1 // Copyright 2013 the V8 project authors. All rights reserved.
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3 // modification, are permitted provided that the following conditions are
4 // met:
5 //
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9 // copyright notice, this list of conditions and the following
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15 //
16 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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21 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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26 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
27
28 #include "hydrogen-dce.h"
29 #include "v8.h"
30
31 namespace v8 {
32 namespace internal {
33
MarkLive(HValue * instr,ZoneList<HValue * > * worklist)34 void HDeadCodeEliminationPhase::MarkLive(
35 HValue* instr, ZoneList<HValue*>* worklist) {
36 if (instr->CheckFlag(HValue::kIsLive)) return; // Already live.
37
38 if (FLAG_trace_dead_code_elimination) PrintLive(NULL, instr);
39
40 // Transitively mark all inputs of live instructions live.
41 worklist->Add(instr, zone());
42 while (!worklist->is_empty()) {
43 HValue* instr = worklist->RemoveLast();
44 instr->SetFlag(HValue::kIsLive);
45 for (int i = 0; i < instr->OperandCount(); ++i) {
46 HValue* input = instr->OperandAt(i);
47 if (!input->CheckFlag(HValue::kIsLive)) {
48 input->SetFlag(HValue::kIsLive);
49 worklist->Add(input, zone());
50 if (FLAG_trace_dead_code_elimination) PrintLive(instr, input);
51 }
52 }
53 }
54 }
55
56
PrintLive(HValue * ref,HValue * instr)57 void HDeadCodeEliminationPhase::PrintLive(HValue* ref, HValue* instr) {
58 HeapStringAllocator allocator;
59 StringStream stream(&allocator);
60 if (ref != NULL) {
61 ref->PrintTo(&stream);
62 } else {
63 stream.Add("root ");
64 }
65 stream.Add(" -> ");
66 instr->PrintTo(&stream);
67 PrintF("[MarkLive %s]\n", *stream.ToCString());
68 }
69
70
MarkLiveInstructions()71 void HDeadCodeEliminationPhase::MarkLiveInstructions() {
72 ZoneList<HValue*> worklist(10, zone());
73
74 // Transitively mark all live instructions, starting from roots.
75 for (int i = 0; i < graph()->blocks()->length(); ++i) {
76 HBasicBlock* block = graph()->blocks()->at(i);
77 for (HInstructionIterator it(block); !it.Done(); it.Advance()) {
78 HInstruction* instr = it.Current();
79 if (instr->CannotBeEliminated()) MarkLive(instr, &worklist);
80 }
81 for (int j = 0; j < block->phis()->length(); j++) {
82 HPhi* phi = block->phis()->at(j);
83 if (phi->CannotBeEliminated()) MarkLive(phi, &worklist);
84 }
85 }
86
87 ASSERT(worklist.is_empty()); // Should have processed everything.
88 }
89
90
RemoveDeadInstructions()91 void HDeadCodeEliminationPhase::RemoveDeadInstructions() {
92 ZoneList<HPhi*> worklist(graph()->blocks()->length(), zone());
93
94 // Remove any instruction not marked kIsLive.
95 for (int i = 0; i < graph()->blocks()->length(); ++i) {
96 HBasicBlock* block = graph()->blocks()->at(i);
97 for (HInstructionIterator it(block); !it.Done(); it.Advance()) {
98 HInstruction* instr = it.Current();
99 if (!instr->CheckFlag(HValue::kIsLive)) {
100 // Instruction has not been marked live, so remove it.
101 instr->DeleteAndReplaceWith(NULL);
102 } else {
103 // Clear the liveness flag to leave the graph clean for the next DCE.
104 instr->ClearFlag(HValue::kIsLive);
105 }
106 }
107 // Collect phis that are dead and remove them in the next pass.
108 for (int j = 0; j < block->phis()->length(); j++) {
109 HPhi* phi = block->phis()->at(j);
110 if (!phi->CheckFlag(HValue::kIsLive)) {
111 worklist.Add(phi, zone());
112 } else {
113 phi->ClearFlag(HValue::kIsLive);
114 }
115 }
116 }
117
118 // Process phis separately to avoid simultaneously mutating the phi list.
119 while (!worklist.is_empty()) {
120 HPhi* phi = worklist.RemoveLast();
121 HBasicBlock* block = phi->block();
122 phi->DeleteAndReplaceWith(NULL);
123 if (phi->HasMergedIndex()) {
124 block->RecordDeletedPhi(phi->merged_index());
125 }
126 }
127 }
128
129 } } // namespace v8::internal
130