DCE.cpp revision 761fbec9c2ace70c12cf41f5d0e97c42a303eeb9 
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/Target/TargetLibraryInfo.h"
26#include "llvm/ADT/Statistic.h"
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      initializeDeadInstEliminationPass(*PassRegistry::getPassRegistry());
40    }
41    virtual bool runOnBasicBlock(BasicBlock &BB) {
42      TargetLibraryInfo *TLI = getAnalysisIfAvailable<TargetLibraryInfo>();
43      bool Changed = false;
44      for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) {
45        Instruction *Inst = DI++;
46        if (isInstructionTriviallyDead(Inst, TLI)) {
47          Inst->eraseFromParent();
48          Changed = true;
49          ++DIEEliminated;
50        }
51      }
52      return Changed;
53    }
54
55    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
56      AU.setPreservesCFG();
57    }
58  };
59}
60
61char DeadInstElimination::ID = 0;
62INITIALIZE_PASS(DeadInstElimination, "die",
63                "Dead Instruction Elimination", false, false)
64
65Pass *llvm::createDeadInstEliminationPass() {
66  return new DeadInstElimination();
67}
68
69
70namespace {
71  //===--------------------------------------------------------------------===//
72  // DeadCodeElimination pass implementation
73  //
74  struct DCE : public FunctionPass {
75    static char ID; // Pass identification, replacement for typeid
76    DCE() : FunctionPass(ID) {
77      initializeDCEPass(*PassRegistry::getPassRegistry());
78    }
79
80    virtual bool runOnFunction(Function &F);
81
82     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
83      AU.setPreservesCFG();
84    }
85 };
86}
87
88char DCE::ID = 0;
89INITIALIZE_PASS(DCE, "dce", "Dead Code Elimination", false, false)
90
91bool DCE::runOnFunction(Function &F) {
92  TargetLibraryInfo *TLI = getAnalysisIfAvailable<TargetLibraryInfo>();
93
94  // Start out with all of the instructions in the worklist...
95  std::vector<Instruction*> WorkList;
96  for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i)
97    WorkList.push_back(&*i);
98
99  // Loop over the worklist finding instructions that are dead.  If they are
100  // dead make them drop all of their uses, making other instructions
101  // potentially dead, and work until the worklist is empty.
102  //
103  bool MadeChange = false;
104  while (!WorkList.empty()) {
105    Instruction *I = WorkList.back();
106    WorkList.pop_back();
107
108    if (isInstructionTriviallyDead(I, TLI)) { // If the instruction is dead.
109      // Loop over all of the values that the instruction uses, if there are
110      // instructions being used, add them to the worklist, because they might
111      // go dead after this one is removed.
112      //
113      for (User::op_iterator OI = I->op_begin(), E = I->op_end(); OI != E; ++OI)
114        if (Instruction *Used = dyn_cast<Instruction>(*OI))
115          WorkList.push_back(Used);
116
117      // Remove the instruction.
118      I->eraseFromParent();
119
120      // Remove the instruction from the worklist if it still exists in it.
121      WorkList.erase(std::remove(WorkList.begin(), WorkList.end(), I), WorkList.end());
122
123      MadeChange = true;
124      ++DCEEliminated;
125    }
126  }
127  return MadeChange;
128}
129
130FunctionPass *llvm::createDeadCodeEliminationPass() {
131  return new DCE();
132}
133
134