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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.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "codegen-dce"
#include "llvm/CodeGen/Passes.h"
#include "llvm/Pass.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;
STATISTIC(NumDeletes, "Number of dead instructions deleted");
namespace {
class DeadMachineInstructionElim : public MachineFunctionPass {
virtual bool runOnMachineFunction(MachineFunction &MF);
const TargetRegisterInfo *TRI;
const MachineRegisterInfo *MRI;
const TargetInstrInfo *TII;
BitVector LivePhysRegs;
public:
static char ID; // Pass identification, replacement for typeid
DeadMachineInstructionElim() : MachineFunctionPass(ID) {
initializeDeadMachineInstructionElimPass(*PassRegistry::getPassRegistry());
}
private:
bool isDead(const MachineInstr *MI) const;
};
}
char DeadMachineInstructionElim::ID = 0;
INITIALIZE_PASS(DeadMachineInstructionElim, "dead-mi-elimination",
"Remove dead machine instructions", false, false)
FunctionPass *llvm::createDeadMachineInstructionElimPass() {
return new DeadMachineInstructionElim();
}
bool DeadMachineInstructionElim::isDead(const MachineInstr *MI) const {
// Technically speaking inline asm without side effects and no defs can still
// be deleted. But there is so much bad inline asm code out there, we should
// let them be.
if (MI->isInlineAsm())
return false;
// Don't delete instructions with side effects.
bool SawStore = false;
if (!MI->isSafeToMove(TII, 0, SawStore) && !MI->isPHI())
return false;
// Examine each operand.
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (MO.isReg() && MO.isDef()) {
unsigned Reg = MO.getReg();
if (TargetRegisterInfo::isPhysicalRegister(Reg) ?
LivePhysRegs[Reg] : !MRI->use_nodbg_empty(Reg)) {
// This def has a non-debug use. Don't delete the instruction!
return false;
}
}
}
// If there are no defs with uses, the instruction is dead.
return true;
}
bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) {
bool AnyChanges = false;
MRI = &MF.getRegInfo();
TRI = MF.getTarget().getRegisterInfo();
TII = MF.getTarget().getInstrInfo();
// Treat reserved registers as always live.
BitVector ReservedRegs = TRI->getReservedRegs(MF);
// 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;
// Start out assuming that reserved registers are live out of this block.
LivePhysRegs = ReservedRegs;
// Also add any explicit live-out physregs for this block.
if (!MBB->empty() && MBB->back().getDesc().isReturn())
for (MachineRegisterInfo::liveout_iterator LOI = MRI->liveout_begin(),
LOE = MRI->liveout_end(); LOI != LOE; ++LOI) {
unsigned Reg = *LOI;
if (TargetRegisterInfo::isPhysicalRegister(Reg))
LivePhysRegs.set(Reg);
}
// FIXME: Add live-ins from sucessors to LivePhysRegs. Normally, physregs
// are not live across blocks, but some targets (x86) can have flags live
// out of a block.
// Now scan the instructions and delete dead ones, tracking physreg
// liveness as we go.
for (MachineBasicBlock::reverse_iterator MII = MBB->rbegin(),
MIE = MBB->rend(); MII != MIE; ) {
MachineInstr *MI = &*MII;
// If the instruction is dead, delete it!
if (isDead(MI)) {
DEBUG(dbgs() << "DeadMachineInstructionElim: DELETING: " << *MI);
// It is possible that some DBG_VALUE instructions refer to this
// instruction. Examine each def operand for such references;
// if found, mark the DBG_VALUE as undef (but don't delete it).
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg() || !MO.isDef())
continue;
unsigned Reg = MO.getReg();
if (!TargetRegisterInfo::isVirtualRegister(Reg))
continue;
MachineRegisterInfo::use_iterator nextI;
for (MachineRegisterInfo::use_iterator I = MRI->use_begin(Reg),
E = MRI->use_end(); I!=E; I=nextI) {
nextI = llvm::next(I); // I is invalidated by the setReg
MachineOperand& Use = I.getOperand();
MachineInstr *UseMI = Use.getParent();
if (UseMI==MI)
continue;
assert(Use.isDebug());
UseMI->getOperand(0).setReg(0U);
}
}
AnyChanges = true;
MI->eraseFromParent();
++NumDeletes;
MIE = MBB->rend();
// MII is now pointing to the next instruction to process,
// so don't increment it.
continue;
}
// Record the physreg defs.
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (MO.isReg() && MO.isDef()) {
unsigned Reg = MO.getReg();
if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
LivePhysRegs.reset(Reg);
// Check the subreg set, not the alias set, because a def
// of a super-register may still be partially live after
// this def.
for (const unsigned *SubRegs = TRI->getSubRegisters(Reg);
*SubRegs; ++SubRegs)
LivePhysRegs.reset(*SubRegs);
}
}
}
// Record the physreg uses, after the defs, in case a physreg is
// both defined and used in the same instruction.
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (MO.isReg() && MO.isUse()) {
unsigned Reg = MO.getReg();
if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
LivePhysRegs.set(Reg);
for (const unsigned *AliasSet = TRI->getAliasSet(Reg);
*AliasSet; ++AliasSet)
LivePhysRegs.set(*AliasSet);
}
}
}
// We didn't delete the current instruction, so increment MII to
// the next one.
++MII;
}
}
LivePhysRegs.clear();
return AnyChanges;
}
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