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