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
|
/*
* Copyright (C) 2014 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#include "ssa_phi_elimination.h"
namespace art {
void SsaDeadPhiElimination::Run() {
// Add to the worklist phis referenced by non-phi instructions.
for (HReversePostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
HBasicBlock* block = it.Current();
for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) {
HPhi* phi = it.Current()->AsPhi();
if (phi->HasEnvironmentUses()) {
// TODO: Do we want to keep that phi alive?
worklist_.Add(phi);
phi->SetLive();
continue;
}
for (HUseIterator<HInstruction> it(phi->GetUses()); !it.Done(); it.Advance()) {
HUseListNode<HInstruction>* current = it.Current();
HInstruction* user = current->GetUser();
if (!user->IsPhi()) {
worklist_.Add(phi);
phi->SetLive();
} else {
phi->SetDead();
}
}
}
}
// Process the worklist by propagating liveness to phi inputs.
while (!worklist_.IsEmpty()) {
HPhi* phi = worklist_.Pop();
for (HInputIterator it(phi); !it.Done(); it.Advance()) {
HInstruction* input = it.Current();
if (input->IsPhi() && input->AsPhi()->IsDead()) {
worklist_.Add(input->AsPhi());
input->AsPhi()->SetLive();
}
}
}
// Remove phis that are not live. Visit in post order so that phis
// that are not inputs of loop phis can be removed when they have
// no users left (dead phis might use dead phis).
for (HPostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
HBasicBlock* block = it.Current();
HInstruction* current = block->GetFirstPhi();
HInstruction* next = nullptr;
while (current != nullptr) {
next = current->GetNext();
if (current->AsPhi()->IsDead()) {
if (current->HasUses()) {
for (HUseIterator<HInstruction> it(current->GetUses()); !it.Done(); it.Advance()) {
HUseListNode<HInstruction>* user_node = it.Current();
HInstruction* user = user_node->GetUser();
DCHECK(user->IsLoopHeaderPhi());
DCHECK(user->AsPhi()->IsDead());
// Just put itself as an input. The phi will be removed in this loop anyway.
user->SetRawInputAt(user_node->GetIndex(), user);
current->RemoveUser(user, user_node->GetIndex());
}
}
block->RemovePhi(current->AsPhi());
}
current = next;
}
}
}
void SsaRedundantPhiElimination::Run() {
// Add all phis in the worklist.
for (HReversePostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
HBasicBlock* block = it.Current();
for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) {
worklist_.Add(it.Current()->AsPhi());
}
}
while (!worklist_.IsEmpty()) {
HPhi* phi = worklist_.Pop();
// If the phi has already been processed, continue.
if (!phi->IsInBlock()) {
continue;
}
// Find if the inputs of the phi are the same instruction.
HInstruction* candidate = phi->InputAt(0);
// A loop phi cannot have itself as the first phi. Note that this
// check relies on our simplification pass ensuring the pre-header
// block is first in the list of predecessors of the loop header.
DCHECK(!phi->IsLoopHeaderPhi() || phi->GetBlock()->IsLoopPreHeaderFirstPredecessor());
DCHECK_NE(phi, candidate);
for (size_t i = 1; i < phi->InputCount(); ++i) {
HInstruction* input = phi->InputAt(i);
// For a loop phi, if the input is the phi, the phi is still candidate for
// elimination.
if (input != candidate && input != phi) {
candidate = nullptr;
break;
}
}
// If the inputs are not the same, continue.
if (candidate == nullptr) {
continue;
}
if (phi->IsInLoop()) {
// Because we're updating the users of this phi, we may have new
// phis candidate for elimination if this phi is in a loop. Add phis that
// used this phi to the worklist.
for (HUseIterator<HInstruction> it(phi->GetUses()); !it.Done(); it.Advance()) {
HUseListNode<HInstruction>* current = it.Current();
HInstruction* user = current->GetUser();
if (user->IsPhi()) {
worklist_.Add(user->AsPhi());
}
}
}
phi->ReplaceWith(candidate);
phi->GetBlock()->RemovePhi(phi);
}
}
} // namespace art
|
