DeadMachineInstructionElim.cpp revision c9df025e33ac435adb3b3318d237c36ca7cec659
1//===- DeadMachineInstructionElim.cpp - Remove dead machine instructions --===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This is an extremely simple MachineInstr-level dead-code-elimination pass. 11// 12//===----------------------------------------------------------------------===// 13 14#define DEBUG_TYPE "codegen-dce" 15#include "llvm/CodeGen/Passes.h" 16#include "llvm/Pass.h" 17#include "llvm/CodeGen/MachineFunctionPass.h" 18#include "llvm/CodeGen/MachineRegisterInfo.h" 19#include "llvm/Support/Debug.h" 20#include "llvm/Support/raw_ostream.h" 21#include "llvm/Target/TargetInstrInfo.h" 22#include "llvm/Target/TargetMachine.h" 23#include "llvm/ADT/Statistic.h" 24using namespace llvm; 25 26STATISTIC(NumDeletes, "Number of dead instructions deleted"); 27 28namespace { 29 class DeadMachineInstructionElim : public MachineFunctionPass { 30 virtual bool runOnMachineFunction(MachineFunction &MF); 31 32 const TargetRegisterInfo *TRI; 33 const MachineRegisterInfo *MRI; 34 const TargetInstrInfo *TII; 35 BitVector LivePhysRegs; 36 37 public: 38 static char ID; // Pass identification, replacement for typeid 39 DeadMachineInstructionElim() : MachineFunctionPass(ID) { 40 initializeDeadMachineInstructionElimPass(*PassRegistry::getPassRegistry()); 41 } 42 43 private: 44 bool isDead(const MachineInstr *MI) const; 45 }; 46} 47char DeadMachineInstructionElim::ID = 0; 48 49INITIALIZE_PASS(DeadMachineInstructionElim, "dead-mi-elimination", 50 "Remove dead machine instructions", false, false) 51 52FunctionPass *llvm::createDeadMachineInstructionElimPass() { 53 return new DeadMachineInstructionElim(); 54} 55 56bool DeadMachineInstructionElim::isDead(const MachineInstr *MI) const { 57 // Technically speaking inline asm without side effects and no defs can still 58 // be deleted. But there is so much bad inline asm code out there, we should 59 // let them be. 60 if (MI->isInlineAsm()) 61 return false; 62 63 // Don't delete instructions with side effects. 64 bool SawStore = false; 65 if (!MI->isSafeToMove(TII, 0, SawStore) && !MI->isPHI()) 66 return false; 67 68 // Examine each operand. 69 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 70 const MachineOperand &MO = MI->getOperand(i); 71 if (MO.isReg() && MO.isDef()) { 72 unsigned Reg = MO.getReg(); 73 if (TargetRegisterInfo::isPhysicalRegister(Reg) ? 74 LivePhysRegs[Reg] : !MRI->use_nodbg_empty(Reg)) { 75 // This def has a non-debug use. Don't delete the instruction! 76 return false; 77 } 78 } 79 } 80 81 // If there are no defs with uses, the instruction is dead. 82 return true; 83} 84 85bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) { 86 bool AnyChanges = false; 87 MRI = &MF.getRegInfo(); 88 TRI = MF.getTarget().getRegisterInfo(); 89 TII = MF.getTarget().getInstrInfo(); 90 91 // Treat reserved registers as always live. 92 BitVector ReservedRegs = TRI->getReservedRegs(MF); 93 94 // Loop over all instructions in all blocks, from bottom to top, so that it's 95 // more likely that chains of dependent but ultimately dead instructions will 96 // be cleaned up. 97 for (MachineFunction::reverse_iterator I = MF.rbegin(), E = MF.rend(); 98 I != E; ++I) { 99 MachineBasicBlock *MBB = &*I; 100 101 // Start out assuming that reserved registers are live out of this block. 