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 #define DEBUG_TYPE "dce"
20 #include "llvm/Transforms/Scalar.h"
21 #include "llvm/Transforms/Utils/Local.h"
22 #include "llvm/Instruction.h"
23 #include "llvm/Pass.h"
24 #include "llvm/Support/InstIterator.h"
25 #include "llvm/ADT/Statistic.h"
29 STATISTIC(DIEEliminated, "Number of insts removed by DIE pass");
30 STATISTIC(DCEEliminated, "Number of insts removed");
33 //===--------------------------------------------------------------------===//
34 // DeadInstElimination pass implementation
36 struct DeadInstElimination : public BasicBlockPass {
37 static char ID; // Pass identification, replacement for typeid
38 DeadInstElimination() : BasicBlockPass(ID) {}
39 virtual bool runOnBasicBlock(BasicBlock &BB) {
41 for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) {
42 Instruction *Inst = DI++;
43 if (isInstructionTriviallyDead(Inst)) {
44 Inst->eraseFromParent();
52 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
58 char DeadInstElimination::ID = 0;
59 INITIALIZE_PASS(DeadInstElimination, "die",
60 "Dead Instruction Elimination", false, false);
62 Pass *llvm::createDeadInstEliminationPass() {
63 return new DeadInstElimination();
68 //===--------------------------------------------------------------------===//
69 // DeadCodeElimination pass implementation
71 struct DCE : public FunctionPass {
72 static char ID; // Pass identification, replacement for typeid
73 DCE() : FunctionPass(ID) {}
75 virtual bool runOnFunction(Function &F);
77 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
84 INITIALIZE_PASS(DCE, "dce", "Dead Code Elimination", false, false);
86 bool DCE::runOnFunction(Function &F) {
87 // Start out with all of the instructions in the worklist...
88 std::vector<Instruction*> WorkList;
89 for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i)
90 WorkList.push_back(&*i);
92 // Loop over the worklist finding instructions that are dead. If they are
93 // dead make them drop all of their uses, making other instructions
94 // potentially dead, and work until the worklist is empty.
96 bool MadeChange = false;
97 while (!WorkList.empty()) {
98 Instruction *I = WorkList.back();
101 if (isInstructionTriviallyDead(I)) { // If the instruction is dead.
102 // Loop over all of the values that the instruction uses, if there are
103 // instructions being used, add them to the worklist, because they might
104 // go dead after this one is removed.
106 for (User::op_iterator OI = I->op_begin(), E = I->op_end(); OI != E; ++OI)
107 if (Instruction *Used = dyn_cast<Instruction>(*OI))
108 WorkList.push_back(Used);
110 // Remove the instruction.
111 I->eraseFromParent();
113 // Remove the instruction from the worklist if it still exists in it.
114 for (std::vector<Instruction*>::iterator WI = WorkList.begin();
115 WI != WorkList.end(); ) {
117 WI = WorkList.erase(WI);
129 FunctionPass *llvm::createDeadCodeEliminationPass() {