v8/src/hydrogen-bce.cc

// 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-bce.h"

namespace v8 {
namespace internal {


// We try to "factor up" HBoundsCheck instructions towards the root of the
// dominator tree.
// For now we handle checks where the index is like "exp + int32value".
// If in the dominator tree we check "exp + v1" and later (dominated)
// "exp + v2", if v2 <= v1 we can safely remove the second check, and if
// v2 > v1 we can use v2 in the 1st check and again remove the second.
// To do so we keep a dictionary of all checks where the key if the pair
// "exp, length".
// The class BoundsCheckKey represents this key.
class BoundsCheckKey : public ZoneObject {
 public:
  HValue* IndexBase() const { return index_base_; }
  HValue* Length() const { return length_; }

  uint32_t Hash() {
    return static_cast<uint32_t>(index_base_->Hashcode() ^ length_->Hashcode());
  }

  static BoundsCheckKey* Create(Zone* zone,
                                HBoundsCheck* check,
                                int32_t* offset) {
    if (!check->index()->representation().IsSmiOrInteger32()) return NULL;

    HValue* index_base = NULL;
    HConstant* constant = NULL;
    bool is_sub = false;

    if (check->index()->IsAdd()) {
      HAdd* index = HAdd::cast(check->index());
      if (index->left()->IsConstant()) {
        constant = HConstant::cast(index->left());
        index_base = index->right();
      } else if (index->right()->IsConstant()) {
        constant = HConstant::cast(index->right());
        index_base = index->left();
      }
    } else if (check->index()->IsSub()) {
      HSub* index = HSub::cast(check->index());
      is_sub = true;
      if (index->right()->IsConstant()) {
        constant = HConstant::cast(index->right());
        index_base = index->left();
      }
    } else if (check->index()->IsConstant()) {
      index_base = check->block()->graph()->GetConstant0();
      constant = HConstant::cast(check->index());
    }

    if (constant != NULL && constant->HasInteger32Value()) {
      *offset = is_sub ? - constant->Integer32Value()
                       : constant->Integer32Value();
    } else {
      *offset = 0;
      index_base = check->index();
    }

    return new(zone) BoundsCheckKey(index_base, check->length());
  }

 private:
  BoundsCheckKey(HValue* index_base, HValue* length)
      : index_base_(index_base),
        length_(length) { }

  HValue* index_base_;
  HValue* length_;

  DISALLOW_COPY_AND_ASSIGN(BoundsCheckKey);
};


// Data about each HBoundsCheck that can be eliminated or moved.
// It is the "value" in the dictionary indexed by "base-index, length"
// (the key is BoundsCheckKey).
// We scan the code with a dominator tree traversal.
// Traversing the dominator tree we keep a stack (implemented as a singly
// linked list) of "data" for each basic block that contains a relevant check
// with the same key (the dictionary holds the head of the list).
// We also keep all the "data" created for a given basic block in a list, and
// use it to "clean up" the dictionary when backtracking in the dominator tree
// traversal.
// Doing this each dictionary entry always directly points to the check that
// is dominating the code being examined now.
// We also track the current "offset" of the index expression and use it to
// decide if any check is already "covered" (so it can be removed) or not.
class BoundsCheckBbData: public ZoneObject {
 public:
  BoundsCheckKey* Key() const { return key_; }
  int32_t LowerOffset() const { return lower_offset_; }
  int32_t UpperOffset() const { return upper_offset_; }
  HBasicBlock* BasicBlock() const { return basic_block_; }
  HBoundsCheck* LowerCheck() const { return lower_check_; }
  HBoundsCheck* UpperCheck() const { return upper_check_; }
  BoundsCheckBbData* NextInBasicBlock() const { return next_in_bb_; }
  BoundsCheckBbData* FatherInDominatorTree() const { return father_in_dt_; }

  bool OffsetIsCovered(int32_t offset) const {
    return offset >= LowerOffset() && offset <= UpperOffset();
  }

  bool HasSingleCheck() { return lower_check_ == upper_check_; }

  void UpdateUpperOffsets(HBoundsCheck* check, int32_t offset) {
    BoundsCheckBbData* data = FatherInDominatorTree();
    while (data != NULL && data->UpperCheck() == check) {
      DCHECK(data->upper_offset_ < offset);
      data->upper_offset_ = offset;
      data = data->FatherInDominatorTree();
    }
  }

  void UpdateLowerOffsets(HBoundsCheck* check, int32_t offset) {
    BoundsCheckBbData* data = FatherInDominatorTree();
    while (data != NULL && data->LowerCheck() == check) {
      DCHECK(data->lower_offset_ > offset);
      data->lower_offset_ = offset;
      data = data->FatherInDominatorTree();
    }
  }

