1 /*
2  * Copyright (C) 2014 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 "dead_code_elimination.h"
18 
19 #include "utils/array_ref.h"
20 #include "base/bit_vector-inl.h"
21 #include "ssa_phi_elimination.h"
22 
23 namespace art {
24 
MarkReachableBlocks(HGraph * graph,ArenaBitVector * visited)25 static void MarkReachableBlocks(HGraph* graph, ArenaBitVector* visited) {
26   ArenaVector<HBasicBlock*> worklist(graph->GetArena()->Adapter(kArenaAllocDCE));
27   constexpr size_t kDefaultWorlistSize = 8;
28   worklist.reserve(kDefaultWorlistSize);
29   visited->SetBit(graph->GetEntryBlock()->GetBlockId());
30   worklist.push_back(graph->GetEntryBlock());
31 
32   while (!worklist.empty()) {
33     HBasicBlock* block = worklist.back();
34     worklist.pop_back();
35     int block_id = block->GetBlockId();
36     DCHECK(visited->IsBitSet(block_id));
37 
38     ArrayRef<HBasicBlock* const> live_successors(block->GetSuccessors());
39     HInstruction* last_instruction = block->GetLastInstruction();
40     if (last_instruction->IsIf()) {
41       HIf* if_instruction = last_instruction->AsIf();
42       HInstruction* condition = if_instruction->InputAt(0);
43       if (condition->IsIntConstant()) {
44         if (condition->AsIntConstant()->IsTrue()) {
45           live_successors = live_successors.SubArray(0u, 1u);
46           DCHECK_EQ(live_successors[0], if_instruction->IfTrueSuccessor());
47         } else {
48           DCHECK(condition->AsIntConstant()->IsFalse()) << condition->AsIntConstant()->GetValue();
49           live_successors = live_successors.SubArray(1u, 1u);
50           DCHECK_EQ(live_successors[0], if_instruction->IfFalseSuccessor());
51         }
52       }
53     } else if (last_instruction->IsPackedSwitch()) {
54       HPackedSwitch* switch_instruction = last_instruction->AsPackedSwitch();
55       HInstruction* switch_input = switch_instruction->InputAt(0);
56       if (switch_input->IsIntConstant()) {
57         int32_t switch_value = switch_input->AsIntConstant()->GetValue();
58         int32_t start_value = switch_instruction->GetStartValue();
59         // Note: Though the spec forbids packed-switch values to wrap around, we leave
60         // that task to the verifier and use unsigned arithmetic with it's "modulo 2^32"
61         // semantics to check if the value is in range, wrapped or not.
62         uint32_t switch_index =
63             static_cast<uint32_t>(switch_value) - static_cast<uint32_t>(start_value);
64         if (switch_index < switch_instruction->GetNumEntries()) {
65           live_successors = live_successors.SubArray(switch_index, 1u);
66           DCHECK_EQ(live_successors[0], block->GetSuccessors()[switch_index]);
67         } else {
68           live_successors = live_successors.SubArray(switch_instruction->GetNumEntries(), 1u);
69           DCHECK_EQ(live_successors[0], switch_instruction->GetDefaultBlock());
70         }
71       }
72     }
73 
74     for (HBasicBlock* successor : live_successors) {
75       // Add only those successors that have not been visited yet.
76       if (!visited->IsBitSet(successor->GetBlockId())) {
77         visited->SetBit(successor->GetBlockId());
78         worklist.push_back(successor);
79       }
80     }
81   }
82 }
83 
MaybeRecordDeadBlock(HBasicBlock * block)84 void HDeadCodeElimination::MaybeRecordDeadBlock(HBasicBlock* block) {
85   if (stats_ != nullptr) {
86     stats_->RecordStat(MethodCompilationStat::kRemovedDeadInstruction,
87                        block->GetPhis().CountSize() + block->GetInstructions().CountSize());
88   }
89 }
90 
RemoveDeadBlocks()91 void HDeadCodeElimination::RemoveDeadBlocks() {
92   if (graph_->HasIrreducibleLoops()) {
93     // Do not eliminate dead blocks if the graph has irreducible loops. We could
94     // support it, but that would require changes in our loop representation to handle
95     // multiple entry points. We decided it was not worth the complexity.
