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/ADT/DepthFirstIterator.h" 20 #include "llvm/ADT/SmallPtrSet.h" 21 #include "llvm/ADT/SmallVector.h" 22 #include "llvm/ADT/Statistic.h" 23 #include "llvm/IR/BasicBlock.h" 24 #include "llvm/IR/Instructions.h" 25 #include "llvm/IR/IntrinsicInst.h" 26 #include "llvm/Pass.h" 27 #include "llvm/Support/CFG.h" 28 #include "llvm/Support/InstIterator.h" 29 using namespace llvm; 30 31 STATISTIC(NumRemoved, "Number of instructions removed"); 32 33 namespace { 34 struct ADCE : public FunctionPass { 35 static char ID; // Pass identification, replacement for typeid 36 ADCE() : FunctionPass(ID) { 37 initializeADCEPass(*PassRegistry::getPassRegistry()); 38 } 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 INITIALIZE_PASS(ADCE, "adce", "Aggressive Dead Code Elimination", false, false) 51 52 bool ADCE::runOnFunction(Function& F) { 53 SmallPtrSet<Instruction*, 128> alive; 54 SmallVector<Instruction*, 128> worklist; 55 56 // Collect the set of "root" instructions that are known live. 57 for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) 58 if (isa<TerminatorInst>(I.getInstructionIterator()) || 59 isa<DbgInfoIntrinsic>(I.getInstructionIterator()) || 60 isa<LandingPadInst>(I.getInstructionIterator()) || 61 I->mayHaveSideEffects()) { 62 alive.insert(I.getInstructionIterator()); 63 worklist.push_back(I.getInstructionIterator()); 64 } 65 66 // Propagate liveness backwards to operands. 67 while (!worklist.empty()) { 68 Instruction* curr = worklist.pop_back_val(); 69 for (Instruction::op_iterator OI = curr->op_begin(), OE = curr->op_end(); 70 OI != OE; ++OI) 71 if (Instruction* Inst = dyn_cast<Instruction>(OI)) 72 if (alive.insert(Inst)) 73 worklist.push_back(Inst); 74 } 75 76 // The inverse of the live set is the dead set. These are those instructions 77 // which have no side effects and do not influence the control flow or return 78 // value of the function, and may therefore be deleted safely. 79 // NOTE: We reuse the worklist vector here for memory efficiency. 80 for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) 81 if (!alive.count(I.getInstructionIterator())) { 82 worklist.push_back(I.getInstructionIterator()); 83 I->dropAllReferences(); 84 } 85 86 for (SmallVector<Instruction*, 1024>::iterator I = worklist.begin(), 87 E = worklist.end(); I != E; ++I) { 88 ++NumRemoved; 89 (*I)->eraseFromParent(); 90 } 91 92 return !worklist.empty(); 93 } 94 95 FunctionPass *llvm::createAggressiveDCEPass() { 96 return new ADCE(); 97 } 98