  // The goal of this method is to modify either upper_offset_ or
  // lower_offset_ so that also new_offset is covered (the covered
  // range grows).
  //
  // The precondition is that new_check follows UpperCheck() and
  // LowerCheck() in the same basic block, and that new_offset is not
  // covered (otherwise we could simply remove new_check).
  //
  // If HasSingleCheck() is true then new_check is added as "second check"
  // (either upper or lower; note that HasSingleCheck() becomes false).
  // Otherwise one of the current checks is modified so that it also covers
  // new_offset, and new_check is removed.
  void CoverCheck(HBoundsCheck* new_check,
                  int32_t new_offset) {
    DCHECK(new_check->index()->representation().IsSmiOrInteger32());
    bool keep_new_check = false;

    if (new_offset > upper_offset_) {
      upper_offset_ = new_offset;
      if (HasSingleCheck()) {
        keep_new_check = true;
        upper_check_ = new_check;
      } else {
        TightenCheck(upper_check_, new_check, new_offset);
        UpdateUpperOffsets(upper_check_, upper_offset_);
      }
    } else if (new_offset < lower_offset_) {
      lower_offset_ = new_offset;
      if (HasSingleCheck()) {
        keep_new_check = true;
        lower_check_ = new_check;
      } else {
        TightenCheck(lower_check_, new_check, new_offset);
        UpdateLowerOffsets(lower_check_, lower_offset_);
      }
    } else {
      // Should never have called CoverCheck() in this case.
      UNREACHABLE();
    }

    if (!keep_new_check) {
      if (FLAG_trace_bce) {
        base::OS::Print("Eliminating check #%d after tightening\n",
                        new_check->id());
      }
      new_check->block()->graph()->isolate()->counters()->
          bounds_checks_eliminated()->Increment();
      new_check->DeleteAndReplaceWith(new_check->ActualValue());
    } else {
      HBoundsCheck* first_check = new_check == lower_check_ ? upper_check_
                                                            : lower_check_;
      if (FLAG_trace_bce) {
        base::OS::Print("Moving second check #%d after first check #%d\n",
                        new_check->id(), first_check->id());
      }
      // The length is guaranteed to be live at first_check.
      DCHECK(new_check->length() == first_check->length());
      HInstruction* old_position = new_check->next();
      new_check->Unlink();
      new_check->InsertAfter(first_check);
      MoveIndexIfNecessary(new_check->index(), new_check, old_position);
    }
  }

  BoundsCheckBbData(BoundsCheckKey* key,
                    int32_t lower_offset,
                    int32_t upper_offset,
                    HBasicBlock* bb,
                    HBoundsCheck* lower_check,
                    HBoundsCheck* upper_check,
                    BoundsCheckBbData* next_in_bb,
                    BoundsCheckBbData* father_in_dt)
      : key_(key),
        lower_offset_(lower_offset),
        upper_offset_(upper_offset),
        basic_block_(bb),
        lower_check_(lower_check),
        upper_check_(upper_check),
        next_in_bb_(next_in_bb),
        father_in_dt_(father_in_dt) { }

 private:
  BoundsCheckKey* key_;
  int32_t lower_offset_;
  int32_t upper_offset_;
  HBasicBlock* basic_block_;
  HBoundsCheck* lower_check_;
  HBoundsCheck* upper_check_;
  BoundsCheckBbData* next_in_bb_;
  BoundsCheckBbData* father_in_dt_;

