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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/hydrogen-bce.h"
      6 
      7 namespace v8 {
      8 namespace internal {
      9 
     10 
     11 // We try to "factor up" HBoundsCheck instructions towards the root of the
     12 // dominator tree.
     13 // For now we handle checks where the index is like "exp + int32value".
     14 // If in the dominator tree we check "exp + v1" and later (dominated)
     15 // "exp + v2", if v2 <= v1 we can safely remove the second check, and if
     16 // v2 > v1 we can use v2 in the 1st check and again remove the second.
     17 // To do so we keep a dictionary of all checks where the key if the pair
     18 // "exp, length".
     19 // The class BoundsCheckKey represents this key.
     20 class BoundsCheckKey : public ZoneObject {
     21  public:
     22   HValue* IndexBase() const { return index_base_; }
     23   HValue* Length() const { return length_; }
     24 
     25   uint32_t Hash() {
     26     return static_cast<uint32_t>(index_base_->Hashcode() ^ length_->Hashcode());
     27   }
     28 
     29   static BoundsCheckKey* Create(Zone* zone,
     30                                 HBoundsCheck* check,
     31                                 int32_t* offset) {
     32     if (!check->index()->representation().IsSmiOrInteger32()) return NULL;
     33 
     34     HValue* index_base = NULL;
     35     HConstant* constant = NULL;
     36     bool is_sub = false;
     37 
     38     if (check->index()->IsAdd()) {
     39       HAdd* index = HAdd::cast(check->index());
     40       if (index->left()->IsConstant()) {
     41         constant = HConstant::cast(index->left());
     42         index_base = index->right();
     43       } else if (index->right()->IsConstant()) {
     44         constant = HConstant::cast(index->right());
     45         index_base = index->left();
     46       }
     47     } else if (check->index()->IsSub()) {
     48       HSub* index = HSub::cast(check->index());
     49       is_sub = true;
     50       if (index->right()->IsConstant()) {
     51         constant = HConstant::cast(index->right());
     52         index_base = index->left();
     53       }
     54     } else if (check->index()->IsConstant()) {
     55       index_base = check->block()->graph()->GetConstant0();
     56       constant = HConstant::cast(check->index());
     57     }
     58 
     59     if (constant != NULL && constant->HasInteger32Value()) {
     60       *offset = is_sub ? - constant->Integer32Value()
     61                        : constant->Integer32Value();
     62     } else {
     63       *offset = 0;
     64       index_base = check->index();
     65     }
     66 
     67     return new(zone) BoundsCheckKey(index_base, check->length());
     68   }
     69 
     70  private:
     71   BoundsCheckKey(HValue* index_base, HValue* length)
     72       : index_base_(index_base),
     73         length_(length) { }
     74 
     75   HValue* index_base_;
     76   HValue* length_;
     77 
     78   DISALLOW_COPY_AND_ASSIGN(BoundsCheckKey);
     79 };
     80 
     81 
     82 // Data about each HBoundsCheck that can be eliminated or moved.
     83 // It is the "value" in the dictionary indexed by "base-index, length"
     84 // (the key is BoundsCheckKey).
     85 // We scan the code with a dominator tree traversal.
     86 // Traversing the dominator tree we keep a stack (implemented as a singly
     87 // linked list) of "data" for each basic block that contains a relevant check
     88 // with the same key (the dictionary holds the head of the list).
     89 // We also keep all the "data" created for a given basic block in a list, and
     90 // use it to "clean up" the dictionary when backtracking in the dominator tree
     91 // traversal.
     92 // Doing this each dictionary entry always directly points to the check that
     93 // is dominating the code being examined now.
     94 // We also track the current "offset" of the index expression and use it to
     95 // decide if any check is already "covered" (so it can be removed) or not.
