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 #include "side_effects_analysis.h"
19
20 namespace art {
21
IsPhiOf(HInstruction * instruction,HBasicBlock * block)22 static bool IsPhiOf(HInstruction* instruction, HBasicBlock* block) {
23 return instruction->IsPhi() && instruction->GetBlock() == block;
24 }
25
26 /**
27 * Returns whether `instruction` has all its inputs and environment defined
28 * before the loop it is in.
29 */
InputsAreDefinedBeforeLoop(HInstruction * instruction)30 static bool InputsAreDefinedBeforeLoop(HInstruction* instruction) {
31 DCHECK(instruction->IsInLoop());
32 HLoopInformation* info = instruction->GetBlock()->GetLoopInformation();
33 for (HInputIterator it(instruction); !it.Done(); it.Advance()) {
34 HLoopInformation* input_loop = it.Current()->GetBlock()->GetLoopInformation();
35 // We only need to check whether the input is defined in the loop. If it is not
36 // it is defined before the loop.
37 if (input_loop != nullptr && input_loop->IsIn(*info)) {
38 return false;
39 }
40 }
41
42 for (HEnvironment* environment = instruction->GetEnvironment();
43 environment != nullptr;
44 environment = environment->GetParent()) {
45 for (size_t i = 0, e = environment->Size(); i < e; ++i) {
46 HInstruction* input = environment->GetInstructionAt(i);
47 if (input != nullptr) {
48 HLoopInformation* input_loop = input->GetBlock()->GetLoopInformation();
49 if (input_loop != nullptr && input_loop->IsIn(*info)) {
50 // We can move an instruction that takes a loop header phi in the environment:
51 // we will just replace that phi with its first input later in `UpdateLoopPhisIn`.
52 bool is_loop_header_phi = IsPhiOf(input, info->GetHeader());
53 if (!is_loop_header_phi) {
54 return false;
55 }
56 }
57 }
58 }
59 }
60 return true;
61 }
62
63 /**
64 * If `environment` has a loop header phi, we replace it with its first input.
65 */
UpdateLoopPhisIn(HEnvironment * environment,HLoopInformation * info)66 static void UpdateLoopPhisIn(HEnvironment* environment, HLoopInformation* info) {
67 for (; environment != nullptr; environment = environment->GetParent()) {
68 for (size_t i = 0, e = environment->Size(); i < e; ++i) {
69 HInstruction* input = environment->GetInstructionAt(i);
70 if (input != nullptr && IsPhiOf(input, info->GetHeader())) {
71 environment->RemoveAsUserOfInput(i);
72 HInstruction* incoming = input->InputAt(0);
73 environment->SetRawEnvAt(i, incoming);
74 incoming->AddEnvUseAt(environment, i);
75 }
76 }
77 }
78 }
79
Run()80 void LICM::Run() {
81 DCHECK(side_effects_.HasRun());
82 // Only used during debug.
83 ArenaBitVector visited(graph_->GetArena(), graph_->GetBlocks().Size(), false);
84
85 // Post order visit to visit inner loops before outer loops.
86 for (HPostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
87 HBasicBlock* block = it.Current();
88 if (!block->IsLoopHeader()) {
89 // Only visit the loop when we reach the header.
90 continue;
91 }
92
93 HLoopInformation* loop_info = block->GetLoopInformation();
94 SideEffects loop_effects = side_effects_.GetLoopEffects(block);
95 HBasicBlock* pre_header = loop_info->GetPreHeader();
96
97 for (HBlocksInLoopIterator it_loop(*loop_info); !it_loop.Done(); it_loop.Advance()) {
98 HBasicBlock* inner = it_loop.Current();
99 DCHECK(inner->IsInLoop());
100 if (inner->GetLoopInformation() != loop_info) {
101 // Thanks to post order visit, inner loops were already visited.
102 DCHECK(visited.IsBitSet(inner->GetBlockId()));
103 continue;
104 }
105 visited.SetBit(inner->GetBlockId());
106
107 // We can move an instruction that can throw only if it is the first
108 // throwing instruction in the loop. Note that the first potentially
109 // throwing instruction encountered that is not hoisted stops this
110 // optimization. Non-throwing instruction can still be hoisted.
111 bool found_first_non_hoisted_throwing_instruction_in_loop = !inner->IsLoopHeader();
112 for (HInstructionIterator inst_it(inner->GetInstructions());
113 !inst_it.Done();
114 inst_it.Advance()) {
115 HInstruction* instruction = inst_it.Current();
116 if (instruction->CanBeMoved()
117 && (!instruction->CanThrow() || !found_first_non_hoisted_throwing_instruction_in_loop)
118 && !instruction->GetSideEffects().DependsOn(loop_effects)
119 && InputsAreDefinedBeforeLoop(instruction)) {
120 // We need to update the environment if the instruction has a loop header
121 // phi in it.
122 if (instruction->NeedsEnvironment()) {
123 UpdateLoopPhisIn(instruction->GetEnvironment(), loop_info);
124 }
125 instruction->MoveBefore(pre_header->GetLastInstruction());
126 } else if (instruction->CanThrow()) {
127 // If `instruction` can throw, we cannot move further instructions
128 // that can throw as well.
129 found_first_non_hoisted_throwing_instruction_in_loop = true;
130 }
131 }
132 }
133 }
134 }
135
136 } // namespace art
137