1 // Copyright 2013 the V8 project authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
4
5 #include "src/crankshaft/hydrogen-dce.h"
6 #include "src/objects-inl.h"
7
8 namespace v8 {
9 namespace internal {
10
MarkLive(HValue * instr,ZoneList<HValue * > * worklist)11 void HDeadCodeEliminationPhase::MarkLive(
12 HValue* instr, ZoneList<HValue*>* worklist) {
13 if (instr->CheckFlag(HValue::kIsLive)) return; // Already live.
14
15 if (FLAG_trace_dead_code_elimination) PrintLive(NULL, instr);
16
17 // Transitively mark all inputs of live instructions live.
18 worklist->Add(instr, zone());
19 while (!worklist->is_empty()) {
20 HValue* instr = worklist->RemoveLast();
21 instr->SetFlag(HValue::kIsLive);
22 for (int i = 0; i < instr->OperandCount(); ++i) {
23 HValue* input = instr->OperandAt(i);
24 if (!input->CheckFlag(HValue::kIsLive)) {
25 input->SetFlag(HValue::kIsLive);
26 worklist->Add(input, zone());
27 if (FLAG_trace_dead_code_elimination) PrintLive(instr, input);
28 }
29 }
30 }
31 }
32
33
PrintLive(HValue * ref,HValue * instr)34 void HDeadCodeEliminationPhase::PrintLive(HValue* ref, HValue* instr) {
35 AllowHandleDereference allow_deref;
36 OFStream os(stdout);
37 os << "[MarkLive ";
38 if (ref != NULL) {
39 os << *ref;
40 } else {
41 os << "root";
42 }
43 os << " -> " << *instr << "]" << std::endl;
44 }
45
46
MarkLiveInstructions()47 void HDeadCodeEliminationPhase::MarkLiveInstructions() {
48 ZoneList<HValue*> worklist(10, zone());
49
50 // Transitively mark all live instructions, starting from roots.
51 for (int i = 0; i < graph()->blocks()->length(); ++i) {
52 HBasicBlock* block = graph()->blocks()->at(i);
53 for (HInstructionIterator it(block); !it.Done(); it.Advance()) {
54 HInstruction* instr = it.Current();
55 if (instr->CannotBeEliminated()) MarkLive(instr, &worklist);
56 }
57 for (int j = 0; j < block->phis()->length(); j++) {
58 HPhi* phi = block->phis()->at(j);
59 if (phi->CannotBeEliminated()) MarkLive(phi, &worklist);
60 }
61 }
62
63 DCHECK(worklist.is_empty()); // Should have processed everything.
64 }
65
66
RemoveDeadInstructions()67 void HDeadCodeEliminationPhase::RemoveDeadInstructions() {
68 ZoneList<HPhi*> worklist(graph()->blocks()->length(), zone());
69
70 // Remove any instruction not marked kIsLive.
71 for (int i = 0; i < graph()->blocks()->length(); ++i) {
72 HBasicBlock* block = graph()->blocks()->at(i);
73 for (HInstructionIterator it(block); !it.Done(); it.Advance()) {
74 HInstruction* instr = it.Current();
75 if (!instr->CheckFlag(HValue::kIsLive)) {
76 // Instruction has not been marked live, so remove it.
77 instr->DeleteAndReplaceWith(NULL);
78 } else {
79 // Clear the liveness flag to leave the graph clean for the next DCE.
80 instr->ClearFlag(HValue::kIsLive);
81 }
82 }
83 // Collect phis that are dead and remove them in the next pass.
84 for (int j = 0; j < block->phis()->length(); j++) {
85 HPhi* phi = block->phis()->at(j);
86 if (!phi->CheckFlag(HValue::kIsLive)) {
87 worklist.Add(phi, zone());
88 } else {
89 phi->ClearFlag(HValue::kIsLive);
90 }
91 }
92 }
93
94 // Process phis separately to avoid simultaneously mutating the phi list.
95 while (!worklist.is_empty()) {
96 HPhi* phi = worklist.RemoveLast();
97 HBasicBlock* block = phi->block();
98 phi->DeleteAndReplaceWith(NULL);
99 if (phi->HasMergedIndex()) {
100 block->RecordDeletedPhi(phi->merged_index());
101 }
102 }
103 }
104
105 } // namespace internal
106 } // namespace v8
107