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#include "llvm/Transforms/Scalar.h"
20#include "llvm/ADT/Statistic.h"
21#include "llvm/IR/InstIterator.h"
22#include "llvm/IR/Instruction.h"
23#include "llvm/Pass.h"
24#include "llvm/Target/TargetLibraryInfo.h"
25#include "llvm/Transforms/Utils/Local.h"
26using namespace llvm;
27
28#define DEBUG_TYPE "dce"
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 DeadInstElimination : public BasicBlockPass {
38    static char ID; // Pass identification, replacement for typeid
39    DeadInstElimination() : BasicBlockPass(ID) {
40      initializeDeadInstEliminationPass(*PassRegistry::getPassRegistry());
41    }
42    bool runOnBasicBlock(BasicBlock &BB) override {
43      if (skipOptnoneFunction(BB))
44        return false;
45      TargetLibraryInfo *TLI = getAnalysisIfAvailable<TargetLibraryInfo>();
46      bool Changed = false;
47      for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) {
48        Instruction *Inst = DI++;
49        if (isInstructionTriviallyDead(Inst, TLI)) {
50          Inst->eraseFromParent();
51          Changed = true;
52          ++DIEEliminated;
53        }
54      }
55      return Changed;
56    }
57
58    void getAnalysisUsage(AnalysisUsage &AU) const override {
59      AU.setPreservesCFG();
60    }
61  };
62}
63
64char DeadInstElimination::ID = 0;
65INITIALIZE_PASS(DeadInstElimination, "die",
66                "Dead Instruction Elimination", false, false)
67
68Pass *llvm::createDeadInstEliminationPass() {
69  return new DeadInstElimination();
70}
71
72
73namespace {
74  //===--------------------------------------------------------------------===//
75  // DeadCodeElimination pass implementation
76  //
77  struct DCE : public FunctionPass {
78    static char ID; // Pass identification, replacement for typeid
79    DCE() : FunctionPass(ID) {
80      initializeDCEPass(*PassRegistry::getPassRegistry());
81    }
82
83    bool runOnFunction(Function &F) override;
84
85    void getAnalysisUsage(AnalysisUsage &AU) const override {
86      AU.setPreservesCFG();
87    }
88 };
89}
90
91char DCE::ID = 0;
92INITIALIZE_PASS(DCE, "dce", "Dead Code Elimination", false, false)
93
94bool DCE::runOnFunction(Function &F) {
95  if (skipOptnoneFunction(F))
96    return false;
97
98  TargetLibraryInfo *TLI = getAnalysisIfAvailable<TargetLibraryInfo>();
99
100  // Start out with all of the instructions in the worklist...
101  std::vector<Instruction*> WorkList;
102  for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i)
103    WorkList.push_back(&*i);
104
105  // Loop over the worklist finding instructions that are dead.  If they are
106  // dead make them drop all of their uses, making other instructions
107  // potentially dead, and work until the worklist is empty.
108  //
109  bool MadeChange = false;
110  while (!WorkList.empty()) {
111    Instruction *I = WorkList.back();
112    WorkList.pop_back();
113
114    if (isInstructionTriviallyDead(I, TLI)) { // If the instruction is dead.
115      // Loop over all of the values that the instruction uses, if there are
116      // instructions being used, add them to the worklist, because they might
117      // go dead after this one is removed.
118      //
119      for (User::op_iterator OI = I->op_begin(), E = I->op_end(); OI != E; ++OI)
120        if (Instruction *Used = dyn_cast<Instruction>(*OI))
121          WorkList.push_back(Used);
122
123      // Remove the instruction.
124      I->eraseFromParent();
125
126      // Remove the instruction from the worklist if it still exists in it.
127      WorkList.erase(std::remove(WorkList.begin(), WorkList.end(), I),
128                     WorkList.end());
129
130      MadeChange = true;
131      ++DCEEliminated;
132    }
133  }
134  return MadeChange;
135}
136
137FunctionPass *llvm::createDeadCodeEliminationPass() {
138  return new DCE();
139}
140
141