CSE2410 Fall 2014 Bounce

// Copyright 2013 the V8 project authors. All rights reserved.

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#include "hydrogen-dce.h"

#include "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) {

  HeapStringAllocator allocator;

  StringStream stream(&allocator);

  if (ref != NULL) {

    ref->PrintTo(&stream);

  } else {

    stream.Add("root ");

  }

  stream.Add(" -> ");

  instr->PrintTo(&stream);

  PrintF("[MarkLive %s]\n", *stream.ToCString());

}

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);

    }

  }

  ASSERT(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 v8::internal