DCE.cpp revision 221345 
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"
26using namespace llvm;
27
28STATISTIC(DIEEliminated, "Number of insts removed by DIE pass");
29STATISTIC(DCEEliminated, "Number of insts removed");
30
31namespace {
32  //===--------------------------------------------------------------------===//
33  // DeadInstElimination pass implementation
34  //
35  struct DeadInstElimination : public BasicBlockPass {
36    static char ID; // Pass identification, replacement for typeid
37    DeadInstElimination() : BasicBlockPass(ID) {
38      initializeDeadInstEliminationPass(*PassRegistry::getPassRegistry());
39    }
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
59char DeadInstElimination::ID = 0;
60INITIALIZE_PASS(DeadInstElimination, "die",
61                "Dead Instruction Elimination", false, false)
62
63Pass *llvm::createDeadInstEliminationPass() {
64  return new DeadInstElimination();
65}
66
67
68namespace {
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      initializeDCEPass(*PassRegistry::getPassRegistry());
76    }
77
78    virtual bool runOnFunction(Function &F);
79
80     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
81      AU.setPreservesCFG();
82    }
83 };
84}
85
86char DCE::ID = 0;
87INITIALIZE_PASS(DCE, "dce", "Dead Code Elimination", false, false)
88
89bool DCE::runOnFunction(Function &F) {
90  // Start out with all of the instructions in the worklist...
91  std::vector<Instruction*> WorkList;
92  for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i)
93    WorkList.push_back(&*i);
94
95  // Loop over the worklist finding instructions that are dead.  If they are
96  // dead make them drop all of their uses, making other instructions
97  // potentially dead, and work until the worklist is empty.
98  //
99  bool MadeChange = false;
100  while (!WorkList.empty()) {
101    Instruction *I = WorkList.back();
102    WorkList.pop_back();
103
104    if (isInstructionTriviallyDead(I)) {       // If the instruction is dead.
105      // Loop over all of the values that the instruction uses, if there are
106      // instructions being used, add them to the worklist, because they might
107      // go dead after this one is removed.
108      //
109      for (User::op_iterator OI = I->op_begin(), E = I->op_end(); OI != E; ++OI)
110        if (Instruction *Used = dyn_cast<Instruction>(*OI))
111          WorkList.push_back(Used);
112
113      // Remove the instruction.
114      I->eraseFromParent();
115
116      // Remove the instruction from the worklist if it still exists in it.
117      for (std::vector<Instruction*>::iterator WI = WorkList.begin();
118           WI != WorkList.end(); ) {
119        if (*WI == I)
120          WI = WorkList.erase(WI);
121        else
122          ++WI;
123      }
124
125      MadeChange = true;
126      ++DCEEliminated;
127    }
128  }
129  return MadeChange;
130}
131
132FunctionPass *llvm::createDeadCodeEliminationPass() {
133  return new DCE();
134}
135
136