1 //===- DCE.cpp - Code to perform dead code elimination --------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source 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/Transforms/Utils/Local.h"
21 #include "llvm/Instruction.h"
22 #include "llvm/Pass.h"
23 #include "llvm/Support/InstIterator.h"
24 #include "Support/Statistic.h"
29 Statistic<> DIEEliminated("die", "Number of insts removed");
30 Statistic<> DCEEliminated("dce", "Number of insts removed");
32 //===--------------------------------------------------------------------===//
33 // DeadInstElimination pass implementation
36 struct DeadInstElimination : public BasicBlockPass {
37 virtual bool runOnBasicBlock(BasicBlock &BB) {
39 for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); )
40 if (dceInstruction(DI)) {
48 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
53 RegisterOpt<DeadInstElimination> X("die", "Dead Instruction Elimination");
56 Pass *llvm::createDeadInstEliminationPass() {
57 return new DeadInstElimination();
61 //===----------------------------------------------------------------------===//
62 // DeadCodeElimination pass implementation
66 struct DCE : public FunctionPass {
67 virtual bool runOnFunction(Function &F);
69 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
74 RegisterOpt<DCE> Y("dce", "Dead Code Elimination");
77 bool DCE::runOnFunction(Function &F) {
78 // Start out with all of the instructions in the worklist...
79 std::vector<Instruction*> WorkList;
80 for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) {
81 WorkList.push_back(&*i);
83 std::set<Instruction*> DeadInsts;
85 // Loop over the worklist finding instructions that are dead. If they are
86 // dead make them drop all of their uses, making other instructions
87 // potentially dead, and work until the worklist is empty.
89 while (!WorkList.empty()) {
90 Instruction *I = WorkList.back();
93 if (isInstructionTriviallyDead(I)) { // If the instruction is dead...
94 // Loop over all of the values that the instruction uses, if there are
95 // instructions being used, add them to the worklist, because they might
96 // go dead after this one is removed.
98 for (User::op_iterator OI = I->op_begin(), E = I->op_end(); OI != E; ++OI)
99 if (Instruction *Used = dyn_cast<Instruction>(*OI))
100 WorkList.push_back(Used);
102 // Tell the instruction to let go of all of the values it uses...
103 I->dropAllReferences();
105 // Keep track of this instruction, because we are going to delete it later
110 // If we found no dead instructions, we haven't changed the function...
111 if (DeadInsts.empty()) return false;
113 // Otherwise, loop over the program, removing and deleting the instructions...
114 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
115 for (BasicBlock::iterator BI = I->begin(); BI != I->end(); )
116 if (DeadInsts.count(BI)) { // Is this instruction dead?
117 BI = I->getInstList().erase(BI); // Yup, remove and delete inst
119 } else { // This instruction is not dead
120 ++BI; // Continue on to the next one...
126 Pass *llvm::createDeadCodeEliminationPass() {