DCE.cpp revision 198090 
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>
27using namespace llvm;
28
29STATISTIC(DIEEliminated, "Number of insts removed by DIE pass");
30STATISTIC(DCEEliminated, "Number of insts removed");
31
32namespace {
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
58char DeadInstElimination::ID = 0;
59static RegisterPass<DeadInstElimination>
60X("die", "Dead Instruction Elimination");
61
62Pass *llvm::createDeadInstEliminationPass() {
63  return new DeadInstElimination();
64}
65
66
67namespace {
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
83char DCE::ID = 0;
84static RegisterPass<DCE> Y("dce", "Dead Code Elimination");
85
86bool 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
129FunctionPass *llvm::createDeadCodeEliminationPass() {
130  return new DCE();
131}
132
133