96     return;
97   }
98   // Classify blocks as reachable/unreachable.
99   ArenaAllocator* allocator = graph_->GetArena();
100   ArenaBitVector live_blocks(allocator, graph_->GetBlocks().size(), false, kArenaAllocDCE);
101 
102   MarkReachableBlocks(graph_, &live_blocks);
103   bool removed_one_or_more_blocks = false;
104   bool rerun_dominance_and_loop_analysis = false;
105 
106   // Remove all dead blocks. Iterate in post order because removal needs the
107   // block's chain of dominators and nested loops need to be updated from the
108   // inside out.
109   for (HPostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
110     HBasicBlock* block  = it.Current();
111     int id = block->GetBlockId();
112     if (!live_blocks.IsBitSet(id)) {
113       MaybeRecordDeadBlock(block);
114       block->DisconnectAndDelete();
115       removed_one_or_more_blocks = true;
116       if (block->IsInLoop()) {
117         rerun_dominance_and_loop_analysis = true;
118       }
119     }
120   }
121 
122   // If we removed at least one block, we need to recompute the full
123   // dominator tree and try block membership.
124   if (removed_one_or_more_blocks) {
125     if (rerun_dominance_and_loop_analysis) {
126       graph_->ClearLoopInformation();
127       graph_->ClearDominanceInformation();
128       graph_->BuildDominatorTree();
129     } else {
130       graph_->ClearDominanceInformation();
131       graph_->ComputeDominanceInformation();
132       graph_->ComputeTryBlockInformation();
133     }
134   }
135 
136   // Connect successive blocks created by dead branches. Order does not matter.
137   for (HReversePostOrderIterator it(*graph_); !it.Done();) {
138     HBasicBlock* block  = it.Current();
139     if (block->IsEntryBlock() || !block->GetLastInstruction()->IsGoto()) {
140       it.Advance();
141       continue;
142     }
143     HBasicBlock* successor = block->GetSingleSuccessor();
144     if (successor->IsExitBlock() || successor->GetPredecessors().size() != 1u) {
145       it.Advance();
146       continue;
147     }
148     block->MergeWith(successor);
149 
150     // Reiterate on this block in case it can be merged with its new successor.
151   }
152 }
153 
RemoveDeadInstructions()154 void HDeadCodeElimination::RemoveDeadInstructions() {
155   // Process basic blocks in post-order in the dominator tree, so that
156   // a dead instruction depending on another dead instruction is removed.
157   for (HPostOrderIterator b(*graph_); !b.Done(); b.Advance()) {
158     HBasicBlock* block = b.Current();
159     // Traverse this block's instructions in backward order and remove
160     // the unused ones.
161     HBackwardInstructionIterator i(block->GetInstructions());
162     // Skip the first iteration, as the last instruction of a block is
163     // a branching instruction.
164     DCHECK(i.Current()->IsControlFlow());
165     for (i.Advance(); !i.Done(); i.Advance()) {
166       HInstruction* inst = i.Current();
167       DCHECK(!inst->IsControlFlow());
168       if (!inst->HasSideEffects()
169           && !inst->CanThrow()
170           && !inst->IsSuspendCheck()
171           && !inst->IsNativeDebugInfo()
172           // If we added an explicit barrier then we should keep it.
173           && !inst->IsMemoryBarrier()
174           && !inst->IsParameterValue()
175           && !inst->HasUses()) {
176         block->RemoveInstruction(inst);
177         MaybeRecordStat(MethodCompilationStat::kRemovedDeadInstruction);
178       }
179     }
180   }
181 }
182 
Run()183 void HDeadCodeElimination::Run() {
184   RemoveDeadBlocks();
185   SsaRedundantPhiElimination(graph_).Run();
186   RemoveDeadInstructions();
187 }
188 
189 }  // namespace art
190