     96 class BoundsCheckBbData: public ZoneObject {
     97  public:
     98   BoundsCheckKey* Key() const { return key_; }
     99   int32_t LowerOffset() const { return lower_offset_; }
    100   int32_t UpperOffset() const { return upper_offset_; }
    101   HBasicBlock* BasicBlock() const { return basic_block_; }
    102   HBoundsCheck* LowerCheck() const { return lower_check_; }
    103   HBoundsCheck* UpperCheck() const { return upper_check_; }
    104   BoundsCheckBbData* NextInBasicBlock() const { return next_in_bb_; }
    105   BoundsCheckBbData* FatherInDominatorTree() const { return father_in_dt_; }
    106 
    107   bool OffsetIsCovered(int32_t offset) const {
    108     return offset >= LowerOffset() && offset <= UpperOffset();
    109   }
    110 
    111   bool HasSingleCheck() { return lower_check_ == upper_check_; }
    112 
    113   void UpdateUpperOffsets(HBoundsCheck* check, int32_t offset) {
    114     BoundsCheckBbData* data = FatherInDominatorTree();
    115     while (data != NULL && data->UpperCheck() == check) {
    116       ASSERT(data->upper_offset_ < offset);
    117       data->upper_offset_ = offset;
    118       data = data->FatherInDominatorTree();
    119     }
    120   }
    121 
    122   void UpdateLowerOffsets(HBoundsCheck* check, int32_t offset) {
    123     BoundsCheckBbData* data = FatherInDominatorTree();
    124     while (data != NULL && data->LowerCheck() == check) {
    125       ASSERT(data->lower_offset_ > offset);
    126       data->lower_offset_ = offset;
    127       data = data->FatherInDominatorTree();
    128     }
    129   }
    130 
    131   // The goal of this method is to modify either upper_offset_ or
    132   // lower_offset_ so that also new_offset is covered (the covered
    133   // range grows).
    134   //
    135   // The precondition is that new_check follows UpperCheck() and
    136   // LowerCheck() in the same basic block, and that new_offset is not
    137   // covered (otherwise we could simply remove new_check).
    138   //
    139   // If HasSingleCheck() is true then new_check is added as "second check"
    140   // (either upper or lower; note that HasSingleCheck() becomes false).
    141   // Otherwise one of the current checks is modified so that it also covers
    142   // new_offset, and new_check is removed.
    143   void CoverCheck(HBoundsCheck* new_check,
    144                   int32_t new_offset) {
    145     ASSERT(new_check->index()->representation().IsSmiOrInteger32());
    146     bool keep_new_check = false;
    147 
    148     if (new_offset > upper_offset_) {
    149       upper_offset_ = new_offset;
    150       if (HasSingleCheck()) {
    151         keep_new_check = true;
    152         upper_check_ = new_check;
    153       } else {
    154         TightenCheck(upper_check_, new_check, new_offset);
    155         UpdateUpperOffsets(upper_check_, upper_offset_);
    156       }
    157     } else if (new_offset < lower_offset_) {
    158       lower_offset_ = new_offset;
    159       if (HasSingleCheck()) {
    160         keep_new_check = true;
    161         lower_check_ = new_check;
    162       } else {
    163         TightenCheck(lower_check_, new_check, new_offset);
    164         UpdateLowerOffsets(lower_check_, lower_offset_);
    165       }
    166     } else {
    167       // Should never have called CoverCheck() in this case.
    168       UNREACHABLE();
    169     }
    170 
    171     if (!keep_new_check) {
    172       if (FLAG_trace_bce) {
    173         OS::Print("Eliminating check #%d after tightening\n",
    174                   new_check->id());
    175       }
    176       new_check->block()->graph()->isolate()->counters()->
    177           bounds_checks_eliminated()->Increment();
    178       new_check->DeleteAndReplaceWith(new_check->ActualValue());
    179     } else {
    180       HBoundsCheck* first_check = new_check == lower_check_ ? upper_check_
    181                                                             : lower_check_;
    182       if (FLAG_trace_bce) {
    183         OS::Print("Moving second check #%d after first check #%d\n",
    184                   new_check->id(), first_check->id());
    185       }
    186       // The length is guaranteed to be live at first_check.
