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 using namespace llvm;
27
28 STATISTIC(DIEEliminated, "Number of insts removed by DIE pass");
29 STATISTIC(DCEEliminated, "Number of insts removed");
30
31 namespace {
32 //===--------------------------------------------------------------------===//
33 // DeadInstElimination pass implementation
34 //
35 struct DeadInstElimination : public BasicBlockPass {
36 static char ID; // Pass identification, replacement for typeid
DeadInstElimination__anon9a06afac0111::DeadInstElimination37 DeadInstElimination() : BasicBlockPass(ID) {
38 initializeDeadInstEliminationPass(*PassRegistry::getPassRegistry());
39 }
runOnBasicBlock__anon9a06afac0111::DeadInstElimination40 virtual bool runOnBasicBlock(BasicBlock &BB) {
41 bool Changed = false;
42 for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) {
43 Instruction *Inst = DI++;
44 if (isInstructionTriviallyDead(Inst)) {
45 Inst->eraseFromParent();
46 Changed = true;
47 ++DIEEliminated;
48 }
49 }
50 return Changed;
51 }
52
getAnalysisUsage__anon9a06afac0111::DeadInstElimination53 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
54 AU.setPreservesCFG();
55 }
56 };
57 }
58
59 char DeadInstElimination::ID = 0;
60 INITIALIZE_PASS(DeadInstElimination, "die",
61 "Dead Instruction Elimination", false, false)
62
createDeadInstEliminationPass()63 Pass *llvm::createDeadInstEliminationPass() {
64 return new DeadInstElimination();
65 }
66
67
68 namespace {
69 //===--------------------------------------------------------------------===//
70 // DeadCodeElimination pass implementation
71 //
72 struct DCE : public FunctionPass {
73 static char ID; // Pass identification, replacement for typeid
DCE__anon9a06afac0211::DCE74 DCE() : FunctionPass(ID) {
75 initializeDCEPass(*PassRegistry::getPassRegistry());
76 }
77
78 virtual bool runOnFunction(Function &F);
79
getAnalysisUsage__anon9a06afac0211::DCE80 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
81 AU.setPreservesCFG();
82 }
83 };
84 }
85
86 char DCE::ID = 0;
87 INITIALIZE_PASS(DCE, "dce", "Dead Code Elimination", false, false)
88
runOnFunction(Function & F)89 bool DCE::runOnFunction(Function &F) {
90 // Start out with all of the instructions in the worklist...
91 std::vector<Instruction*> WorkList;
92 for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i)
93 WorkList.push_back(&*i);
94
95 // Loop over the worklist finding instructions that are dead. If they are
96 // dead make them drop all of their uses, making other instructions
97 // potentially dead, and work until the worklist is empty.
98 //
99 bool MadeChange = false;
100 while (!WorkList.empty()) {
101 Instruction *I = WorkList.back();
102 WorkList.pop_back();
103
104 if (isInstructionTriviallyDead(I)) { // If the instruction is dead.
105 // Loop over all of the values that the instruction uses, if there are
106 // instructions being used, add them to the worklist, because they might
107 // go dead after this one is removed.
108 //
109 for (User::op_iterator OI = I->op_begin(), E = I->op_end(); OI != E; ++OI)
110 if (Instruction *Used = dyn_cast<Instruction>(*OI))
111 WorkList.push_back(Used);
112
113 // Remove the instruction.
114 I->eraseFromParent();
115
116 // Remove the instruction from the worklist if it still exists in it.
117 for (std::vector<Instruction*>::iterator WI = WorkList.begin();
118 WI != WorkList.end(); ) {
119 if (*WI == I)
120 WI = WorkList.erase(WI);
121 else
122 ++WI;
123 }
124
125 MadeChange = true;
126 ++DCEEliminated;
127 }
128 }
129 return MadeChange;
130 }
131
createDeadCodeEliminationPass()132 FunctionPass *llvm::createDeadCodeEliminationPass() {
133 return new DCE();
134 }
135
136