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/hydrogen-dce.h"
6 #include "src/v8.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 OFStream os(stdout);
36 os << "[MarkLive ";
37 if (ref != NULL) {
38 os << *ref;
39 } else {
40 os << "root ";
41 }
42 os << " -> " << *instr << "]" << endl;
43 }
44
45
MarkLiveInstructions()46 void HDeadCodeEliminationPhase::MarkLiveInstructions() {
47 ZoneList<HValue*> worklist(10, zone());
48
49 // Transitively mark all live instructions, starting from roots.
50 for (int i = 0; i < graph()->blocks()->length(); ++i) {
51 HBasicBlock* block = graph()->blocks()->at(i);
52 for (HInstructionIterator it(block); !it.Done(); it.Advance()) {
53 HInstruction* instr = it.Current();
54 if (instr->CannotBeEliminated()) MarkLive(instr, &worklist);
55 }
56 for (int j = 0; j < block->phis()->length(); j++) {
57 HPhi* phi = block->phis()->at(j);
58 if (phi->CannotBeEliminated()) MarkLive(phi, &worklist);
59 }
60 }
61
62 DCHECK(worklist.is_empty()); // Should have processed everything.
63 }
64
65
RemoveDeadInstructions()66 void HDeadCodeEliminationPhase::RemoveDeadInstructions() {
67 ZoneList<HPhi*> worklist(graph()->blocks()->length(), zone());
68
69 // Remove any instruction not marked kIsLive.
70 for (int i = 0; i < graph()->blocks()->length(); ++i) {
71 HBasicBlock* block = graph()->blocks()->at(i);
72 for (HInstructionIterator it(block); !it.Done(); it.Advance()) {
73 HInstruction* instr = it.Current();
74 if (!instr->CheckFlag(HValue::kIsLive)) {
75 // Instruction has not been marked live, so remove it.
76 instr->DeleteAndReplaceWith(NULL);
77 } else {
78 // Clear the liveness flag to leave the graph clean for the next DCE.
79 instr->ClearFlag(HValue::kIsLive);
80 }
81 }
82 // Collect phis that are dead and remove them in the next pass.
83 for (int j = 0; j < block->phis()->length(); j++) {
84 HPhi* phi = block->phis()->at(j);
85 if (!phi->CheckFlag(HValue::kIsLive)) {
86 worklist.Add(phi, zone());
87 } else {
88 phi->ClearFlag(HValue::kIsLive);
89 }
90 }
91 }
92
93 // Process phis separately to avoid simultaneously mutating the phi list.
94 while (!worklist.is_empty()) {
95 HPhi* phi = worklist.RemoveLast();
96 HBasicBlock* block = phi->block();
97 phi->DeleteAndReplaceWith(NULL);
98 if (phi->HasMergedIndex()) {
99 block->RecordDeletedPhi(phi->merged_index());
100 }
101 }
102 }
103
104 } } // namespace v8::internal
105