  void MoveIndexIfNecessary(HValue* index_raw,
                            HBoundsCheck* insert_before,
                            HInstruction* end_of_scan_range) {
    // index_raw can be HAdd(index_base, offset), HSub(index_base, offset),
    // HConstant(offset) or index_base directly.
    // In the latter case, no need to move anything.
    if (index_raw->IsAdd() || index_raw->IsSub()) {
      HArithmeticBinaryOperation* index =
          HArithmeticBinaryOperation::cast(index_raw);
      HValue* left_input = index->left();
      HValue* right_input = index->right();
      bool must_move_index = false;
      bool must_move_left_input = false;
      bool must_move_right_input = false;
      for (HInstruction* cursor = end_of_scan_range; cursor != insert_before;) {
        if (cursor == left_input) must_move_left_input = true;
        if (cursor == right_input) must_move_right_input = true;
        if (cursor == index) must_move_index = true;
        if (cursor->previous() == NULL) {
          cursor = cursor->block()->dominator()->end();
        } else {
          cursor = cursor->previous();
        }
      }
      if (must_move_index) {
        index->Unlink();
        index->InsertBefore(insert_before);
      }
      // The BCE algorithm only selects mergeable bounds checks that share
      // the same "index_base", so we'll only ever have to move constants.
      if (must_move_left_input) {
        HConstant::cast(left_input)->Unlink();
        HConstant::cast(left_input)->InsertBefore(index);
      }
      if (must_move_right_input) {
        HConstant::cast(right_input)->Unlink();
        HConstant::cast(right_input)->InsertBefore(index);
      }
    } else if (index_raw->IsConstant()) {
      HConstant* index = HConstant::cast(index_raw);
      bool must_move = false;
      for (HInstruction* cursor = end_of_scan_range; cursor != insert_before;) {
        if (cursor == index) must_move = true;
        if (cursor->previous() == NULL) {
          cursor = cursor->block()->dominator()->end();
        } else {
          cursor = cursor->previous();
        }
      }
      if (must_move) {
        index->Unlink();
        index->InsertBefore(insert_before);
      }
    }
  }

  void TightenCheck(HBoundsCheck* original_check,
                    HBoundsCheck* tighter_check,
                    int32_t new_offset) {
    DCHECK(original_check->length() == tighter_check->length());
    MoveIndexIfNecessary(tighter_check->index(), original_check, tighter_check);
    original_check->ReplaceAllUsesWith(original_check->index());
    original_check->SetOperandAt(0, tighter_check->index());
    if (FLAG_trace_bce) {
      base::OS::Print("Tightened check #%d with offset %d from #%d\n",
                      original_check->id(), new_offset, tighter_check->id());
    }
  }

  DISALLOW_COPY_AND_ASSIGN(BoundsCheckBbData);
};


static bool BoundsCheckKeyMatch(void* key1, void* key2) {
  BoundsCheckKey* k1 = static_cast<BoundsCheckKey*>(key1);
  BoundsCheckKey* k2 = static_cast<BoundsCheckKey*>(key2);
  return k1->IndexBase() == k2->IndexBase() && k1->Length() == k2->Length();
}


BoundsCheckTable::BoundsCheckTable(Zone* zone)
    : ZoneHashMap(BoundsCheckKeyMatch, ZoneHashMap::kDefaultHashMapCapacity,
                  ZoneAllocationPolicy(zone)) { }


BoundsCheckBbData** BoundsCheckTable::LookupOrInsert(BoundsCheckKey* key,
                                                     Zone* zone) {
  return reinterpret_cast<BoundsCheckBbData**>(
      &(Lookup(key, key->Hash(), true, ZoneAllocationPolicy(zone))->value));
}


void BoundsCheckTable::Insert(BoundsCheckKey* key,
                              BoundsCheckBbData* data,
                              Zone* zone) {
  Lookup(key, key->Hash(), true, ZoneAllocationPolicy(zone))->value = data;
}


void BoundsCheckTable::Delete(BoundsCheckKey* key) {
  Remove(key, key->Hash());
}


class HBoundsCheckEliminationState {
 public:
  HBasicBlock* block_;
  BoundsCheckBbData* bb_data_list_;
  int index_;
};


// Eliminates checks in bb and recursively in the dominated blocks.
// Also replace the results of check instructions with the original value, if
// the result is used. This is safe now, since we don't do code motion after
// this point. It enables better register allocation since the value produced
// by check instructions is really a copy of the original value.
void HBoundsCheckEliminationPhase::EliminateRedundantBoundsChecks(
    HBasicBlock* entry) {
  // Allocate the stack.
  HBoundsCheckEliminationState* stack =
    zone()->NewArray<HBoundsCheckEliminationState>(graph()->blocks()->length());

  // Explicitly push the entry block.
  stack[0].block_ = entry;
  stack[0].bb_data_list_ = PreProcessBlock(entry);
  stack[0].index_ = 0;
  int stack_depth = 1;

