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/DCE.h"
20 #include "llvm/ADT/SetVector.h"
21 #include "llvm/ADT/Statistic.h"
22 #include "llvm/IR/InstIterator.h"
23 #include "llvm/IR/Instruction.h"
24 #include "llvm/Pass.h"
25 #include "llvm/Analysis/TargetLibraryInfo.h"
26 #include "llvm/Transforms/Scalar.h"
27 #include "llvm/Transforms/Utils/Local.h"
28 using namespace llvm;
29
30 #define DEBUG_TYPE "dce"
31
32 STATISTIC(DIEEliminated, "Number of insts removed by DIE pass");
33 STATISTIC(DCEEliminated, "Number of insts removed");
34
35 namespace {
36 //===--------------------------------------------------------------------===//
37 // DeadInstElimination pass implementation
38 //
39 struct DeadInstElimination : public BasicBlockPass {
40 static char ID; // Pass identification, replacement for typeid
DeadInstElimination__anon79d7f90a0111::DeadInstElimination41 DeadInstElimination() : BasicBlockPass(ID) {
42 initializeDeadInstEliminationPass(*PassRegistry::getPassRegistry());
43 }
runOnBasicBlock__anon79d7f90a0111::DeadInstElimination44 bool runOnBasicBlock(BasicBlock &BB) override {
45 if (skipBasicBlock(BB))
46 return false;
47 auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
48 TargetLibraryInfo *TLI = TLIP ? &TLIP->getTLI() : nullptr;
49 bool Changed = false;
50 for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) {
51 Instruction *Inst = &*DI++;
52 if (isInstructionTriviallyDead(Inst, TLI)) {
53 Inst->eraseFromParent();
54 Changed = true;
55 ++DIEEliminated;
56 }
57 }
58 return Changed;
59 }
60
getAnalysisUsage__anon79d7f90a0111::DeadInstElimination61 void getAnalysisUsage(AnalysisUsage &AU) const override {
62 AU.setPreservesCFG();
63 }
64 };
65 }
66
67 char DeadInstElimination::ID = 0;
68 INITIALIZE_PASS(DeadInstElimination, "die",
69 "Dead Instruction Elimination", false, false)
70
createDeadInstEliminationPass()71 Pass *llvm::createDeadInstEliminationPass() {
72 return new DeadInstElimination();
73 }
74
DCEInstruction(Instruction * I,SmallSetVector<Instruction *,16> & WorkList,const TargetLibraryInfo * TLI)75 static bool DCEInstruction(Instruction *I,
76 SmallSetVector<Instruction *, 16> &WorkList,
77 const TargetLibraryInfo *TLI) {
78 if (isInstructionTriviallyDead(I, TLI)) {
79 // Null out all of the instruction's operands to see if any operand becomes
80 // dead as we go.
81 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
82 Value *OpV = I->getOperand(i);
83 I->setOperand(i, nullptr);
84
85 if (!OpV->use_empty() || I == OpV)
86 continue;
87
88 // If the operand is an instruction that became dead as we nulled out the
89 // operand, and if it is 'trivially' dead, delete it in a future loop
90 // iteration.
91 if (Instruction *OpI = dyn_cast<Instruction>(OpV))
92 if (isInstructionTriviallyDead(OpI, TLI))
93 WorkList.insert(OpI);
94 }
95
96 I->eraseFromParent();
97 ++DCEEliminated;
98 return true;
99 }
100 return false;
101 }
102
eliminateDeadCode(Function & F,TargetLibraryInfo * TLI)103 static bool eliminateDeadCode(Function &F, TargetLibraryInfo *TLI) {
104 bool MadeChange = false;
105 SmallSetVector<Instruction *, 16> WorkList;
106 // Iterate over the original function, only adding insts to the worklist
107 // if they actually need to be revisited. This avoids having to pre-init
108 // the worklist with the entire function's worth of instructions.
109 for (inst_iterator FI = inst_begin(F), FE = inst_end(F); FI != FE;) {
110 Instruction *I = &*FI;
111 ++FI;
112
113 // We're visiting this instruction now, so make sure it's not in the
114 // worklist from an earlier visit.
115 if (!WorkList.count(I))
116 MadeChange |= DCEInstruction(I, WorkList, TLI);
117 }
118
119 while (!WorkList.empty()) {
120 Instruction *I = WorkList.pop_back_val();
121 MadeChange |= DCEInstruction(I, WorkList, TLI);
122 }
123 return MadeChange;
124 }
125
run(Function & F,AnalysisManager<Function> & AM)126 PreservedAnalyses DCEPass::run(Function &F, AnalysisManager<Function> &AM) {
127 if (eliminateDeadCode(F, AM.getCachedResult<TargetLibraryAnalysis>(F)))
128 return PreservedAnalyses::none();
129 return PreservedAnalyses::all();
130 }
131
132 namespace {
133 struct DCELegacyPass : public FunctionPass {
134 static char ID; // Pass identification, replacement for typeid
DCELegacyPass__anon79d7f90a0211::DCELegacyPass135 DCELegacyPass() : FunctionPass(ID) {
136 initializeDCELegacyPassPass(*PassRegistry::getPassRegistry());
137 }
138
runOnFunction__anon79d7f90a0211::DCELegacyPass139 bool runOnFunction(Function &F) override {
140 if (skipFunction(F))
141 return false;
142
143 auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
144 TargetLibraryInfo *TLI = TLIP ? &TLIP->getTLI() : nullptr;
145
146 return eliminateDeadCode(F, TLI);
147 }
148
getAnalysisUsage__anon79d7f90a0211::DCELegacyPass149 void getAnalysisUsage(AnalysisUsage &AU) const override {
150 AU.setPreservesCFG();
151 }
152 };
153 }
154
155 char DCELegacyPass::ID = 0;
156 INITIALIZE_PASS(DCELegacyPass, "dce", "Dead Code Elimination", false, false)
157
createDeadCodeEliminationPass()158 FunctionPass *llvm::createDeadCodeEliminationPass() {
159 return new DCELegacyPass();
160 }
161