1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
|
//===- DeadMachineInstructionElim.cpp - Remove dead machine instructions --===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This is an extremely simple MachineInstr-level dead-code-elimination pass.
//
//===----------------------------------------------------------------------===//
#include "llvm/CodeGen/DeadMachineInstructionElim.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/LiveRegUnits.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
#define DEBUG_TYPE "dead-mi-elimination"
STATISTIC(NumDeletes, "Number of dead instructions deleted");
namespace {
class DeadMachineInstructionElimImpl {
const MachineRegisterInfo *MRI = nullptr;
const TargetInstrInfo *TII = nullptr;
LiveRegUnits LivePhysRegs;
public:
bool runImpl(MachineFunction &MF);
private:
bool isDead(const MachineInstr *MI) const;
bool eliminateDeadMI(MachineFunction &MF);
};
class DeadMachineInstructionElim : public MachineFunctionPass {
public:
static char ID; // Pass identification, replacement for typeid
DeadMachineInstructionElim() : MachineFunctionPass(ID) {
initializeDeadMachineInstructionElimPass(*PassRegistry::getPassRegistry());
}
bool runOnMachineFunction(MachineFunction &MF) override {
if (skipFunction(MF.getFunction()))
return false;
return DeadMachineInstructionElimImpl().runImpl(MF);
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
MachineFunctionPass::getAnalysisUsage(AU);
}
};
} // namespace
PreservedAnalyses
DeadMachineInstructionElimPass::run(MachineFunction &MF,
MachineFunctionAnalysisManager &) {
if (!DeadMachineInstructionElimImpl().runImpl(MF))
return PreservedAnalyses::all();
PreservedAnalyses PA = getMachineFunctionPassPreservedAnalyses();
PA.preserveSet<CFGAnalyses>();
return PA;
}
char DeadMachineInstructionElim::ID = 0;
char &llvm::DeadMachineInstructionElimID = DeadMachineInstructionElim::ID;
INITIALIZE_PASS(DeadMachineInstructionElim, DEBUG_TYPE,
"Remove dead machine instructions", false, false)
bool DeadMachineInstructionElimImpl::isDead(const MachineInstr *MI) const {
// Technically speaking inline asm without side effects and no defs can still
// be deleted. But there is so much bad inline asm code out there, we should
// let them be.
if (MI->isInlineAsm())
return false;
// Don't delete frame allocation labels.
if (MI->getOpcode() == TargetOpcode::LOCAL_ESCAPE)
return false;
// Don't delete instructions with side effects.
bool SawStore = false;
if (!MI->isSafeToMove(nullptr, SawStore) && !MI->isPHI())
return false;
// Examine each operand.
for (const MachineOperand &MO : MI->all_defs()) {
Register Reg = MO.getReg();
if (Reg.isPhysical()) {
// Don't delete live physreg defs, or any reserved register defs.
if (!LivePhysRegs.available(Reg) || MRI->isReserved(Reg))
return false;
} else {
if (MO.isDead()) {
#ifndef NDEBUG
// Basic check on the register. All of them should be 'undef'.
for (auto &U : MRI->use_nodbg_operands(Reg))
assert(U.isUndef() && "'Undef' use on a 'dead' register is found!");
#endif
continue;
}
for (const MachineInstr &Use : MRI->use_nodbg_instructions(Reg)) {
if (&Use != MI)
// This def has a non-debug use. Don't delete the instruction!
return false;
}
}
}
// If there are no defs with uses, the instruction is dead.
return true;
}
bool DeadMachineInstructionElimImpl::runImpl(MachineFunction &MF) {
MRI = &MF.getRegInfo();
const TargetSubtargetInfo &ST = MF.getSubtarget();
TII = ST.getInstrInfo();
LivePhysRegs.init(*ST.getRegisterInfo());
bool AnyChanges = eliminateDeadMI(MF);
while (AnyChanges && eliminateDeadMI(MF))
;
return AnyChanges;
}
bool DeadMachineInstructionElimImpl::eliminateDeadMI(MachineFunction &MF) {
bool AnyChanges = false;
// Loop over all instructions in all blocks, from bottom to top, so that it's
// more likely that chains of dependent but ultimately dead instructions will
// be cleaned up.
for (MachineBasicBlock *MBB : post_order(&MF)) {
LivePhysRegs.addLiveOuts(*MBB);
// Now scan the instructions and delete dead ones, tracking physreg
// liveness as we go.
for (MachineInstr &MI : make_early_inc_range(reverse(*MBB))) {
// If the instruction is dead, delete it!
if (isDead(&MI)) {
LLVM_DEBUG(dbgs() << "DeadMachineInstructionElim: DELETING: " << MI);
// It is possible that some DBG_VALUE instructions refer to this
// instruction. They will be deleted in the live debug variable
// analysis.
MI.eraseFromParent();
AnyChanges = true;
++NumDeletes;
continue;
}
LivePhysRegs.stepBackward(MI);
}
}
LivePhysRegs.clear();
return AnyChanges;
}
|
