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 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(&ID) {} 38 39 virtual bool runOnFunction(Function& F); 40 41 virtual void getAnalysisUsage(AnalysisUsage& AU) const { 42 AU.setPreservesCFG(); 43 } 44 45 }; 46 } 47 48 char ADCE::ID = 0; 49 static RegisterPass<ADCE> X("adce", "Aggressive Dead Code Elimination"); 50 51 bool ADCE::runOnFunction(Function& F) { 52 SmallPtrSet<Instruction*, 128> alive; 53 SmallVector<Instruction*, 128> worklist; 54 55 // Collect the set of "root" instructions that are known live. 56 for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) 57 if (isa<TerminatorInst>(I.getInstructionIterator()) || 58 I->mayWriteToMemory()) { 59 alive.insert(I.getInstructionIterator()); 60 worklist.push_back(I.getInstructionIterator()); 61 } 62 63 // Propagate liveness backwards to operands. 64 while (!worklist.empty()) { 65 Instruction* curr = worklist.back(); 66 worklist.pop_back(); 67 68 for (Instruction::op_iterator OI = curr->op_begin(), OE = curr->op_end(); 69 OI != OE; ++OI) 70 if (Instruction* Inst = dyn_cast<Instruction>(OI)) 71 if (alive.insert(Inst)) 72 worklist.push_back(Inst); 73 } 74 75 // The inverse of the live set is the dead set. These are those instructions 76 // which have no side effects and do not influence the control flow or return 77 // value of the function, and may therefore be deleted safely. 78 // NOTE: We reuse the worklist vector here for memory efficiency. 79 for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) 80 if (!alive.count(I.getInstructionIterator())) { 81 worklist.push_back(I.getInstructionIterator()); 82 I->dropAllReferences(); 83 } 84 85 for (SmallVector<Instruction*, 1024>::iterator I = worklist.begin(), 86 E = worklist.end(); I != E; ++I) { 87 NumRemoved++; 88 (*I)->eraseFromParent(); 89 } 90 91 return !worklist.empty(); 92 } 93 94 FunctionPass *llvm::createAggressiveDCEPass() { 95 return new ADCE(); 96 } 97