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