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/Analysis/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      auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
46      TargetLibraryInfo *TLI = TLIP ? &TLIP->getTLI() : nullptr;
47      bool Changed = false;
48      for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) {
49        Instruction *Inst = DI++;
50        if (isInstructionTriviallyDead(Inst, TLI)) {
51          Inst->eraseFromParent();
52          Changed = true;
53          ++DIEEliminated;
54        }
55      }
56      return Changed;
57    }
58
59    void getAnalysisUsage(AnalysisUsage &AU) const override {
60      AU.setPreservesCFG();
61    }
62  };
63}
64
65char DeadInstElimination::ID = 0;
66INITIALIZE_PASS(DeadInstElimination, "die",
67                "Dead Instruction Elimination", false, false)
68
69Pass *llvm::createDeadInstEliminationPass() {
70  return new DeadInstElimination();
71}
72
73
74namespace {
75  //===--------------------------------------------------------------------===//
76  // DeadCodeElimination pass implementation
77  //
78  struct DCE : public FunctionPass {
79    static char ID; // Pass identification, replacement for typeid
80    DCE() : FunctionPass(ID) {
81      initializeDCEPass(*PassRegistry::getPassRegistry());
82    }
83
84    bool runOnFunction(Function &F) override;
85
86    void getAnalysisUsage(AnalysisUsage &AU) const override {
87      AU.setPreservesCFG();
88    }
89 };
90}
91
92char DCE::ID = 0;
93INITIALIZE_PASS(DCE, "dce", "Dead Code Elimination", false, false)
94
95bool DCE::runOnFunction(Function &F) {
96  if (skipOptnoneFunction(F))
97    return false;
98
99  auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
100  TargetLibraryInfo *TLI = TLIP ? &TLIP->getTLI() : nullptr;
101
102  // Start out with all of the instructions in the worklist...
103  std::vector<Instruction*> WorkList;
104  for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i)
105    WorkList.push_back(&*i);
106
107  // Loop over the worklist finding instructions that are dead.  If they are
108  // dead make them drop all of their uses, making other instructions
109  // potentially dead, and work until the worklist is empty.
110  //
111  bool MadeChange = false;
112  while (!WorkList.empty()) {
113    Instruction *I = WorkList.back();
114    WorkList.pop_back();
115
116    if (isInstructionTriviallyDead(I, TLI)) { // If the instruction is dead.
117      // Loop over all of the values that the instruction uses, if there are
118      // instructions being used, add them to the worklist, because they might
119      // go dead after this one is removed.
120      //
121      for (User::op_iterator OI = I->op_begin(), E = I->op_end(); OI != E; ++OI)
122        if (Instruction *Used = dyn_cast<Instruction>(*OI))
123          WorkList.push_back(Used);
124
125      // Remove the instruction.
126      I->eraseFromParent();
127
128      // Remove the instruction from the worklist if it still exists in it.
129      WorkList.erase(std::remove(WorkList.begin(), WorkList.end(), I),
130                     WorkList.end());
131
132      MadeChange = true;
133      ++DCEEliminated;
134    }
135  }
136  return MadeChange;
137}
138
139FunctionPass *llvm::createDeadCodeEliminationPass() {
140  return new DCE();
141}
142
143