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//===- DCE.cpp - Code to perform dead code elimination --------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements dead inst elimination and dead code elimination.
//
// Dead Inst Elimination performs a single pass over the function removing
// instructions that are obviously dead. Dead Code Elimination is similar, but
// it rechecks instructions that were used by removed instructions to see if
// they are newly dead.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "dce"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Instruction.h"
#include "llvm/Pass.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/InstIterator.h"
#include "llvm/ADT/Statistic.h"
#include <set>
using namespace llvm;
STATISTIC(DIEEliminated, "Number of insts removed by DIE pass");
STATISTIC(DCEEliminated, "Number of insts removed");
namespace {
//===--------------------------------------------------------------------===//
// DeadInstElimination pass implementation
//
struct VISIBILITY_HIDDEN DeadInstElimination : public BasicBlockPass {
static char ID; // Pass identification, replacement for typeid
DeadInstElimination() : BasicBlockPass(&ID) {}
virtual bool runOnBasicBlock(BasicBlock &BB) {
bool Changed = false;
for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) {
Instruction *Inst = DI++;
if (isInstructionTriviallyDead(Inst)) {
Inst->eraseFromParent();
Changed = true;
++DIEEliminated;
}
}
return Changed;
}
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
}
};
}
char DeadInstElimination::ID = 0;
static RegisterPass<DeadInstElimination>
X("die", "Dead Instruction Elimination");
Pass *llvm::createDeadInstEliminationPass() {
return new DeadInstElimination();
}
namespace {
//===--------------------------------------------------------------------===//
// DeadCodeElimination pass implementation
//
struct DCE : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
DCE() : FunctionPass(&ID) {}
virtual bool runOnFunction(Function &F);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
}
};
}
char DCE::ID = 0;
static RegisterPass<DCE> Y("dce", "Dead Code Elimination");
bool DCE::runOnFunction(Function &F) {
// Start out with all of the instructions in the worklist...
std::vector<Instruction*> WorkList;
for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i)
WorkList.push_back(&*i);
// Loop over the worklist finding instructions that are dead. If they are
// dead make them drop all of their uses, making other instructions
// potentially dead, and work until the worklist is empty.
//
bool MadeChange = false;
while (!WorkList.empty()) {
Instruction *I = WorkList.back();
WorkList.pop_back();
if (isInstructionTriviallyDead(I)) { // If the instruction is dead.
// Loop over all of the values that the instruction uses, if there are
// instructions being used, add them to the worklist, because they might
// go dead after this one is removed.
//
for (User::op_iterator OI = I->op_begin(), E = I->op_end(); OI != E; ++OI)
if (Instruction *Used = dyn_cast<Instruction>(*OI))
WorkList.push_back(Used);
// Remove the instruction.
I->eraseFromParent();
// Remove the instruction from the worklist if it still exists in it.
for (std::vector<Instruction*>::iterator WI = WorkList.begin();
WI != WorkList.end(); ) {
if (*WI == I)
WI = WorkList.erase(WI);
else
++WI;
}
MadeChange = true;
++DCEEliminated;
}
}
return MadeChange;
}
FunctionPass *llvm::createDeadCodeEliminationPass() {
return new DCE();
}
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