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"
26 using namespace llvm;
27
28 #define DEBUG_TYPE "dce"
29
30 STATISTIC(DIEEliminated, "Number of insts removed by DIE pass");
31 STATISTIC(DCEEliminated, "Number of insts removed");
32
33 namespace {
34 //===--------------------------------------------------------------------===//
35 // DeadInstElimination pass implementation
36 //
37 struct DeadInstElimination : public BasicBlockPass {
38 static char ID; // Pass identification, replacement for typeid
DeadInstElimination__anon487e53010111::DeadInstElimination39 DeadInstElimination() : BasicBlockPass(ID) {
40 initializeDeadInstEliminationPass(*PassRegistry::getPassRegistry());
41 }
runOnBasicBlock__anon487e53010111::DeadInstElimination42 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
getAnalysisUsage__anon487e53010111::DeadInstElimination59 void getAnalysisUsage(AnalysisUsage &AU) const override {
60 AU.setPreservesCFG();
61 }
62 };
63 }
64
65 char DeadInstElimination::ID = 0;
66 INITIALIZE_PASS(DeadInstElimination, "die",
67 "Dead Instruction Elimination", false, false)
68
createDeadInstEliminationPass()69 Pass *llvm::createDeadInstEliminationPass() {
70 return new DeadInstElimination();
71 }
72
73
74 namespace {
75 //===--------------------------------------------------------------------===//
76 // DeadCodeElimination pass implementation
77 //
78 struct DCE : public FunctionPass {
79 static char ID; // Pass identification, replacement for typeid
DCE__anon487e53010211::DCE80 DCE() : FunctionPass(ID) {
81 initializeDCEPass(*PassRegistry::getPassRegistry());
82 }
83
84 bool runOnFunction(Function &F) override;
85
getAnalysisUsage__anon487e53010211::DCE86 void getAnalysisUsage(AnalysisUsage &AU) const override {
87 AU.setPreservesCFG();
88 }
89 };
90 }
91
92 char DCE::ID = 0;
93 INITIALIZE_PASS(DCE, "dce", "Dead Code Elimination", false, false)
94
runOnFunction(Function & F)95 bool 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
createDeadCodeEliminationPass()139 FunctionPass *llvm::createDeadCodeEliminationPass() {
140 return new DCE();
141 }
142
143