    187       ASSERT(new_check->length() == first_check->length());
    188       HInstruction* old_position = new_check->next();
    189       new_check->Unlink();
    190       new_check->InsertAfter(first_check);
    191       MoveIndexIfNecessary(new_check->index(), new_check, old_position);
    192     }
    193   }
    194 
    195   BoundsCheckBbData(BoundsCheckKey* key,
    196                     int32_t lower_offset,
    197                     int32_t upper_offset,
    198                     HBasicBlock* bb,
    199                     HBoundsCheck* lower_check,
    200                     HBoundsCheck* upper_check,
    201                     BoundsCheckBbData* next_in_bb,
    202                     BoundsCheckBbData* father_in_dt)
    203       : key_(key),
    204         lower_offset_(lower_offset),
    205         upper_offset_(upper_offset),
    206         basic_block_(bb),
    207         lower_check_(lower_check),
    208         upper_check_(upper_check),
    209         next_in_bb_(next_in_bb),
    210         father_in_dt_(father_in_dt) { }
    211 
    212  private:
    213   BoundsCheckKey* key_;
    214   int32_t lower_offset_;
    215   int32_t upper_offset_;
    216   HBasicBlock* basic_block_;
    217   HBoundsCheck* lower_check_;
    218   HBoundsCheck* upper_check_;
    219   BoundsCheckBbData* next_in_bb_;
    220   BoundsCheckBbData* father_in_dt_;
    221 
    222   void MoveIndexIfNecessary(HValue* index_raw,
    223                             HBoundsCheck* insert_before,
    224                             HInstruction* end_of_scan_range) {
    225     // index_raw can be HAdd(index_base, offset), HSub(index_base, offset),
    226     // HConstant(offset) or index_base directly.
    227     // In the latter case, no need to move anything.
    228     if (index_raw->IsAdd() || index_raw->IsSub()) {
    229       HArithmeticBinaryOperation* index =
    230           HArithmeticBinaryOperation::cast(index_raw);
    231       HValue* left_input = index->left();
    232       HValue* right_input = index->right();
    233       bool must_move_index = false;
    234       bool must_move_left_input = false;
    235       bool must_move_right_input = false;
    236       for (HInstruction* cursor = end_of_scan_range; cursor != insert_before;) {
    237         if (cursor == left_input) must_move_left_input = true;
    238         if (cursor == right_input) must_move_right_input = true;
    239         if (cursor == index) must_move_index = true;
    240         if (cursor->previous() == NULL) {
    241           cursor = cursor->block()->dominator()->end();
    242         } else {
    243           cursor = cursor->previous();
    244         }
    245       }
    246       if (must_move_index) {
    247         index->Unlink();
    248         index->InsertBefore(insert_before);
    249       }
    250       // The BCE algorithm only selects mergeable bounds checks that share
    251       // the same "index_base", so we'll only ever have to move constants.
    252       if (must_move_left_input) {
    253         HConstant::cast(left_input)->Unlink();
    254         HConstant::cast(left_input)->InsertBefore(index);
    255       }
    256       if (must_move_right_input) {
    257         HConstant::cast(right_input)->Unlink();
    258         HConstant::cast(right_input)->InsertBefore(index);
    259       }
    260     } else if (index_raw->IsConstant()) {
    261       HConstant* index = HConstant::cast(index_raw);
    262       bool must_move = false;
    263       for (HInstruction* cursor = end_of_scan_range; cursor != insert_before;) {
    264         if (cursor == index) must_move = true;
    265         if (cursor->previous() == NULL) {
    266           cursor = cursor->block()->dominator()->end();
    267         } else {
    268           cursor = cursor->previous();
    269         }
    270       }
    271       if (must_move) {
    272         index->Unlink();
