v8/src/hydrogen-environment-liveness.cc

// Copyright 2013 the V8 project authors. All rights reserved.
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//     * Redistributions of source code must retain the above copyright
//       notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above
//       copyright notice, this list of conditions and the following
//       disclaimer in the documentation and/or other materials provided
//       with the distribution.
//     * Neither the name of Google Inc. nor the names of its
//       contributors may be used to endorse or promote products derived
//       from this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.


#include "hydrogen-environment-liveness.h"


namespace v8 {
namespace internal {


EnvironmentSlotLivenessAnalyzer::EnvironmentSlotLivenessAnalyzer(
    HGraph* graph)
    : graph_(graph),
      zone_(graph->isolate()),
      zone_scope_(&zone_, DELETE_ON_EXIT),
      block_count_(graph->blocks()->length()),
      maximum_environment_size_(graph->maximum_environment_size()),
      collect_markers_(true),
      last_simulate_(NULL) {
  if (maximum_environment_size_ == 0) return;

  live_at_block_start_ =
      new(zone()) ZoneList<BitVector*>(block_count_, zone());
  first_simulate_ = new(zone()) ZoneList<HSimulate*>(block_count_, zone());
  first_simulate_invalid_for_index_ =
      new(zone()) ZoneList<BitVector*>(block_count_, zone());
  markers_ = new(zone())
      ZoneList<HEnvironmentMarker*>(maximum_environment_size_, zone());
  went_live_since_last_simulate_ =
      new(zone()) BitVector(maximum_environment_size_, zone());

  for (int i = 0; i < block_count_; ++i) {
    live_at_block_start_->Add(
        new(zone()) BitVector(maximum_environment_size_, zone()), zone());
    first_simulate_->Add(NULL, zone());
    first_simulate_invalid_for_index_->Add(
        new(zone()) BitVector(maximum_environment_size_, zone()), zone());
  }
}


void EnvironmentSlotLivenessAnalyzer::ZapEnvironmentSlot(int index,
                                                         HSimulate* simulate) {
  int operand_index = simulate->ToOperandIndex(index);
  if (operand_index == -1) {
    simulate->AddAssignedValue(index, graph_->GetConstantUndefined());
  } else {
    simulate->SetOperandAt(operand_index, graph_->GetConstantUndefined());
  }
}


void EnvironmentSlotLivenessAnalyzer::ZapEnvironmentSlotsInSuccessors(
    HBasicBlock* block,
    BitVector* live) {
  // When a value is live in successor A but dead in B, we must
  // explicitly zap it in B.
  for (HSuccessorIterator it(block->end()); !it.Done(); it.Advance()) {
    HBasicBlock* successor = it.Current();
    int successor_id = successor->block_id();
    BitVector* live_in_successor = live_at_block_start_->at(successor_id);
    if (live_in_successor->Equals(*live)) continue;
    for (int i = 0; i < live->length(); ++i) {
      if (!live->Contains(i)) continue;
      if (live_in_successor->Contains(i)) continue;
      if (first_simulate_invalid_for_index_->at(successor_id)->Contains(i)) {
        continue;
      }
      HSimulate* simulate = first_simulate_->at(successor_id);
      if (simulate == NULL) continue;
      ASSERT(simulate->closure().is_identical_to(
                 block->last_environment()->closure()));
      ZapEnvironmentSlot(i, simulate);
    }
  }
}


void EnvironmentSlotLivenessAnalyzer::ZapEnvironmentSlotsForInstruction(
    HEnvironmentMarker* marker) {
  if (!marker->CheckFlag(HValue::kEndsLiveRange)) return;
  HSimulate* simulate = marker->next_simulate();
  if (simulate != NULL) {
    ASSERT(simulate->closure().is_identical_to(marker->closure()));
    ZapEnvironmentSlot(marker->index(), simulate);
  }
}


void EnvironmentSlotLivenessAnalyzer::UpdateLivenessAtBlockEnd(
    HBasicBlock* block,
    BitVector* live) {
  // Liveness at the end of each block: union of liveness in successors.
  live->Clear();
  for (HSuccessorIterator it(block->end()); !it.Done(); it.Advance()) {
    live->Union(*live_at_block_start_->at(it.Current()->block_id()));
  }
}


void EnvironmentSlotLivenessAnalyzer::UpdateLivenessAtInstruction(
    HInstruction* instr,
    BitVector* live) {
  switch (instr->opcode()) {
    case HValue::kEnvironmentMarker: {
      HEnvironmentMarker* marker = HEnvironmentMarker::cast(instr);
      int index = marker->index();
      if (!live->Contains(index)) {
        marker->SetFlag(HValue::kEndsLiveRange);
      } else {
        marker->ClearFlag(HValue::kEndsLiveRange);
      }
      if (!went_live_since_last_simulate_->Contains(index)) {
        marker->set_next_simulate(last_simulate_);
      }
      if (marker->kind() == HEnvironmentMarker::LOOKUP) {
        live->Add(index);
      } else {
        ASSERT(marker->kind() == HEnvironmentMarker::BIND);
        live->Remove(index);
        went_live_since_last_simulate_->Add(index);
      }
      if (collect_markers_) {
        // Populate |markers_| list during the first pass.
        markers_->Add(marker, &zone_);
      }
      break;
    }
    case HValue::kLeaveInlined:
      // No environment values are live at the end of an inlined section.
      live->Clear();
      last_simulate_ = NULL;

