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/ADT/PostOrderIterator.h"
14 #include "llvm/ADT/Statistic.h"
15 #include "llvm/CodeGen/LiveRegUnits.h"
16 #include "llvm/CodeGen/MachineFunctionPass.h"
17 #include "llvm/CodeGen/MachineRegisterInfo.h"
18 #include "llvm/CodeGen/TargetSubtargetInfo.h"
19 #include "llvm/InitializePasses.h"
20 #include "llvm/Pass.h"
21 #include "llvm/Support/Debug.h"
22 #include "llvm/Support/raw_ostream.h"
23
24 using namespace llvm;
25
26 #define DEBUG_TYPE "dead-mi-elimination"
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 MachineRegisterInfo *MRI;
35 const TargetInstrInfo *TII;
36 LiveRegUnits LivePhysRegs;
37
38 public:
39 static char ID; // Pass identification, replacement for typeid
DeadMachineInstructionElim()40 DeadMachineInstructionElim() : MachineFunctionPass(ID) {
41 initializeDeadMachineInstructionElimPass(*PassRegistry::getPassRegistry());
42 }
43
getAnalysisUsage(AnalysisUsage & AU) const44 void getAnalysisUsage(AnalysisUsage &AU) const override {
45 AU.setPreservesCFG();
46 MachineFunctionPass::getAnalysisUsage(AU);
47 }
48
49 private:
50 bool isDead(const MachineInstr *MI) const;
51
52 bool eliminateDeadMI(MachineFunction &MF);
53 };
54 }
55 char DeadMachineInstructionElim::ID = 0;
56 char &llvm::DeadMachineInstructionElimID = DeadMachineInstructionElim::ID;
57
58 INITIALIZE_PASS(DeadMachineInstructionElim, DEBUG_TYPE,
59 "Remove dead machine instructions", false, false)
60
isDead(const MachineInstr * MI) const61 bool DeadMachineInstructionElim::isDead(const MachineInstr *MI) const {
62 // Technically speaking inline asm without side effects and no defs can still
63 // be deleted. But there is so much bad inline asm code out there, we should
64 // let them be.
65 if (MI->isInlineAsm())
66 return false;
67
68 // Don't delete frame allocation labels.
69 if (MI->getOpcode() == TargetOpcode::LOCAL_ESCAPE)
70 return false;
71
72 // Don't delete instructions with side effects.
73 bool SawStore = false;
74 if (!MI->isSafeToMove(nullptr, SawStore) && !MI->isPHI())
75 return false;
76
77 // Examine each operand.
78 for (const MachineOperand &MO : MI->operands()) {
79 if (MO.isReg() && MO.isDef()) {
80 Register Reg = MO.getReg();
81 if (Reg.isPhysical()) {
82 // Don't delete live physreg defs, or any reserved register defs.
83 if (!LivePhysRegs.available(Reg) || MRI->isReserved(Reg))
84 return false;
85 } else {
86 if (MO.isDead()) {
87 #ifndef NDEBUG
88 // Basic check on the register. All of them should be 'undef'.
89 for (auto &U : MRI->use_nodbg_operands(Reg))
90 assert(U.isUndef() && "'Undef' use on a 'dead' register is found!");
91 #endif
92 continue;
93 }
94 for (const MachineInstr &Use : MRI->use_nodbg_instructions(Reg)) {
95 if (&Use != MI)
96 // This def has a non-debug use. Don't delete the instruction!
97 return false;
98 }
99 }
100 }
101 }
102
103 // If there are no defs with uses, the instruction is dead.
104 return true;
105 }
106
runOnMachineFunction(MachineFunction & MF)107 bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) {
108 if (skipFunction(MF.getFunction()))
109 return false;
110
111 MRI = &MF.getRegInfo();
112
113 const TargetSubtargetInfo &ST = MF.getSubtarget();
114 TII = ST.getInstrInfo();
115 LivePhysRegs.init(*ST.getRegisterInfo());
116
117 bool AnyChanges = eliminateDeadMI(MF);
118 while (AnyChanges && eliminateDeadMI(MF))
119 ;
120 return AnyChanges;
121 }
122
eliminateDeadMI(MachineFunction & MF)123 bool DeadMachineInstructionElim::eliminateDeadMI(MachineFunction &MF) {
124 bool AnyChanges = false;
125
126 // Loop over all instructions in all blocks, from bottom to top, so that it's
127 // more likely that chains of dependent but ultimately dead instructions will
128 // be cleaned up.
129 for (MachineBasicBlock *MBB : post_order(&MF)) {
130 LivePhysRegs.addLiveOuts(*MBB);
131
132 // Now scan the instructions and delete dead ones, tracking physreg
133 // liveness as we go.
134 for (MachineInstr &MI : make_early_inc_range(reverse(*MBB))) {
135 // If the instruction is dead, delete it!
136 if (isDead(&MI)) {
137 LLVM_DEBUG(dbgs() << "DeadMachineInstructionElim: DELETING: " << MI);
138 // It is possible that some DBG_VALUE instructions refer to this
139 // instruction. They will be deleted in the live debug variable
140 // analysis.
141 MI.eraseFromParent();
142 AnyChanges = true;
143 ++NumDeletes;
144 continue;
145 }
146
147 LivePhysRegs.stepBackward(MI);
148 }
149 }
150
151 LivePhysRegs.clear();
152 return AnyChanges;
153 }
154