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
8namespace v8 {
9namespace internal {
10
11void 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
34void 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
46void 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
66void 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