lib/Transforms/Scalar/DCE.cpp

   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 dead inst elimination and dead code elimination.
  11 //
  12 // Dead Inst Elimination performs a single pass over the function removing
  13 // instructions that are obviously dead.  Dead Code Elimination is similar, but
  14 // it rechecks instructions that were used by removed instructions to see if
  15 // they are newly dead.
  16 //
  17 //===----------------------------------------------------------------------===//
  18
  19 #define DEBUG_TYPE "dce"
  20 #include "llvm/Transforms/Scalar.h"
  21 #include "llvm/Transforms/Utils/Local.h"
  22 #include "llvm/Instruction.h"
  23 #include "llvm/Pass.h"
  24 #include "llvm/Support/InstIterator.h"
  25 #include "llvm/ADT/Statistic.h"
  26 #include <set>
  27 using namespace llvm;
  28
  29 STATISTIC(DIEEliminated, "Number of insts removed by DIE pass");
  30 STATISTIC(DCEEliminated, "Number of insts removed");
  31
  32 namespace {
  33   //===--------------------------------------------------------------------===//
  34   // DeadInstElimination pass implementation
  35   //
  36   struct DeadInstElimination : public BasicBlockPass {
  37     static char ID; // Pass identification, replacement for typeid
  38     DeadInstElimination() : BasicBlockPass(ID) {
  39       initializeDeadInstEliminationPass(*PassRegistry::getPassRegistry());
  40     }
  41     virtual bool runOnBasicBlock(BasicBlock &BB) {
  42       bool Changed = false;
  43       for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) {
  44         Instruction *Inst = DI++;
  45         if (isInstructionTriviallyDead(Inst)) {
  46           Inst->eraseFromParent();
  47           Changed = true;
  48           ++DIEEliminated;
  49         }
  50       }
  51       return Changed;
  52     }
  53
  54     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  55       AU.setPreservesCFG();
  56     }
  57   };
  58 }
  59
  60 char DeadInstElimination::ID = 0;
  61 INITIALIZE_PASS(DeadInstElimination, "die",
  62                 "Dead Instruction Elimination", false, false)
  63
  64 Pass *llvm::createDeadInstEliminationPass() {
  65   return new DeadInstElimination();
  66 }
  67
  68
  69 namespace {
  70   //===--------------------------------------------------------------------===//
  71   // DeadCodeElimination pass implementation
  72   //
  73   struct DCE : public FunctionPass {
  74     static char ID; // Pass identification, replacement for typeid
  75     DCE() : FunctionPass(ID) {
  76       initializeDCEPass(*PassRegistry::getPassRegistry());
  77     }
  78
  79     virtual bool runOnFunction(Function &F);
  80
  81      virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  82       AU.setPreservesCFG();
  83     }
  84  };
  85 }
  86
  87 char DCE::ID = 0;
  88 INITIALIZE_PASS(DCE, "dce", "Dead Code Elimination", false, false)
  89
  90 bool DCE::runOnFunction(Function &F) {
  91   // Start out with all of the instructions in the worklist...
  92   std::vector<Instruction*> WorkList;
  93   for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i)
  94     WorkList.push_back(&*i);
  95
  96   // Loop over the worklist finding instructions that are dead.  If they are
  97   // dead make them drop all of their uses, making other instructions
  98   // potentially dead, and work until the worklist is empty.
  99   //
 100   bool MadeChange = false;
 101   while (!WorkList.empty()) {
 102     Instruction *I = WorkList.back();
 103     WorkList.pop_back();
 104
 105     if (isInstructionTriviallyDead(I)) {       // If the instruction is dead.
 106       // Loop over all of the values that the instruction uses, if there are
 107       // instructions being used, add them to the worklist, because they might
 108       // go dead after this one is removed.
 109       //
 110       for (User::op_iterator OI = I->op_begin(), E = I->op_end(); OI != E; ++OI)
 111         if (Instruction *Used = dyn_cast<Instruction>(*OI))
 112           WorkList.push_back(Used);
 113
 114       // Remove the instruction.
 115       I->eraseFromParent();
 116
 117       // Remove the instruction from the worklist if it still exists in it.
 118       for (std::vector<Instruction*>::iterator WI = WorkList.begin();
 119            WI != WorkList.end(); ) {
 120         if (*WI == I)
 121           WI = WorkList.erase(WI);
 122         else
 123           ++WI;
 124       }
 125
 126       MadeChange = true;
 127       ++DCEEliminated;
 128     }
 129   }
 130   return MadeChange;
 131 }
 132
 133 FunctionPass *llvm::createDeadCodeEliminationPass() {
 134   return new DCE();
 135 }
 136