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//===- DeadMachineInstructionElim.cpp - Remove dead machine instructions --===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This is an extremely simple MachineInstr-level dead-code-elimination pass.
//
//===----------------------------------------------------------------------===//
#include "llvm/CodeGen/Passes.h"
#include "llvm/Pass.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
using namespace llvm;
namespace {
class VISIBILITY_HIDDEN DeadMachineInstructionElim :
public MachineFunctionPass {
virtual bool runOnMachineFunction(MachineFunction &MF);
public:
static char ID; // Pass identification, replacement for typeid
DeadMachineInstructionElim() : MachineFunctionPass(&ID) {}
};
}
char DeadMachineInstructionElim::ID = 0;
static RegisterPass<DeadMachineInstructionElim>
Y("dead-mi-elimination",
"Remove dead machine instructions");
FunctionPass *llvm::createDeadMachineInstructionElimPass() {
return new DeadMachineInstructionElim();
}
bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) {
bool AnyChanges = false;
const MachineRegisterInfo &MRI = MF.getRegInfo();
const TargetInstrInfo &TII = *MF.getTarget().getInstrInfo();
bool SawStore = true;
// Loop over all instructions in all blocks, from bottom to top, so that it's
// more likely that chains of dependent but ultimately dead instructions will
// be cleaned up.
for (MachineFunction::reverse_iterator I = MF.rbegin(), E = MF.rend();
I != E; ++I) {
MachineBasicBlock *MBB = &*I;
for (MachineBasicBlock::reverse_iterator MII = MBB->rbegin(),
MIE = MBB->rend(); MII != MIE; ) {
MachineInstr *MI = &*MII;
// Don't delete instructions with side effects.
if (MI->isSafeToMove(&TII, SawStore)) {
// Examine each operand.
bool AllDefsDead = true;
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (MO.isRegister() && MO.isDef()) {
unsigned Reg = MO.getReg();
if ((!MO.isImplicit() &&
TargetRegisterInfo::isPhysicalRegister(Reg)) ||
!MRI.use_empty(Reg)) {
// This def has a use. Don't delete the instruction!
AllDefsDead = false;
break;
}
}
}
// If there are no defs with uses, the instruction is dead.
if (AllDefsDead) {
// Clear out the operands to take the registers out of their
// use chains.
while (unsigned Num = MI->getNumOperands())
MI->RemoveOperand(Num-1);
// Delete the actual instruction.
AnyChanges = true;
MI->eraseFromParent();
MIE = MBB->rend();
// MII is now pointing to the next instruction to process,
// so don't increment it.
continue;
}
}
// We didn't delete the current instruction, so increment MII to
// the next one.
++MII;
}
}
return AnyChanges;
}
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