1 //===- DCE.cpp - Code to perform dead code elimination --------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements dead inst elimination and dead code elimination.
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.
17 //===----------------------------------------------------------------------===//
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"
28 #define DEBUG_TYPE "dce"
30 STATISTIC(DIEEliminated, "Number of insts removed by DIE pass");
31 STATISTIC(DCEEliminated, "Number of insts removed");
34 //===--------------------------------------------------------------------===//
35 // DeadInstElimination pass implementation
37 struct DeadInstElimination : public BasicBlockPass {
38 static char ID; // Pass identification, replacement for typeid
39 DeadInstElimination() : BasicBlockPass(ID) {
40 initializeDeadInstEliminationPass(*PassRegistry::getPassRegistry());
42 bool runOnBasicBlock(BasicBlock &BB) override {
43 if (skipOptnoneFunction(BB))
45 auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
46 TargetLibraryInfo *TLI = TLIP ? &TLIP->getTLI() : nullptr;
48 for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) {
49 Instruction *Inst = DI++;
50 if (isInstructionTriviallyDead(Inst, TLI)) {
51 Inst->eraseFromParent();
59 void getAnalysisUsage(AnalysisUsage &AU) const override {
65 char DeadInstElimination::ID = 0;
66 INITIALIZE_PASS(DeadInstElimination, "die",
67 "Dead Instruction Elimination", false, false)
69 Pass *llvm::createDeadInstEliminationPass() {
70 return new DeadInstElimination();
75 //===--------------------------------------------------------------------===//
76 // DeadCodeElimination pass implementation
78 struct DCE : public FunctionPass {
79 static char ID; // Pass identification, replacement for typeid
80 DCE() : FunctionPass(ID) {
81 initializeDCEPass(*PassRegistry::getPassRegistry());
84 bool runOnFunction(Function &F) override;
86 void getAnalysisUsage(AnalysisUsage &AU) const override {
93 INITIALIZE_PASS(DCE, "dce", "Dead Code Elimination", false, false)
95 bool DCE::runOnFunction(Function &F) {
96 if (skipOptnoneFunction(F))
99 auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
100 TargetLibraryInfo *TLI = TLIP ? &TLIP->getTLI() : nullptr;
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);
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.
111 bool MadeChange = false;
112 while (!WorkList.empty()) {
113 Instruction *I = WorkList.back();
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.
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);
125 // Remove the instruction.
126 I->eraseFromParent();
128 // Remove the instruction from the worklist if it still exists in it.
129 WorkList.erase(std::remove(WorkList.begin(), WorkList.end(), I),
139 FunctionPass *llvm::createDeadCodeEliminationPass() {