102 LivePhysRegs = ReservedRegs; 103 104 // Also add any explicit live-out physregs for this block. 105 if (!MBB->empty() && MBB->back().getDesc().isReturn()) 106 for (MachineRegisterInfo::liveout_iterator LOI = MRI->liveout_begin(), 107 LOE = MRI->liveout_end(); LOI != LOE; ++LOI) { 108 unsigned Reg = *LOI; 109 if (TargetRegisterInfo::isPhysicalRegister(Reg)) 110 LivePhysRegs.set(Reg); 111 } 112 113 // FIXME: Add live-ins from sucessors to LivePhysRegs. Normally, physregs 114 // are not live across blocks, but some targets (x86) can have flags live 115 // out of a block. 116 117 // Now scan the instructions and delete dead ones, tracking physreg 118 // liveness as we go. 119 for (MachineBasicBlock::reverse_iterator MII = MBB->rbegin(), 120 MIE = MBB->rend(); MII != MIE; ) { 121 MachineInstr *MI = &*MII; 122 123 // If the instruction is dead, delete it! 124 if (isDead(MI)) { 125 DEBUG(dbgs() << "DeadMachineInstructionElim: DELETING: " << *MI); 126 // It is possible that some DBG_VALUE instructions refer to this 127 // instruction. Examine each def operand for such references; 128 // if found, mark the DBG_VALUE as undef (but don't delete it). 129 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 130 const MachineOperand &MO = MI->getOperand(i); 131 if (!MO.isReg() || !MO.isDef()) 132 continue; 133 unsigned Reg = MO.getReg(); 134 if (!TargetRegisterInfo::isVirtualRegister(Reg)) 135 continue; 136 MachineRegisterInfo::use_iterator nextI; 137 for (MachineRegisterInfo::use_iterator I = MRI->use_begin(Reg), 138 E = MRI->use_end(); I!=E; I=nextI) { 139 nextI = llvm::next(I); // I is invalidated by the setReg 140 MachineOperand& Use = I.getOperand(); 141 MachineInstr *UseMI = Use.getParent(); 142 if (UseMI==MI) 143 continue; 144 assert(Use.isDebug()); 145 UseMI->getOperand(0).setReg(0U); 146 } 147 } 148 AnyChanges = true; 149 MI->eraseFromParent(); 150 ++NumDeletes; 151 MIE = MBB->rend(); 152 // MII is now pointing to the next instruction to process, 153 // so don't increment it. 154 continue; 155 } 156 157 // Record the physreg defs. 158 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 159 const MachineOperand &MO = MI->getOperand(i); 160 if (MO.isReg() && MO.isDef()) { 161 unsigned Reg = MO.getReg(); 162 if (TargetRegisterInfo::isPhysicalRegister(Reg)) { 163 LivePhysRegs.reset(Reg); 164 // Check the subreg set, not the alias set, because a def 165 // of a super-register may still be partially live after 166 // this def. 167 for (const unsigned *SubRegs = TRI->getSubRegisters(Reg); 168 *SubRegs; ++SubRegs) 169 LivePhysRegs.reset(*SubRegs); 170 } 171 } 172 } 173 // Record the physreg uses, after the defs, in case a physreg is 174 // both defined and used in the same instruction. 175 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 176 const MachineOperand &MO = MI->getOperand(i); 177 if (MO.isReg() && MO.isUse()) { 178 unsigned Reg = MO.getReg(); 179 if (TargetRegisterInfo::isPhysicalRegister(Reg)) { 180 LivePhysRegs.set(Reg); 181 for (const unsigned *AliasSet = TRI->getAliasSet(Reg); 182 *AliasSet; ++AliasSet) 183 LivePhysRegs.set(*AliasSet); 184 } 185 } 186 } 187 188 // We didn't delete the current instruction, so increment MII to 189 // the next one. 190 ++MII; 191 } 192 } 193 194 LivePhysRegs.clear(); 195 return AnyChanges; 196} 197