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/Instruction.h"
23 #include "llvm/Pass.h"
24 #include "llvm/Support/InstIterator.h"
25 #include "llvm/Target/TargetLibraryInfo.h"
26 #include "llvm/Transforms/Utils/Local.h"
27 using namespace llvm;
28
29 STATISTIC(DIEEliminated, "Number of insts removed by DIE pass");
30 STATISTIC(DCEEliminated, "Number of insts removed");
31
32 namespace {
33 //===--------------------------------------------------------------------===//
34 // DeadInstElimination pass implementation
35 //
36 struct DeadInstElimination : public BasicBlockPass {
37 static char ID; // Pass identification, replacement for typeid
DeadInstElimination__anon066bc06b0111::DeadInstElimination38 DeadInstElimination() : BasicBlockPass(ID) {
39 initializeDeadInstEliminationPass(*PassRegistry::getPassRegistry());
40 }
runOnBasicBlock__anon066bc06b0111::DeadInstElimination41 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
getAnalysisUsage__anon066bc06b0111::DeadInstElimination55 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
56 AU.setPreservesCFG();
57 }
58 };
59 }
60
61 char DeadInstElimination::ID = 0;
62 INITIALIZE_PASS(DeadInstElimination, "die",
63 "Dead Instruction Elimination", false, false)
64
createDeadInstEliminationPass()65 Pass *llvm::createDeadInstEliminationPass() {
66 return new DeadInstElimination();
67 }
68
69
70 namespace {
71 //===--------------------------------------------------------------------===//
72 // DeadCodeElimination pass implementation
73 //
74 struct DCE : public FunctionPass {
75 static char ID; // Pass identification, replacement for typeid
DCE__anon066bc06b0211::DCE76 DCE() : FunctionPass(ID) {
77 initializeDCEPass(*PassRegistry::getPassRegistry());
78 }
79
80 virtual bool runOnFunction(Function &F);
81
getAnalysisUsage__anon066bc06b0211::DCE82 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
83 AU.setPreservesCFG();
84 }
85 };
86 }
87
88 char DCE::ID = 0;
89 INITIALIZE_PASS(DCE, "dce", "Dead Code Elimination", false, false)
90
runOnFunction(Function & F)91 bool 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),
122 WorkList.end());
123
124 MadeChange = true;
125 ++DCEEliminated;
126 }
127 }
128 return MadeChange;
129 }
130
createDeadCodeEliminationPass()131 FunctionPass *llvm::createDeadCodeEliminationPass() {
132 return new DCE();
133 }
134
135