licm.cc revision 82091dad38f3e5bfaf3b6984c9ab73069fb68310 
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  if (instruction->HasEnvironment()) {
43    HEnvironment* environment = instruction->GetEnvironment();
44    for (size_t i = 0, e = environment->Size(); i < e; ++i) {
45      HInstruction* input = environment->GetInstructionAt(i);
46      if (input != nullptr) {
47        HLoopInformation* input_loop = input->GetBlock()->GetLoopInformation();
48        if (input_loop != nullptr && input_loop->IsIn(*info)) {
49          // We can move an instruction that takes a loop header phi in the environment:
50          // we will just replace that phi with its first input later in `UpdateLoopPhisIn`.
51          bool is_loop_header_phi = IsPhiOf(input, info->GetHeader());
52          if (!is_loop_header_phi) {
53            return false;
54          }
55        }
56      }
57    }
58  }
59  return true;
60}
61
62/**
63 * If `environment` has a loop header phi, we replace it with its first input.
64 */
65static void UpdateLoopPhisIn(HEnvironment* environment, HLoopInformation* info) {
66  for (size_t i = 0, e = environment->Size(); i < e; ++i) {
67    HInstruction* input = environment->GetInstructionAt(i);
68    if (input != nullptr && IsPhiOf(input, info->GetHeader())) {
69      HUseListNode<HEnvironment*>* env_use = environment->GetInstructionEnvUseAt(i);
70      input->RemoveEnvironmentUser(env_use);
71      HInstruction* incoming = input->InputAt(0);
72      environment->SetRawEnvAt(i, incoming);
73      incoming->AddEnvUseAt(environment, i);
74    }
75  }
76}
77
78void LICM::Run() {
79  DCHECK(side_effects_.HasRun());
80  // Only used during debug.
81  ArenaBitVector visited(graph_->GetArena(), graph_->GetBlocks().Size(), false);
82
83  // Post order visit to visit inner loops before outer loops.
84  for (HPostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
85    HBasicBlock* block = it.Current();
86    if (!block->IsLoopHeader()) {
87      // Only visit the loop when we reach the header.
88      continue;
89    }
90
91    HLoopInformation* loop_info = block->GetLoopInformation();
92    SideEffects loop_effects = side_effects_.GetLoopEffects(block);
93    HBasicBlock* pre_header = loop_info->GetPreHeader();
94
95    for (HBlocksInLoopIterator it_loop(*loop_info); !it_loop.Done(); it_loop.Advance()) {
96      HBasicBlock* inner = it_loop.Current();
97      DCHECK(inner->IsInLoop());
98      if (inner->GetLoopInformation() != loop_info) {
99        // Thanks to post order visit, inner loops were already visited.
100        DCHECK(visited.IsBitSet(inner->GetBlockId()));
101        continue;
102      }
103      visited.SetBit(inner->GetBlockId());
104
105      // We can move an instruction that can throw only if it is the first
106      // throwing instruction in the loop. Note that the first potentially
107      // throwing instruction encountered that is not hoisted stops this
108      // optimization. Non-throwing instruction can still be hoisted.
109      bool found_first_non_hoisted_throwing_instruction_in_loop = !inner->IsLoopHeader();
110      for (HInstructionIterator inst_it(inner->GetInstructions());
111           !inst_it.Done();
112           inst_it.Advance()) {
113        HInstruction* instruction = inst_it.Current();
114        if (instruction->CanBeMoved()
115            && (!instruction->CanThrow() || !found_first_non_hoisted_throwing_instruction_in_loop)
116            && !instruction->GetSideEffects().DependsOn(loop_effects)
117            && InputsAreDefinedBeforeLoop(instruction)) {
118          // We need to update the environment if the instruction has a loop header
119          // phi in it.
120          if (instruction->NeedsEnvironment()) {
121            UpdateLoopPhisIn(instruction->GetEnvironment(), loop_info);
122          }
123          instruction->MoveBefore(pre_header->GetLastInstruction());
124        } else if (instruction->CanThrow()) {
125          // If `instruction` can throw, we cannot move further instructions
126          // that can throw as well.
127          found_first_non_hoisted_throwing_instruction_in_loop = true;
128        }
129      }
130    }
131  }
132}
133
134}  // namespace art
135