1193323Sed//===- DCE.cpp - Code to perform dead code elimination --------------------===// 2193323Sed// 3193323Sed// The LLVM Compiler Infrastructure 4193323Sed// 5193323Sed// This file is distributed under the University of Illinois Open Source 6193323Sed// License. See LICENSE.TXT for details. 7193323Sed// 8193323Sed//===----------------------------------------------------------------------===// 9193323Sed// 10193323Sed// This file implements the Aggressive Dead Code Elimination pass. This pass 11193323Sed// optimistically assumes that all instructions are dead until proven otherwise, 12239462Sdim// allowing it to eliminate dead computations that other DCE passes do not 13193323Sed// catch, particularly involving loop computations. 14193323Sed// 15193323Sed//===----------------------------------------------------------------------===// 16193323Sed 17193323Sed#define DEBUG_TYPE "adce" 18193323Sed#include "llvm/Transforms/Scalar.h" 19193323Sed#include "llvm/ADT/DepthFirstIterator.h" 20193323Sed#include "llvm/ADT/SmallPtrSet.h" 21193323Sed#include "llvm/ADT/SmallVector.h" 22193323Sed#include "llvm/ADT/Statistic.h" 23249423Sdim#include "llvm/IR/BasicBlock.h" 24249423Sdim#include "llvm/IR/Instructions.h" 25249423Sdim#include "llvm/IR/IntrinsicInst.h" 26249423Sdim#include "llvm/Pass.h" 27249423Sdim#include "llvm/Support/CFG.h" 28249423Sdim#include "llvm/Support/InstIterator.h" 29193323Sedusing namespace llvm; 30193323Sed 31193323SedSTATISTIC(NumRemoved, "Number of instructions removed"); 32193323Sed 33193323Sednamespace { 34198090Srdivacky struct ADCE : public FunctionPass { 35193323Sed static char ID; // Pass identification, replacement for typeid 36218893Sdim ADCE() : FunctionPass(ID) { 37218893Sdim initializeADCEPass(*PassRegistry::getPassRegistry()); 38218893Sdim } 39239462Sdim 40193323Sed virtual bool runOnFunction(Function& F); 41239462Sdim 42193323Sed virtual void getAnalysisUsage(AnalysisUsage& AU) const { 43193323Sed AU.setPreservesCFG(); 44193323Sed } 45239462Sdim 46193323Sed }; 47193323Sed} 48193323Sed 49193323Sedchar ADCE::ID = 0; 50218893SdimINITIALIZE_PASS(ADCE, "adce", "Aggressive Dead Code Elimination", false, false) 51193323Sed 52193323Sedbool ADCE::runOnFunction(Function& F) { 53193323Sed SmallPtrSet<Instruction*, 128> alive; 54193323Sed SmallVector<Instruction*, 128> worklist; 55239462Sdim 56193323Sed // Collect the set of "root" instructions that are known live. 57193323Sed for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) 58193323Sed if (isa<TerminatorInst>(I.getInstructionIterator()) || 59193323Sed isa<DbgInfoIntrinsic>(I.getInstructionIterator()) || 60226633Sdim isa<LandingPadInst>(I.getInstructionIterator()) || 61193323Sed I->mayHaveSideEffects()) { 62193323Sed alive.insert(I.getInstructionIterator()); 63193323Sed worklist.push_back(I.getInstructionIterator()); 64193323Sed } 65239462Sdim 66193323Sed // Propagate liveness backwards to operands. 67193323Sed while (!worklist.empty()) { 68202375Srdivacky Instruction* curr = worklist.pop_back_val(); 69193323Sed for (Instruction::op_iterator OI = curr->op_begin(), OE = curr->op_end(); 70193323Sed OI != OE; ++OI) 71193323Sed if (Instruction* Inst = dyn_cast<Instruction>(OI)) 72193323Sed if (alive.insert(Inst)) 73193323Sed worklist.push_back(Inst); 74193323Sed } 75239462Sdim 76193323Sed // The inverse of the live set is the dead set. These are those instructions 77193323Sed // which have no side effects and do not influence the control flow or return 78193323Sed // value of the function, and may therefore be deleted safely. 79193323Sed // NOTE: We reuse the worklist vector here for memory efficiency. 80193323Sed for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) 81193323Sed if (!alive.count(I.getInstructionIterator())) { 82193323Sed worklist.push_back(I.getInstructionIterator()); 83193323Sed I->dropAllReferences(); 84193323Sed } 85239462Sdim 86263508Sdim for (SmallVectorImpl<Instruction *>::iterator I = worklist.begin(), 87193323Sed E = worklist.end(); I != E; ++I) { 88210299Sed ++NumRemoved; 89193323Sed (*I)->eraseFromParent(); 90193323Sed } 91193323Sed 92193323Sed return !worklist.empty(); 93193323Sed} 94193323Sed 95193323SedFunctionPass *llvm::createAggressiveDCEPass() { 96193323Sed return new ADCE(); 97193323Sed} 98