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 #include "llvm/CodeGen/Passes.h"
15 #include "llvm/ADT/Statistic.h"
16 #include "llvm/CodeGen/MachineFunctionPass.h"
17 #include "llvm/CodeGen/MachineRegisterInfo.h"
18 #include "llvm/Pass.h"
19 #include "llvm/Support/Debug.h"
20 #include "llvm/Support/raw_ostream.h"
21 #include "llvm/Target/TargetInstrInfo.h"
22 #include "llvm/Target/TargetSubtargetInfo.h"
23
24 using namespace llvm;
25
26 #define DEBUG_TYPE "codegen-dce"
27
28 STATISTIC(NumDeletes, "Number of dead instructions deleted");
29
30 namespace {
31 class DeadMachineInstructionElim : public MachineFunctionPass {
32 bool runOnMachineFunction(MachineFunction &MF) override;
33
34 const TargetRegisterInfo *TRI;
35 const MachineRegisterInfo *MRI;
36 const TargetInstrInfo *TII;
37 BitVector LivePhysRegs;
38
39 public:
40 static char ID; // Pass identification, replacement for typeid
DeadMachineInstructionElim()41 DeadMachineInstructionElim() : MachineFunctionPass(ID) {
42 initializeDeadMachineInstructionElimPass(*PassRegistry::getPassRegistry());
43 }
44
45 private:
46 bool isDead(const MachineInstr *MI) const;
47 };
48 }
49 char DeadMachineInstructionElim::ID = 0;
50 char &llvm::DeadMachineInstructionElimID = DeadMachineInstructionElim::ID;
51
52 INITIALIZE_PASS(DeadMachineInstructionElim, "dead-mi-elimination",
53 "Remove dead machine instructions", false, false)
54
isDead(const MachineInstr * MI) const55 bool DeadMachineInstructionElim::isDead(const MachineInstr *MI) const {
56 // Technically speaking inline asm without side effects and no defs can still
57 // be deleted. But there is so much bad inline asm code out there, we should
58 // let them be.
59 if (MI->isInlineAsm())
60 return false;
61
62 // Don't delete frame allocation labels.
63 if (MI->getOpcode() == TargetOpcode::LOCAL_ESCAPE)
64 return false;
65
66 // Don't delete instructions with side effects.
67 bool SawStore = false;
68 if (!MI->isSafeToMove(nullptr, SawStore) && !MI->isPHI())
69 return false;
70
71 // Examine each operand.
72 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
73 const MachineOperand &MO = MI->getOperand(i);
74 if (MO.isReg() && MO.isDef()) {
75 unsigned Reg = MO.getReg();
76 if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
77 // Don't delete live physreg defs, or any reserved register defs.
78 if (LivePhysRegs.test(Reg) || MRI->isReserved(Reg))
79 return false;
80 } else {
81 if (!MRI->use_nodbg_empty(Reg))
82 // This def has a non-debug use. Don't delete the instruction!
83 return false;
84 }
85 }
86 }
87
88 // If there are no defs with uses, the instruction is dead.
89 return true;
90 }
91
runOnMachineFunction(MachineFunction & MF)92 bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) {
93 if (skipOptnoneFunction(*MF.getFunction()))
94 return false;
95
96 bool AnyChanges = false;
97 MRI = &MF.getRegInfo();
98 TRI = MF.getSubtarget().getRegisterInfo();
99 TII = MF.getSubtarget().getInstrInfo();
100
101 // Loop over all instructions in all blocks, from bottom to top, so that it's
102 // more likely that chains of dependent but ultimately dead instructions will
103 // be cleaned up.
104 for (MachineBasicBlock &MBB : make_range(MF.rbegin(), MF.rend())) {
105 // Start out assuming that reserved registers are live out of this block.
106 LivePhysRegs = MRI->getReservedRegs();
107
108 // Add live-ins from sucessors to LivePhysRegs. Normally, physregs are not
109 // live across blocks, but some targets (x86) can have flags live out of a
110 // block.
111 for (MachineBasicBlock::succ_iterator S = MBB.succ_begin(),
112 E = MBB.succ_end(); S != E; S++)
113 for (const auto &LI : (*S)->liveins())
114 LivePhysRegs.set(LI.PhysReg);
115
116 // Now scan the instructions and delete dead ones, tracking physreg
117 // liveness as we go.
118 for (MachineBasicBlock::reverse_iterator MII = MBB.rbegin(),
119 MIE = MBB.rend(); MII != MIE; ) {
120 MachineInstr *MI = &*MII;
121
122 // If the instruction is dead, delete it!
123 if (isDead(MI)) {
124 DEBUG(dbgs() << "DeadMachineInstructionElim: DELETING: " << *MI);
125 // It is possible that some DBG_VALUE instructions refer to this
126 // instruction. They get marked as undef and will be deleted
127 // in the live debug variable analysis.
128 MI->eraseFromParentAndMarkDBGValuesForRemoval();
129 AnyChanges = true;
130 ++NumDeletes;
131 MIE = MBB.rend();
132 // MII is now pointing to the next instruction to process,
133 // so don't increment it.
134 continue;
135 }
136
137 // Record the physreg defs.
138 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
139 const MachineOperand &MO = MI->getOperand(i);
140 if (MO.isReg() && MO.isDef()) {
141 unsigned Reg = MO.getReg();
142 if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
143 // Check the subreg set, not the alias set, because a def
144 // of a super-register may still be partially live after
145 // this def.
146 for (MCSubRegIterator SR(Reg, TRI,/*IncludeSelf=*/true);
147 SR.isValid(); ++SR)
148 LivePhysRegs.reset(*SR);
149 }
150 } else if (MO.isRegMask()) {
151 // Register mask of preserved registers. All clobbers are dead.
152 LivePhysRegs.clearBitsNotInMask(MO.getRegMask());
153 }
154 }
155 // Record the physreg uses, after the defs, in case a physreg is
156 // both defined and used in the same instruction.
157 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
158 const MachineOperand &MO = MI->getOperand(i);
159 if (MO.isReg() && MO.isUse()) {
160 unsigned Reg = MO.getReg();
161 if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
162 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
163 LivePhysRegs.set(*AI);
164 }
165 }
166 }
167
168 // We didn't delete the current instruction, so increment MII to
169 // the next one.
170 ++MII;
171 }
172 }
173
174 LivePhysRegs.clear();
175 return AnyChanges;
176 }
177