DCE.cpp revision 36b56886974eae4f9c5ebc96befd3e7bfe5de338 
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/ADT/Statistic.h"
22#include "llvm/IR/InstIterator.h"
23#include "llvm/IR/Instruction.h"
24#include "llvm/Pass.h"
25#include "llvm/Target/TargetLibraryInfo.h"
26#include "llvm/Transforms/Utils/Local.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    bool runOnBasicBlock(BasicBlock &BB) override {
42      if (skipOptnoneFunction(BB))
43        return false;
44      TargetLibraryInfo *TLI = getAnalysisIfAvailable<TargetLibraryInfo>();
45      bool Changed = false;
46      for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) {
47        Instruction *Inst = DI++;
48        if (isInstructionTriviallyDead(Inst, TLI)) {
49          Inst->eraseFromParent();
50          Changed = true;
51          ++DIEEliminated;
52        }
53      }
54      return Changed;
55    }
56
57    void getAnalysisUsage(AnalysisUsage &AU) const override {
58      AU.setPreservesCFG();
59    }
60  };
61}
62
63char DeadInstElimination::ID = 0;
64INITIALIZE_PASS(DeadInstElimination, "die",
65                "Dead Instruction Elimination", false, false)
66
67Pass *llvm::createDeadInstEliminationPass() {
68  return new DeadInstElimination();
69}
70
71
72namespace {
73  //===--------------------------------------------------------------------===//
74  // DeadCodeElimination pass implementation
75  //
76  struct DCE : public FunctionPass {
77    static char ID; // Pass identification, replacement for typeid
78    DCE() : FunctionPass(ID) {
79      initializeDCEPass(*PassRegistry::getPassRegistry());
80    }
81
82    bool runOnFunction(Function &F) override;
83
84    void getAnalysisUsage(AnalysisUsage &AU) const override {
85      AU.setPreservesCFG();
86    }
87 };
88}
89
90char DCE::ID = 0;
91INITIALIZE_PASS(DCE, "dce", "Dead Code Elimination", false, false)
92
93bool DCE::runOnFunction(Function &F) {
94  if (skipOptnoneFunction(F))
95    return false;
96
97  TargetLibraryInfo *TLI = getAnalysisIfAvailable<TargetLibraryInfo>();
98
99  // Start out with all of the instructions in the worklist...
100  std::vector<Instruction*> WorkList;
101  for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i)
102    WorkList.push_back(&*i);
103
104  // Loop over the worklist finding instructions that are dead.  If they are
105  // dead make them drop all of their uses, making other instructions
106  // potentially dead, and work until the worklist is empty.
107  //
108  bool MadeChange = false;
109  while (!WorkList.empty()) {
110    Instruction *I = WorkList.back();
111    WorkList.pop_back();
112
113    if (isInstructionTriviallyDead(I, TLI)) { // If the instruction is dead.
114      // Loop over all of the values that the instruction uses, if there are
115      // instructions being used, add them to the worklist, because they might
116      // go dead after this one is removed.
117      //
118      for (User::op_iterator OI = I->op_begin(), E = I->op_end(); OI != E; ++OI)
119        if (Instruction *Used = dyn_cast<Instruction>(*OI))
120          WorkList.push_back(Used);
121
122      // Remove the instruction.
123      I->eraseFromParent();
124
125      // Remove the instruction from the worklist if it still exists in it.
126      WorkList.erase(std::remove(WorkList.begin(), WorkList.end(), I),
127                     WorkList.end());
128
129      MadeChange = true;
130      ++DCEEliminated;
131    }
132  }
133  return MadeChange;
134}
135
136FunctionPass *llvm::createDeadCodeEliminationPass() {
137  return new DCE();
138}
139
140