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
170
171
172
173
174
| //===- 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/ADT/Statistic.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetSubtargetInfo.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 DeadMachineInstructionElim : public MachineFunctionPass {
bool runOnMachineFunction(MachineFunction &MF) override;
const TargetRegisterInfo *TRI;
const MachineRegisterInfo *MRI;
const TargetInstrInfo *TII;
BitVector LivePhysRegs;
public:
static char ID; // Pass identification, replacement for typeid
DeadMachineInstructionElim() : MachineFunctionPass(ID) {
initializeDeadMachineInstructionElimPass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
MachineFunctionPass::getAnalysisUsage(AU);
}
private:
bool isDead(const MachineInstr *MI) const;
};
}
char DeadMachineInstructionElim::ID = 0;
char &llvm::DeadMachineInstructionElimID = DeadMachineInstructionElim::ID;
INITIALIZE_PASS(DeadMachineInstructionElim, DEBUG_TYPE,
"Remove dead machine instructions", false, false)
bool DeadMachineInstructionElim::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 (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (MO.isReg() && MO.isDef()) {
Register Reg = MO.getReg();
if (Register::isPhysicalRegister(Reg)) {
// Don't delete live physreg defs, or any reserved register defs.
if (LivePhysRegs.test(Reg) || MRI->isReserved(Reg))
return false;
} else {
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 DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) {
if (skipFunction(MF.getFunction()))
return false;
bool AnyChanges = false;
MRI = &MF.getRegInfo();
TRI = MF.getSubtarget().getRegisterInfo();
TII = MF.getSubtarget().getInstrInfo();
// 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 : make_range(MF.rbegin(), MF.rend())) {
// Start out assuming that reserved registers are live out of this block.
LivePhysRegs = MRI->getReservedRegs();
// Add live-ins from successors to LivePhysRegs. Normally, physregs are not
// live across blocks, but some targets (x86) can have flags live out of a
// block.
for (MachineBasicBlock::succ_iterator S = MBB.succ_begin(),
E = MBB.succ_end(); S != E; S++)
for (const auto &LI : (*S)->liveins())
LivePhysRegs.set(LI.PhysReg);
// Now scan the instructions and delete dead ones, tracking physreg
// liveness as we go.
for (MachineBasicBlock::reverse_iterator MII = MBB.rbegin(),
MIE = MBB.rend(); MII != MIE; ) {
MachineInstr *MI = &*MII++;
// 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 get marked as undef and will be deleted
// in the live debug variable analysis.
MI->eraseFromParentAndMarkDBGValuesForRemoval();
AnyChanges = true;
++NumDeletes;
continue;
}
// Record the physreg defs.
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (MO.isReg() && MO.isDef()) {
Register Reg = MO.getReg();
if (Register::isPhysicalRegister(Reg)) {
// Check the subreg set, not the alias set, because a def
// of a super-register may still be partially live after
// this def.
for (MCSubRegIterator SR(Reg, TRI,/*IncludeSelf=*/true);
SR.isValid(); ++SR)
LivePhysRegs.reset(*SR);
}
} else if (MO.isRegMask()) {
// Register mask of preserved registers. All clobbers are dead.
LivePhysRegs.clearBitsNotInMask(MO.getRegMask());
}
}
// Record the physreg uses, after the defs, in case a physreg is
// both defined and used in the same instruction.
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (MO.isReg() && MO.isUse()) {
Register Reg = MO.getReg();
if (Register::isPhysicalRegister(Reg)) {
for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
LivePhysRegs.set(*AI);
}
}
}
}
}
LivePhysRegs.clear();
return AnyChanges;
}
|