1 //===- DeadMachineInstructionElim.cpp - Remove dead machine instructions --===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This is an extremely simple MachineInstr-level dead-code-elimination pass.
10 //
11 //===----------------------------------------------------------------------===//
12
13 #include "llvm/CodeGen/DeadMachineInstructionElim.h"
14 #include "llvm/ADT/PostOrderIterator.h"
15 #include "llvm/ADT/Statistic.h"
16 #include "llvm/CodeGen/LiveRegUnits.h"
17 #include "llvm/CodeGen/MachineFunctionPass.h"
18 #include "llvm/CodeGen/MachineRegisterInfo.h"
19 #include "llvm/CodeGen/TargetSubtargetInfo.h"
20 #include "llvm/InitializePasses.h"
21 #include "llvm/Pass.h"
22 #include "llvm/Support/Debug.h"
23 #include "llvm/Support/raw_ostream.h"
24
25 using namespace llvm;
26
27 #define DEBUG_TYPE "dead-mi-elimination"
28
29 STATISTIC(NumDeletes, "Number of dead instructions deleted");
30
31 namespace {
32 class DeadMachineInstructionElimImpl {
33 const MachineRegisterInfo *MRI = nullptr;
34 const TargetInstrInfo *TII = nullptr;
35 LiveRegUnits LivePhysRegs;
36
37 public:
38 bool runImpl(MachineFunction &MF);
39
40 private:
41 bool isDead(const MachineInstr *MI) const;
42 bool eliminateDeadMI(MachineFunction &MF);
43 };
44
45 class DeadMachineInstructionElim : public MachineFunctionPass {
46 public:
47 static char ID; // Pass identification, replacement for typeid
48
DeadMachineInstructionElim()49 DeadMachineInstructionElim() : MachineFunctionPass(ID) {
50 initializeDeadMachineInstructionElimPass(*PassRegistry::getPassRegistry());
51 }
52
runOnMachineFunction(MachineFunction & MF)53 bool runOnMachineFunction(MachineFunction &MF) override {
54 if (skipFunction(MF.getFunction()))
55 return false;
56 return DeadMachineInstructionElimImpl().runImpl(MF);
57 }
58
getAnalysisUsage(AnalysisUsage & AU) const59 void getAnalysisUsage(AnalysisUsage &AU) const override {
60 AU.setPreservesCFG();
61 MachineFunctionPass::getAnalysisUsage(AU);
62 }
63 };
64 } // namespace
65
66 PreservedAnalyses
run(MachineFunction & MF,MachineFunctionAnalysisManager &)67 DeadMachineInstructionElimPass::run(MachineFunction &MF,
68 MachineFunctionAnalysisManager &) {
69 if (!DeadMachineInstructionElimImpl().runImpl(MF))
70 return PreservedAnalyses::all();
71 PreservedAnalyses PA = getMachineFunctionPassPreservedAnalyses();
72 PA.preserveSet<CFGAnalyses>();
73 return PA;
74 }
75
76 char DeadMachineInstructionElim::ID = 0;
77 char &llvm::DeadMachineInstructionElimID = DeadMachineInstructionElim::ID;
78
79 INITIALIZE_PASS(DeadMachineInstructionElim, DEBUG_TYPE,
80 "Remove dead machine instructions", false, false)
81
isDead(const MachineInstr * MI) const82 bool DeadMachineInstructionElimImpl::isDead(const MachineInstr *MI) const {
83 // Technically speaking inline asm without side effects and no defs can still
84 // be deleted. But there is so much bad inline asm code out there, we should
85 // let them be.
86 if (MI->isInlineAsm())
87 return false;
88
89 // Don't delete frame allocation labels.
90 if (MI->getOpcode() == TargetOpcode::LOCAL_ESCAPE)
91 return false;
92
93 // Don't delete instructions with side effects.
94 bool SawStore = false;
95 if (!MI->isSafeToMove(nullptr, SawStore) && !MI->isPHI())
96 return false;
97
98 // Examine each operand.
99 for (const MachineOperand &MO : MI->all_defs()) {
100 Register Reg = MO.getReg();
101 if (Reg.isPhysical()) {
102 // Don't delete live physreg defs, or any reserved register defs.
103 if (!LivePhysRegs.available(Reg) || MRI->isReserved(Reg))
104 return false;
105 } else {
106 if (MO.isDead()) {
107 #ifndef NDEBUG
108 // Basic check on the register. All of them should be 'undef'.
109 for (auto &U : MRI->use_nodbg_operands(Reg))
110 assert(U.isUndef() && "'Undef' use on a 'dead' register is found!");
111 #endif
112 continue;
113 }
114 for (const MachineInstr &Use : MRI->use_nodbg_instructions(Reg)) {
115 if (&Use != MI)
116 // This def has a non-debug use. Don't delete the instruction!
117 return false;
118 }
119 }
120 }
121
122 // If there are no defs with uses, the instruction is dead.
123 return true;
124 }
125
runImpl(MachineFunction & MF)126 bool DeadMachineInstructionElimImpl::runImpl(MachineFunction &MF) {
127 MRI = &MF.getRegInfo();
128
129 const TargetSubtargetInfo &ST = MF.getSubtarget();
130 TII = ST.getInstrInfo();
131 LivePhysRegs.init(*ST.getRegisterInfo());
132
133 bool AnyChanges = eliminateDeadMI(MF);
134 while (AnyChanges && eliminateDeadMI(MF))
135 ;
136 return AnyChanges;
137 }
138
eliminateDeadMI(MachineFunction & MF)139 bool DeadMachineInstructionElimImpl::eliminateDeadMI(MachineFunction &MF) {
140 bool AnyChanges = false;
141
142 // Loop over all instructions in all blocks, from bottom to top, so that it's
143 // more likely that chains of dependent but ultimately dead instructions will
144 // be cleaned up.
145 for (MachineBasicBlock *MBB : post_order(&MF)) {
146 LivePhysRegs.addLiveOuts(*MBB);
147
148 // Now scan the instructions and delete dead ones, tracking physreg
149 // liveness as we go.
150 for (MachineInstr &MI : make_early_inc_range(reverse(*MBB))) {
151 // If the instruction is dead, delete it!
152 if (isDead(&MI)) {
153 LLVM_DEBUG(dbgs() << "DeadMachineInstructionElim: DELETING: " << MI);
154 // It is possible that some DBG_VALUE instructions refer to this
155 // instruction. They will be deleted in the live debug variable
156 // analysis.
157 MI.eraseFromParent();
158 AnyChanges = true;
159 ++NumDeletes;
160 continue;
161 }
162
163 LivePhysRegs.stepBackward(MI);
164 }
165 }
166
167 LivePhysRegs.clear();
168 return AnyChanges;
169 }
170