  // Implement depth-first traversal with a stack.
  while (stack_depth > 0) {
    int current = stack_depth - 1;
    HBoundsCheckEliminationState* state = &stack[current];
    const ZoneList<HBasicBlock*>* children = state->block_->dominated_blocks();

    if (state->index_ < children->length()) {
      // Recursively visit children blocks.
      HBasicBlock* child = children->at(state->index_++);
      int next = stack_depth++;
      stack[next].block_ = child;
      stack[next].bb_data_list_ = PreProcessBlock(child);
      stack[next].index_ = 0;
    } else {
      // Finished with all children; post process the block.
      PostProcessBlock(state->block_, state->bb_data_list_);
      stack_depth--;
    }
  }
}


BoundsCheckBbData* HBoundsCheckEliminationPhase::PreProcessBlock(
    HBasicBlock* bb) {
  BoundsCheckBbData* bb_data_list = NULL;

  for (HInstructionIterator it(bb); !it.Done(); it.Advance()) {
    HInstruction* i = it.Current();
    if (!i->IsBoundsCheck()) continue;

    HBoundsCheck* check = HBoundsCheck::cast(i);
    int32_t offset;
    BoundsCheckKey* key =
        BoundsCheckKey::Create(zone(), check, &offset);
    if (key == NULL) continue;
    BoundsCheckBbData** data_p = table_.LookupOrInsert(key, zone());
    BoundsCheckBbData* data = *data_p;
    if (data == NULL) {
      bb_data_list = new(zone()) BoundsCheckBbData(key,
                                                   offset,
                                                   offset,
                                                   bb,
                                                   check,
                                                   check,
                                                   bb_data_list,
                                                   NULL);
      *data_p = bb_data_list;
      if (FLAG_trace_bce) {
        base::OS::Print("Fresh bounds check data for block #%d: [%d]\n",
                        bb->block_id(), offset);
      }
    } else if (data->OffsetIsCovered(offset)) {
      bb->graph()->isolate()->counters()->
          bounds_checks_eliminated()->Increment();
      if (FLAG_trace_bce) {
        base::OS::Print("Eliminating bounds check #%d, offset %d is covered\n",
                        check->id(), offset);
      }
      check->DeleteAndReplaceWith(check->ActualValue());
    } else if (data->BasicBlock() == bb) {
      // TODO(jkummerow): I think the following logic would be preferable:
      // if (data->Basicblock() == bb ||
      //     graph()->use_optimistic_licm() ||
      //     bb->IsLoopSuccessorDominator()) {
      //   data->CoverCheck(check, offset)
      // } else {
      //   /* add pristine BCBbData like in (data == NULL) case above */
      // }
      // Even better would be: distinguish between read-only dominator-imposed
      // knowledge and modifiable upper/lower checks.
      // What happens currently is that the first bounds check in a dominated
      // block will stay around while any further checks are hoisted out,
      // which doesn't make sense. Investigate/fix this in a future CL.
      data->CoverCheck(check, offset);
    } else if (graph()->use_optimistic_licm() ||
               bb->IsLoopSuccessorDominator()) {
      int32_t new_lower_offset = offset < data->LowerOffset()
          ? offset
          : data->LowerOffset();
      int32_t new_upper_offset = offset > data->UpperOffset()
          ? offset
          : data->UpperOffset();
      bb_data_list = new(zone()) BoundsCheckBbData(key,
                                                   new_lower_offset,
                                                   new_upper_offset,
                                                   bb,
                                                   data->LowerCheck(),
                                                   data->UpperCheck(),
                                                   bb_data_list,
                                                   data);
      if (FLAG_trace_bce) {
        base::OS::Print("Updated bounds check data for block #%d: [%d - %d]\n",
                        bb->block_id(), new_lower_offset, new_upper_offset);
      }
      table_.Insert(key, bb_data_list, zone());
    }
  }

  return bb_data_list;
}


void HBoundsCheckEliminationPhase::PostProcessBlock(
    HBasicBlock* block, BoundsCheckBbData* data) {
  while (data != NULL) {
    if (data->FatherInDominatorTree()) {
      table_.Insert(data->Key(), data->FatherInDominatorTree(), zone());
    } else {
      table_.Delete(data->Key());
    }
    data = data->NextInBasicBlock();
  }
}

} }  // namespace v8::internal