    273         index->InsertBefore(insert_before);
    274       }
    275     }
    276   }
    277 
    278   void TightenCheck(HBoundsCheck* original_check,
    279                     HBoundsCheck* tighter_check,
    280                     int32_t new_offset) {
    281     ASSERT(original_check->length() == tighter_check->length());
    282     MoveIndexIfNecessary(tighter_check->index(), original_check, tighter_check);
    283     original_check->ReplaceAllUsesWith(original_check->index());
    284     original_check->SetOperandAt(0, tighter_check->index());
    285     if (FLAG_trace_bce) {
    286       OS::Print("Tightened check #%d with offset %d from #%d\n",
    287                 original_check->id(), new_offset, tighter_check->id());
    288     }
    289   }
    290 
    291   DISALLOW_COPY_AND_ASSIGN(BoundsCheckBbData);
    292 };
    293 
    294 
    295 static bool BoundsCheckKeyMatch(void* key1, void* key2) {
    296   BoundsCheckKey* k1 = static_cast<BoundsCheckKey*>(key1);
    297   BoundsCheckKey* k2 = static_cast<BoundsCheckKey*>(key2);
    298   return k1->IndexBase() == k2->IndexBase() && k1->Length() == k2->Length();
    299 }
    300 
    301 
    302 BoundsCheckTable::BoundsCheckTable(Zone* zone)
    303     : ZoneHashMap(BoundsCheckKeyMatch, ZoneHashMap::kDefaultHashMapCapacity,
    304                   ZoneAllocationPolicy(zone)) { }
    305 
    306 
    307 BoundsCheckBbData** BoundsCheckTable::LookupOrInsert(BoundsCheckKey* key,
    308                                                      Zone* zone) {
    309   return reinterpret_cast<BoundsCheckBbData**>(
    310       &(Lookup(key, key->Hash(), true, ZoneAllocationPolicy(zone))->value));
    311 }
    312 
    313 
    314 void BoundsCheckTable::Insert(BoundsCheckKey* key,
    315                               BoundsCheckBbData* data,
    316                               Zone* zone) {
    317   Lookup(key, key->Hash(), true, ZoneAllocationPolicy(zone))->value = data;
    318 }
    319 
    320 
    321 void BoundsCheckTable::Delete(BoundsCheckKey* key) {
    322   Remove(key, key->Hash());
    323 }
    324 
    325 
    326 class HBoundsCheckEliminationState {
    327  public:
    328   HBasicBlock* block_;
    329   BoundsCheckBbData* bb_data_list_;
    330   int index_;
    331 };
    332 
    333 
    334 // Eliminates checks in bb and recursively in the dominated blocks.
    335 // Also replace the results of check instructions with the original value, if
    336 // the result is used. This is safe now, since we don't do code motion after
    337 // this point. It enables better register allocation since the value produced
    338 // by check instructions is really a copy of the original value.
    339 void HBoundsCheckEliminationPhase::EliminateRedundantBoundsChecks(
    340     HBasicBlock* entry) {
    341   // Allocate the stack.
    342   HBoundsCheckEliminationState* stack =
    343     zone()->NewArray<HBoundsCheckEliminationState>(graph()->blocks()->length());
    344 
    345   // Explicitly push the entry block.
    346   stack[0].block_ = entry;
    347   stack[0].bb_data_list_ = PreProcessBlock(entry);
    348   stack[0].index_ = 0;
    349   int stack_depth = 1;
    350 
    351   // Implement depth-first traversal with a stack.
    352   while (stack_depth > 0) {
    353     int current = stack_depth - 1;
    354     HBoundsCheckEliminationState* state = &stack[current];
    355     const ZoneList<HBasicBlock*>* children = state->block_->dominated_blocks();
    356 
    357     if (state->index_ < children->length()) {
    358       // Recursively visit children blocks.
    359       HBasicBlock* child = children->at(state->index_++);
    360       int next = stack_depth++;
    361       stack[next].block_ = child;
    362       stack[next].bb_data_list_ = PreProcessBlock(child);
    363       stack[next].index_ = 0;
    364     } else {
    365       // Finished with all children; post process the block.
