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 #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 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 RegisterPass<DeadInstElimination> X("die", "Dead Instruction Elimination");
56 Pass *llvm::createDeadInstEliminationPass() {
57 return new DeadInstElimination();
62 //===--------------------------------------------------------------------===//
63 // DeadCodeElimination pass implementation
65 struct DCE : public FunctionPass {
66 virtual bool runOnFunction(Function &F);
68 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
73 RegisterPass<DCE> Y("dce", "Dead Code Elimination");
76 bool DCE::runOnFunction(Function &F) {
77 // Start out with all of the instructions in the worklist...
78 std::vector<Instruction*> WorkList;
79 for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i)
80 WorkList.push_back(&*i);
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 bool MadeChange = false;
87 while (!WorkList.empty()) {
88 Instruction *I = WorkList.back();
91 if (isInstructionTriviallyDead(I)) { // If the instruction is dead.
92 // Loop over all of the values that the instruction uses, if there are
93 // instructions being used, add them to the worklist, because they might
94 // go dead after this one is removed.
96 for (User::op_iterator OI = I->op_begin(), E = I->op_end(); OI != E; ++OI)
97 if (Instruction *Used = dyn_cast<Instruction>(*OI))
98 WorkList.push_back(Used);
100 // Remove the instruction.
101 I->eraseFromParent();
103 // Remove the instruction from the worklist if it still exists in it.
104 for (std::vector<Instruction*>::iterator WI = WorkList.begin(),
105 E = WorkList.end(); WI != E; ++WI)
119 FunctionPass *llvm::createDeadCodeEliminationPass() {