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      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 
      7 namespace v8 {
      8 namespace internal {
      9 
     10 void HDeadCodeEliminationPhase::MarkLive(
     11     HValue* instr, ZoneList<HValue*>* worklist) {
     12   if (instr->CheckFlag(HValue::kIsLive)) return;  // Already live.
     13 
     14   if (FLAG_trace_dead_code_elimination) PrintLive(NULL, instr);
     15 
     16   // Transitively mark all inputs of live instructions live.
     17   worklist->Add(instr, zone());
     18   while (!worklist->is_empty()) {
     19     HValue* instr = worklist->RemoveLast();
     20     instr->SetFlag(HValue::kIsLive);
     21     for (int i = 0; i < instr->OperandCount(); ++i) {
     22       HValue* input = instr->OperandAt(i);
     23       if (!input->CheckFlag(HValue::kIsLive)) {
     24         input->SetFlag(HValue::kIsLive);
     25         worklist->Add(input, zone());
     26         if (FLAG_trace_dead_code_elimination) PrintLive(instr, input);
     27       }
     28     }
     29   }
     30 }
     31 
     32 
     33 void HDeadCodeEliminationPhase::PrintLive(HValue* ref, HValue* instr) {
     34   AllowHandleDereference allow_deref;
     35   OFStream os(stdout);
     36   os << "[MarkLive ";
     37   if (ref != NULL) {
     38     os << *ref;
     39   } else {
     40     os << "root";
     41   }
     42   os << " -> " << *instr << "]" << std::endl;
     43 }
     44 
     45 
     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 
     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 internal
    105 }  // namespace v8
    106