1 //===- DeadMachineInstructionElim.cpp - Remove dead machine instructions --===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This is an extremely simple MachineInstr-level dead-code-elimination pass.
12 //===----------------------------------------------------------------------===//
14 #include "llvm/CodeGen/Passes.h"
15 #include "llvm/Pass.h"
16 #include "llvm/CodeGen/MachineFunctionPass.h"
17 #include "llvm/CodeGen/MachineRegisterInfo.h"
18 #include "llvm/Support/Debug.h"
19 #include "llvm/Support/raw_ostream.h"
20 #include "llvm/Target/TargetInstrInfo.h"
21 #include "llvm/Target/TargetMachine.h"
25 class DeadMachineInstructionElim : public MachineFunctionPass {
26 virtual bool runOnMachineFunction(MachineFunction &MF);
28 const TargetRegisterInfo *TRI;
29 const MachineRegisterInfo *MRI;
30 const TargetInstrInfo *TII;
31 BitVector LivePhysRegs;
34 static char ID; // Pass identification, replacement for typeid
35 DeadMachineInstructionElim() : MachineFunctionPass(&ID) {}
38 bool isDead(const MachineInstr *MI) const;
41 char DeadMachineInstructionElim::ID = 0;
43 static RegisterPass<DeadMachineInstructionElim>
44 Y("dead-mi-elimination",
45 "Remove dead machine instructions");
47 FunctionPass *llvm::createDeadMachineInstructionElimPass() {
48 return new DeadMachineInstructionElim();
51 bool DeadMachineInstructionElim::isDead(const MachineInstr *MI) const {
52 // Don't delete instructions with side effects.
53 bool SawStore = false;
54 if (!MI->isSafeToMove(TII, SawStore, 0))
57 // Examine each operand.
58 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
59 const MachineOperand &MO = MI->getOperand(i);
60 if (MO.isReg() && MO.isDef()) {
61 unsigned Reg = MO.getReg();
62 if (TargetRegisterInfo::isPhysicalRegister(Reg) ?
63 LivePhysRegs[Reg] : !MRI->use_empty(Reg)) {
64 // This def has a use. Don't delete the instruction!
70 // If there are no defs with uses, the instruction is dead.
74 bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) {
75 bool AnyChanges = false;
76 MRI = &MF.getRegInfo();
77 TRI = MF.getTarget().getRegisterInfo();
78 TII = MF.getTarget().getInstrInfo();
80 // Compute a bitvector to represent all non-allocatable physregs.
81 BitVector NonAllocatableRegs = TRI->getAllocatableSet(MF);
82 NonAllocatableRegs.flip();
84 // Loop over all instructions in all blocks, from bottom to top, so that it's
85 // more likely that chains of dependent but ultimately dead instructions will
87 for (MachineFunction::reverse_iterator I = MF.rbegin(), E = MF.rend();
89 MachineBasicBlock *MBB = &*I;
91 // Start out assuming that all non-allocatable registers are live
93 LivePhysRegs = NonAllocatableRegs;
95 // Also add any explicit live-out physregs for this block.
96 if (!MBB->empty() && MBB->back().getDesc().isReturn())
97 for (MachineRegisterInfo::liveout_iterator LOI = MRI->liveout_begin(),
98 LOE = MRI->liveout_end(); LOI != LOE; ++LOI) {
100 if (TargetRegisterInfo::isPhysicalRegister(Reg))
101 LivePhysRegs.set(Reg);
104 // Now scan the instructions and delete dead ones, tracking physreg
105 // liveness as we go.
106 for (MachineBasicBlock::reverse_iterator MII = MBB->rbegin(),
107 MIE = MBB->rend(); MII != MIE; ) {
108 MachineInstr *MI = &*MII;
110 if (MI->getOpcode()==TargetInstrInfo::DEBUG_VALUE) {
111 // Don't delete the DEBUG_VALUE itself, but if its Value operand is
112 // a vreg and this is the only use, substitute an undef operand;
113 // the former operand will then be deleted normally.
114 if (MI->getNumOperands()==3 && MI->getOperand(0).isReg()) {
115 unsigned Reg = MI->getOperand(0).getReg();
116 MachineRegisterInfo::use_iterator I = MRI->use_begin(Reg);
117 assert(I != MRI->use_end());
118 if (++I == MRI->use_end())
119 // only one use, which must be this DEBUG_VALUE.
120 MI->getOperand(0).setReg(0U);
124 // If the instruction is dead, delete it!
126 DEBUG(dbgs() << "DeadMachineInstructionElim: DELETING: " << *MI);
128 MI->eraseFromParent();
130 // MII is now pointing to the next instruction to process,
131 // so don't increment it.
135 // Record the physreg defs.
136 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
137 const MachineOperand &MO = MI->getOperand(i);
138 if (MO.isReg() && MO.isDef()) {
139 unsigned Reg = MO.getReg();
140 if (Reg != 0 && TargetRegisterInfo::isPhysicalRegister(Reg)) {
141 LivePhysRegs.reset(Reg);
142 // Check the subreg set, not the alias set, because a def
143 // of a super-register may still be partially live after
145 for (const unsigned *SubRegs = TRI->getSubRegisters(Reg);
147 LivePhysRegs.reset(*SubRegs);
151 // Record the physreg uses, after the defs, in case a physreg is
152 // both defined and used in the same instruction.
153 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
154 const MachineOperand &MO = MI->getOperand(i);
155 if (MO.isReg() && MO.isUse()) {
156 unsigned Reg = MO.getReg();
157 if (Reg != 0 && TargetRegisterInfo::isPhysicalRegister(Reg)) {
158 LivePhysRegs.set(Reg);
159 for (const unsigned *AliasSet = TRI->getAliasSet(Reg);
160 *AliasSet; ++AliasSet)
161 LivePhysRegs.set(*AliasSet);
166 // We didn't delete the current instruction, so increment MII to
172 LivePhysRegs.clear();