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 #include "llvm/Transforms/Scalar.h" 18 #include "llvm/ADT/DepthFirstIterator.h" 19 #include "llvm/ADT/SmallPtrSet.h" 20 #include "llvm/ADT/SmallVector.h" 21 #include "llvm/ADT/Statistic.h" 22 #include "llvm/IR/BasicBlock.h" 23 #include "llvm/IR/CFG.h" 24 #include "llvm/IR/InstIterator.h" 25 #include "llvm/IR/Instructions.h" 26 #include "llvm/IR/IntrinsicInst.h" 27 #include "llvm/Pass.h" 28 using namespace llvm; 29 30 #define DEBUG_TYPE "adce" 31 32 STATISTIC(NumRemoved, "Number of instructions removed"); 33 34 namespace { 35 struct ADCE : public FunctionPass { 36 static char ID; // Pass identification, replacement for typeid 37 ADCE() : FunctionPass(ID) { 38 initializeADCEPass(*PassRegistry::getPassRegistry()); 39 } 40 41 bool runOnFunction(Function& F) override; 42 43 void getAnalysisUsage(AnalysisUsage& AU) const override { 44 AU.setPreservesCFG(); 45 } 46 47 }; 48 } 49 50 char ADCE::ID = 0; 51 INITIALIZE_PASS(ADCE, "adce", "Aggressive Dead Code Elimination", false, false) 52 53 bool ADCE::runOnFunction(Function& F) { 54 if (skipOptnoneFunction(F)) 55 return false; 56 57 SmallPtrSet<Instruction*, 128> alive; 58 SmallVector<Instruction*, 128> worklist; 59 60 // Collect the set of "root" instructions that are known live. 61 for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) 62 if (isa<TerminatorInst>(I.getInstructionIterator()) || 63 isa<DbgInfoIntrinsic>(I.getInstructionIterator()) || 64 isa<LandingPadInst>(I.getInstructionIterator()) || 65 I->mayHaveSideEffects()) { 66 alive.insert(I.getInstructionIterator()); 67 worklist.push_back(I.getInstructionIterator()); 68 } 69 70 // Propagate liveness backwards to operands. 71 while (!worklist.empty()) { 72 Instruction* curr = worklist.pop_back_val(); 73 for (Instruction::op_iterator OI = curr->op_begin(), OE = curr->op_end(); 74 OI != OE; ++OI) 75 if (Instruction* Inst = dyn_cast<Instruction>(OI)) 76 if (alive.insert(Inst)) 77 worklist.push_back(Inst); 78 } 79 80 // The inverse of the live set is the dead set. These are those instructions 81 // which have no side effects and do not influence the control flow or return 82 // value of the function, and may therefore be deleted safely. 83 // NOTE: We reuse the worklist vector here for memory efficiency. 84 for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) 85 if (!alive.count(I.getInstructionIterator())) { 86 worklist.push_back(I.getInstructionIterator()); 87 I->dropAllReferences(); 88 } 89 90 for (SmallVectorImpl<Instruction *>::iterator I = worklist.begin(), 91 E = worklist.end(); I != E; ++I) { 92 ++NumRemoved; 93 (*I)->eraseFromParent(); 94 } 95 96 return !worklist.empty(); 97 } 98 99 FunctionPass *llvm::createAggressiveDCEPass() { 100 return new ADCE(); 101 } 102
