1 /*
2 * Copyright (C) 2015 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17 #include "licm.h"
18
19 #include "side_effects_analysis.h"
20
21 namespace art HIDDEN {
22
IsPhiOf(HInstruction * instruction,HBasicBlock * block)23 static bool IsPhiOf(HInstruction* instruction, HBasicBlock* block) {
24 return instruction->IsPhi() && instruction->GetBlock() == block;
25 }
26
27 /**
28 * Returns whether `instruction` has all its inputs and environment defined
29 * before the loop it is in.
30 */
InputsAreDefinedBeforeLoop(HInstruction * instruction)31 static bool InputsAreDefinedBeforeLoop(HInstruction* instruction) {
32 DCHECK(instruction->IsInLoop());
33 HLoopInformation* info = instruction->GetBlock()->GetLoopInformation();
34 for (const HInstruction* input : instruction->GetInputs()) {
35 HLoopInformation* input_loop = input->GetBlock()->GetLoopInformation();
36 // We only need to check whether the input is defined in the loop. If it is not
37 // it is defined before the loop.
38 if (input_loop != nullptr && input_loop->IsIn(*info)) {
39 return false;
40 }
41 }
42
43 for (HEnvironment* environment = instruction->GetEnvironment();
44 environment != nullptr;
45 environment = environment->GetParent()) {
46 for (size_t i = 0, e = environment->Size(); i < e; ++i) {
47 HInstruction* input = environment->GetInstructionAt(i);
48 if (input != nullptr) {
49 HLoopInformation* input_loop = input->GetBlock()->GetLoopInformation();
50 if (input_loop != nullptr && input_loop->IsIn(*info)) {
51 // We can move an instruction that takes a loop header phi in the environment:
52 // we will just replace that phi with its first input later in `UpdateLoopPhisIn`.
53 bool is_loop_header_phi = IsPhiOf(input, info->GetHeader());
54 if (!is_loop_header_phi) {
55 return false;
56 }
57 }
58 }
59 }
60 }
61 return true;
62 }
63
64 /**
65 * If `environment` has a loop header phi, we replace it with its first input.
66 */
UpdateLoopPhisIn(ArenaAllocator * allocator,HEnvironment * environment,HLoopInformation * info)67 static void UpdateLoopPhisIn(ArenaAllocator* allocator,
68 HEnvironment* environment,
69 HLoopInformation* info) {
70 for (; environment != nullptr; environment = environment->GetParent()) {
71 for (size_t i = 0, e = environment->Size(); i < e; ++i) {
72 HInstruction* input = environment->GetInstructionAt(i);
73 if (input != nullptr && IsPhiOf(input, info->GetHeader())) {
74 environment->RemoveAsUserOfInput(i);
75 HInstruction* incoming = input->InputAt(0);
76 environment->SetRawEnvAt(i, incoming);
77 incoming->AddEnvUseAt(allocator, environment, i);
78 }
79 }
80 }
81 }
82
Run()83 bool LICM::Run() {
84 bool didLICM = false;
85 DCHECK(side_effects_.HasRun());
86
87 // Only used during debug.
88 ArenaBitVector* visited = nullptr;
89 if (kIsDebugBuild) {
90 visited = new (graph_->GetAllocator()) ArenaBitVector(graph_->GetAllocator(),
91 graph_->GetBlocks().size(),
92 false,
93 kArenaAllocLICM);
94 }
95
96 // Post order visit to visit inner loops before outer loops.
97 for (HBasicBlock* block : graph_->GetPostOrder()) {
98 if (!block->IsLoopHeader()) {
99 // Only visit the loop when we reach the header.
100 continue;
101 }
102
103 HLoopInformation* loop_info = block->GetLoopInformation();
104 SideEffects loop_effects = side_effects_.GetLoopEffects(block);
105 HBasicBlock* pre_header = loop_info->GetPreHeader();
106
107 for (HBlocksInLoopIterator it_loop(*loop_info); !it_loop.Done(); it_loop.Advance()) {
108 HBasicBlock* inner = it_loop.Current();
109 DCHECK(inner->IsInLoop());
110 if (inner->GetLoopInformation() != loop_info) {
111 // Thanks to post order visit, inner loops were already visited.
112 DCHECK(visited->IsBitSet(inner->GetBlockId()));
113 continue;
114 }
115 if (kIsDebugBuild) {
116 visited->SetBit(inner->GetBlockId());
117 }
118
119 if (loop_info->ContainsIrreducibleLoop()) {
120 // We cannot licm in an irreducible loop, or in a natural loop containing an
121 // irreducible loop.
122 continue;
123 }
124 DCHECK(!loop_info->IsIrreducible());
125
126 // We can move an instruction that can throw only as long as it is the first visible
127 // instruction (throw or write) in the loop. Note that the first potentially visible
128 // instruction that is not hoisted stops this optimization. Non-throwing instructions,
129 // on the other hand, can still be hoisted.
130 bool found_first_non_hoisted_visible_instruction_in_loop = !inner->IsLoopHeader();
131 for (HInstructionIterator inst_it(inner->GetInstructions());
132 !inst_it.Done();
133 inst_it.Advance()) {
134 HInstruction* instruction = inst_it.Current();
135 bool can_move = false;
136 if (instruction->CanBeMoved() && InputsAreDefinedBeforeLoop(instruction)) {
137 if (instruction->CanThrow()) {
138 if (!found_first_non_hoisted_visible_instruction_in_loop) {
139 DCHECK(instruction->GetBlock()->IsLoopHeader());
140 if (instruction->IsClinitCheck()) {
141 // clinit is only done once, and since all visible instructions
142 // in the loop header so far have been hoisted out, we can hoist
143 // the clinit check out also.
144 can_move = true;
145 } else if (!instruction->GetSideEffects().MayDependOn(loop_effects)) {
146 can_move = true;
147 }
148 }
149 } else if (!instruction->GetSideEffects().MayDependOn(loop_effects)) {
150 can_move = true;
151 }
152 }
153 if (can_move) {
154 // We need to update the environment if the instruction has a loop header
155 // phi in it.
156 if (instruction->NeedsEnvironment()) {
157 UpdateLoopPhisIn(graph_->GetAllocator(), instruction->GetEnvironment(), loop_info);
158 } else {
159 DCHECK(!instruction->HasEnvironment());
160 }
161 instruction->MoveBefore(pre_header->GetLastInstruction());
162 MaybeRecordStat(stats_, MethodCompilationStat::kLoopInvariantMoved);
163 didLICM = true;
164 }
165
166 if (!can_move && (instruction->CanThrow() || instruction->DoesAnyWrite())) {
167 // If `instruction` can do something visible (throw or write),
168 // we cannot move further instructions that can throw.
169 found_first_non_hoisted_visible_instruction_in_loop = true;
170 }
171 }
172 }
173 }
174 return didLICM;
175 }
176
177 } // namespace art
178