      // The following ASSERTs guard the assumption used in case
      // kEnterInlined below:
      ASSERT(instr->next()->IsSimulate());
      ASSERT(instr->next()->next()->IsGoto());

      break;
    case HValue::kEnterInlined: {
      // Those environment values are live that are live at any return
      // target block. Here we make use of the fact that the end of an
      // inline sequence always looks like this: HLeaveInlined, HSimulate,
      // HGoto (to return_target block), with no environment lookups in
      // between (see ASSERTs above).
      HEnterInlined* enter = HEnterInlined::cast(instr);
      live->Clear();
      for (int i = 0; i < enter->return_targets()->length(); ++i) {
        int return_id = enter->return_targets()->at(i)->block_id();
        // When an AbnormalExit is involved, it can happen that the return
        // target block doesn't actually exist.
        if (return_id < live_at_block_start_->length()) {
          live->Union(*live_at_block_start_->at(return_id));
        }
      }
      last_simulate_ = NULL;
      break;
    }
    case HValue::kDeoptimize: {
      // Keep all environment slots alive.
      HDeoptimize* deopt = HDeoptimize::cast(instr);
      for (int i = deopt->first_local_index();
           i < deopt->first_expression_index(); ++i) {
        live->Add(i);
      }
      break;
    }
    case HValue::kSimulate:
      last_simulate_ = HSimulate::cast(instr);
      went_live_since_last_simulate_->Clear();
      break;
    default:
      break;
  }
}


void EnvironmentSlotLivenessAnalyzer::AnalyzeAndTrim() {
  HPhase phase("H_EnvironmentLivenessAnalysis", graph_);
  if (maximum_environment_size_ == 0) return;

  // Main iteration. Compute liveness of environment slots, and store it
  // for each block until it doesn't change any more. For efficiency, visit
  // blocks in reverse order and walk backwards through each block. We
  // need several iterations to propagate liveness through nested loops.
  BitVector* live = new(zone()) BitVector(maximum_environment_size_, zone());
  BitVector* worklist = new(zone()) BitVector(block_count_, zone());
  for (int i = 0; i < block_count_; ++i) {
    worklist->Add(i);
  }
  while (!worklist->IsEmpty()) {
    for (int block_id = block_count_ - 1; block_id >= 0; --block_id) {
      if (!worklist->Contains(block_id)) {
        continue;
      }
      worklist->Remove(block_id);
      last_simulate_ = NULL;

      HBasicBlock* block = graph_->blocks()->at(block_id);
      UpdateLivenessAtBlockEnd(block, live);

      for (HInstruction* instr = block->last(); instr != NULL;
           instr = instr->previous()) {
        UpdateLivenessAtInstruction(instr, live);
      }

      // Reached the start of the block, do necessary bookkeeping:
      // store computed information for this block and add predecessors
      // to the work list as necessary.
      first_simulate_->Set(block_id, last_simulate_);
      first_simulate_invalid_for_index_->at(block_id)->CopyFrom(
          *went_live_since_last_simulate_);
      if (live_at_block_start_->at(block_id)->UnionIsChanged(*live)) {
        for (int i = 0; i < block->predecessors()->length(); ++i) {
          worklist->Add(block->predecessors()->at(i)->block_id());
        }
        if (block->IsInlineReturnTarget()) {
          worklist->Add(block->inlined_entry_block()->block_id());
        }
      }
    }
    // Only collect bind/lookup instructions during the first pass.
    collect_markers_ = false;
  }

  // Analysis finished. Zap dead environment slots.
  for (int i = 0; i < markers_->length(); ++i) {
    ZapEnvironmentSlotsForInstruction(markers_->at(i));
  }
  for (int block_id = block_count_ - 1; block_id >= 0; --block_id) {
    HBasicBlock* block = graph_->blocks()->at(block_id);
    UpdateLivenessAtBlockEnd(block, live);
    ZapEnvironmentSlotsInSuccessors(block, live);
  }

  // Finally, remove the HEnvironment{Bind,Lookup} markers.
  for (int i = 0; i < markers_->length(); ++i) {
    markers_->at(i)->DeleteAndReplaceWith(NULL);
  }
}

} }  // namespace v8::internal