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// Copyright 2013 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "src/hydrogen-dce.h"
#include "src/v8.h"
namespace v8 {
namespace internal {
void HDeadCodeEliminationPhase::MarkLive(
HValue* instr, ZoneList<HValue*>* worklist) {
if (instr->CheckFlag(HValue::kIsLive)) return; // Already live.
if (FLAG_trace_dead_code_elimination) PrintLive(NULL, instr);
// Transitively mark all inputs of live instructions live.
worklist->Add(instr, zone());
while (!worklist->is_empty()) {
HValue* instr = worklist->RemoveLast();
instr->SetFlag(HValue::kIsLive);
for (int i = 0; i < instr->OperandCount(); ++i) {
HValue* input = instr->OperandAt(i);
if (!input->CheckFlag(HValue::kIsLive)) {
input->SetFlag(HValue::kIsLive);
worklist->Add(input, zone());
if (FLAG_trace_dead_code_elimination) PrintLive(instr, input);
}
}
}
}
void HDeadCodeEliminationPhase::PrintLive(HValue* ref, HValue* instr) {
AllowHandleDereference allow_deref;
OFStream os(stdout);
os << "[MarkLive ";
if (ref != NULL) {
os << *ref;
} else {
os << "root";
}
os << " -> " << *instr << "]" << std::endl;
}
void HDeadCodeEliminationPhase::MarkLiveInstructions() {
ZoneList<HValue*> worklist(10, zone());
// Transitively mark all live instructions, starting from roots.
for (int i = 0; i < graph()->blocks()->length(); ++i) {
HBasicBlock* block = graph()->blocks()->at(i);
for (HInstructionIterator it(block); !it.Done(); it.Advance()) {
HInstruction* instr = it.Current();
if (instr->CannotBeEliminated()) MarkLive(instr, &worklist);
}
for (int j = 0; j < block->phis()->length(); j++) {
HPhi* phi = block->phis()->at(j);
if (phi->CannotBeEliminated()) MarkLive(phi, &worklist);
}
}
DCHECK(worklist.is_empty()); // Should have processed everything.
}
void HDeadCodeEliminationPhase::RemoveDeadInstructions() {
ZoneList<HPhi*> worklist(graph()->blocks()->length(), zone());
// Remove any instruction not marked kIsLive.
for (int i = 0; i < graph()->blocks()->length(); ++i) {
HBasicBlock* block = graph()->blocks()->at(i);
for (HInstructionIterator it(block); !it.Done(); it.Advance()) {
HInstruction* instr = it.Current();
if (!instr->CheckFlag(HValue::kIsLive)) {
// Instruction has not been marked live, so remove it.
instr->DeleteAndReplaceWith(NULL);
} else {
// Clear the liveness flag to leave the graph clean for the next DCE.
instr->ClearFlag(HValue::kIsLive);
}
}
// Collect phis that are dead and remove them in the next pass.
for (int j = 0; j < block->phis()->length(); j++) {
HPhi* phi = block->phis()->at(j);
if (!phi->CheckFlag(HValue::kIsLive)) {
worklist.Add(phi, zone());
} else {
phi->ClearFlag(HValue::kIsLive);
}
}
}
// Process phis separately to avoid simultaneously mutating the phi list.
while (!worklist.is_empty()) {
HPhi* phi = worklist.RemoveLast();
HBasicBlock* block = phi->block();
phi->DeleteAndReplaceWith(NULL);
if (phi->HasMergedIndex()) {
block->RecordDeletedPhi(phi->merged_index());
}
}
}
} // namespace internal
} // namespace v8
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