    366       PostProcessBlock(state->block_, state->bb_data_list_);
    367       stack_depth--;
    368     }
    369   }
    370 }
    371 
    372 
    373 BoundsCheckBbData* HBoundsCheckEliminationPhase::PreProcessBlock(
    374     HBasicBlock* bb) {
    375   BoundsCheckBbData* bb_data_list = NULL;
    376 
    377   for (HInstructionIterator it(bb); !it.Done(); it.Advance()) {
    378     HInstruction* i = it.Current();
    379     if (!i->IsBoundsCheck()) continue;
    380 
    381     HBoundsCheck* check = HBoundsCheck::cast(i);
    382     int32_t offset;
    383     BoundsCheckKey* key =
    384         BoundsCheckKey::Create(zone(), check, &offset);
    385     if (key == NULL) continue;
    386     BoundsCheckBbData** data_p = table_.LookupOrInsert(key, zone());
    387     BoundsCheckBbData* data = *data_p;
    388     if (data == NULL) {
    389       bb_data_list = new(zone()) BoundsCheckBbData(key,
    390                                                    offset,
    391                                                    offset,
    392                                                    bb,
    393                                                    check,
    394                                                    check,
    395                                                    bb_data_list,
    396                                                    NULL);
    397       *data_p = bb_data_list;
    398       if (FLAG_trace_bce) {
    399         OS::Print("Fresh bounds check data for block #%d: [%d]\n",
    400                   bb->block_id(), offset);
    401       }
    402     } else if (data->OffsetIsCovered(offset)) {
    403       bb->graph()->isolate()->counters()->
    404           bounds_checks_eliminated()->Increment();
    405       if (FLAG_trace_bce) {
    406         OS::Print("Eliminating bounds check #%d, offset %d is covered\n",
    407                   check->id(), offset);
    408       }
    409       check->DeleteAndReplaceWith(check->ActualValue());
    410     } else if (data->BasicBlock() == bb) {
    411       // TODO(jkummerow): I think the following logic would be preferable:
    412       // if (data->Basicblock() == bb ||
    413       //     graph()->use_optimistic_licm() ||
    414       //     bb->IsLoopSuccessorDominator()) {
    415       //   data->CoverCheck(check, offset)
    416       // } else {
    417       //   /* add pristine BCBbData like in (data == NULL) case above */
    418       // }
    419       // Even better would be: distinguish between read-only dominator-imposed
    420       // knowledge and modifiable upper/lower checks.
    421       // What happens currently is that the first bounds check in a dominated
    422       // block will stay around while any further checks are hoisted out,
    423       // which doesn't make sense. Investigate/fix this in a future CL.
    424       data->CoverCheck(check, offset);
    425     } else if (graph()->use_optimistic_licm() ||
    426                bb->IsLoopSuccessorDominator()) {
    427       int32_t new_lower_offset = offset < data->LowerOffset()
    428           ? offset
    429           : data->LowerOffset();
    430       int32_t new_upper_offset = offset > data->UpperOffset()
    431           ? offset
    432           : data->UpperOffset();
    433       bb_data_list = new(zone()) BoundsCheckBbData(key,
    434                                                    new_lower_offset,
    435                                                    new_upper_offset,
    436                                                    bb,
    437                                                    data->LowerCheck(),
    438                                                    data->UpperCheck(),
    439                                                    bb_data_list,
    440                                                    data);
    441       if (FLAG_trace_bce) {
    442         OS::Print("Updated bounds check data for block #%d: [%d - %d]\n",
    443                   bb->block_id(), new_lower_offset, new_upper_offset);
    444       }
    445       table_.Insert(key, bb_data_list, zone());
    446     }
    447   }
    448 
    449   return bb_data_list;
    450 }
    451 
    452 
    453 void HBoundsCheckEliminationPhase::PostProcessBlock(
    454     HBasicBlock* block, BoundsCheckBbData* data) {
    455   while (data != NULL) {
    456     if (data->FatherInDominatorTree()) {
    457       table_.Insert(data->Key(), data->FatherInDominatorTree(), zone());
    458     } else {
    459       table_.Delete(data->Key());
    460     }
    461     data = data->NextInBasicBlock();
    462   }
    463 }
    464 
    465 } }  // namespace v8::internal
    466