DCE.cpp revision 193323 
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>
28using namespace llvm;
29
30STATISTIC(DIEEliminated, "Number of insts removed by DIE pass");
31STATISTIC(DCEEliminated, "Number of insts removed");
32
33namespace {
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
59char DeadInstElimination::ID = 0;
60static RegisterPass<DeadInstElimination>
61X("die", "Dead Instruction Elimination");
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
76    virtual bool runOnFunction(Function &F);
77
78     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
79      AU.setPreservesCFG();
80    }
81 };
82}
83
84char DCE::ID = 0;
85static RegisterPass<DCE> Y("dce", "Dead Code Elimination");
86
87bool 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
130FunctionPass *llvm::createDeadCodeEliminationPass() {
131  return new DCE();
132}
133
134