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