructions removed"); 33 34 namespace { 35 struct VISIBILITY_HIDDEN ADCE : public FunctionPass { 36 static char ID; // Pass identification, replacement for typeid 37 ADCE() : FunctionPass((intptr_t)&ID) {} 38 39 DenseSet<Instruction*> alive; 40 SmallVector<Instruction*, 1024> worklist; 41 42 DenseSet<BasicBlock*> reachable; 43 SmallVector<BasicBlock*, 1024> unreachable; 44 45 virtual bool runOnFunction(Function& F); 46 47 virtual void getAnalysisUsage(AnalysisUsage& AU) const { 48 AU.setPreservesCFG(); 49 } 50 51 }; 52 } 53 54 char ADCE::ID = 0; 55 static RegisterPass<ADCE> X("adce", "Aggressive Dead Code Elimination"); 56 57 bool ADCE::runOnFunction(Function& F) { 58 alive.clear(); 59 worklist.clear(); 60 reachable.clear(); 61 unreachable.clear(); 62 63 // First, collect the set of reachable blocks ... 64 for (df_iterator<BasicBlock*> DI = df_begin(&F.getEntryBlock()), 65 DE = df_end(&F.getEntryBlock()); DI != DE; ++DI) 66 reachable.insert(*DI); 67 68 // ... and then invert it into the list of unreachable ones. These 69 // blocks will be removed from the function. 70 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) 71 if (!reachable.count(FI)) 72 unreachable.push_back(FI); 73 74 // Prepare to remove blocks by removing the PHI node entries for those blocks 75 // in their successors, and remove them from reference counting. 76 for (SmallVector<BasicBlock*, 1024>::iterator UI = unreachable.begin(), 77 UE = unreachable.end(); UI != UE; ++UI) { 78 BasicBlock* BB = *UI; 79 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); 80 SI != SE; ++SI) { 81 BasicBlock* succ = *SI; 82 BasicBlock::iterator succ_inst = succ->begin(); 83 while (PHINode* P = dyn_cast<PHINode>(succ_inst)) { 84 P->removeIncomingValue(BB); 85 ++succ_inst; 86 } 87 } 88 89 BB->dropAllReferences(); 90 } 91 92 // Finally, erase the unreachable blocks. 93 for (SmallVector<BasicBlock*, 1024>::iterator UI = unreachable.begin(), 94 UE = unreachable.end(); UI != UE; ++UI) 95 (*UI)->eraseFromParent(); 96 97 // Collect the set of "root" instructions that are known live. 98 for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) 99 if (isa<TerminatorInst>(I.getInstructionIterator()) || 100 I->mayWriteToMemory()) { 101 alive.insert(I.getInstructionIterator()); 102 worklist.push_back(I.getInstructionIterator()); 103 } 104 105 // Propagate liveness backwards to operands. 106 while (!worklist.empty()) { 107 Instruction* curr = worklist.back(); 108 worklist.pop_back(); 109 110 for (Instruction::op_iterator OI = curr->op_begin(), OE = curr->op_end(); 111 OI != OE; ++OI) 112 if (Instruction* Inst = dyn_cast<Instruction>(OI)) 113 if (alive.insert(Inst)) 114 worklist.push_back(Inst); 115 } 116 117 // The inverse of the live set is the dead set. These are those instructions 118 // which have no side effects and do not influence the control flow or return 119 // value of the function, and may therefore be deleted safely. 120 // NOTE: We reuse the worklist vector here for memory efficiency. 121 for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) 122 if (!alive.count(I.getInstructionIterator())) { 123 worklist.push_back(I.getInstructionIterator()); 124 I->dropAllReferences(); 125 } 126 127 for (SmallVector<Instruction*, 1024>::iterator I = worklist.begin(), 128 E = worklist.end(); I != E; ++I) { 129 NumRemoved++; 130 (*I)->eraseFromParent(); 131 } 132 133 return !worklist.empty(); 134 } 135 136 FunctionPass *llvm::createAggressiveDCEPass() { 137 return new ADCE(); 138 } 139