licm.cc revision 771e5cc519665ce0cc76985bb4803f0dd50c3b40 
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
20namespace art {
21
22static 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 */
30static 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 */
66static 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
80void 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().MayDependOn(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