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