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"
28 Statistic<> DIEEliminated("die", "Number of insts removed");
29 Statistic<> DCEEliminated("dce", "Number of insts removed");
31 //===--------------------------------------------------------------------===//
32 // DeadInstElimination pass implementation
35 struct DeadInstElimination : public BasicBlockPass {
36 virtual bool runOnBasicBlock(BasicBlock &BB) {
38 for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); )
39 if (dceInstruction(DI)) {
47 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
52 RegisterOpt<DeadInstElimination> X("die", "Dead Instruction Elimination");
55 Pass *createDeadInstEliminationPass() {
56 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(inst_begin(F), inst_end(F));
80 std::set<Instruction*> DeadInsts;
82 // Loop over the worklist finding instructions that are dead. If they are
83 // dead make them drop all of their uses, making other instructions
84 // potentially dead, and work until the worklist is empty.
86 while (!WorkList.empty()) {
87 Instruction *I = WorkList.back();
90 if (isInstructionTriviallyDead(I)) { // If the instruction is dead...
91 // Loop over all of the values that the instruction uses, if there are
92 // instructions being used, add them to the worklist, because they might
93 // go dead after this one is removed.
95 for (User::use_iterator UI = I->use_begin(), UE = I->use_end();
97 if (Instruction *Used = dyn_cast<Instruction>(*UI))
98 WorkList.push_back(Used);
100 // Tell the instruction to let go of all of the values it uses...
101 I->dropAllReferences();
103 // Keep track of this instruction, because we are going to delete it later
108 // If we found no dead instructions, we haven't changed the function...
109 if (DeadInsts.empty()) return false;
111 // Otherwise, loop over the program, removing and deleting the instructions...
112 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
113 for (BasicBlock::iterator BI = I->begin(); BI != I->end(); )
114 if (DeadInsts.count(BI)) { // Is this instruction dead?
115 BI = I->getInstList().erase(BI); // Yup, remove and delete inst
117 } else { // This instruction is not dead
118 ++BI; // Continue on to the next one...
124 Pass *createDeadCodeEliminationPass() {