Turn redundant bounds checks elimination into a proper HPhase.

src/hydrogen-bce.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-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->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();
+      }
+    }
+
+    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_; }
+
+  // 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.
+  //
+  // If the check cannot be modified because the context is unknown it
+  // returns false, otherwise it returns true.
+  bool CoverCheck(HBoundsCheck* new_check,
+                  int32_t new_offset) {
+    ASSERT(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 {
+        bool result = BuildOffsetAdd(upper_check_,
+                                     &added_upper_index_,
+                                     &added_upper_offset_,
+                                     Key()->IndexBase(),
+                                     new_check->index()->representation(),
+                                     new_offset);
+        if (!result) return false;
+        upper_check_->ReplaceAllUsesWith(upper_check_->index());
+        upper_check_->SetOperandAt(0, added_upper_index_);
+      }
+    } else if (new_offset < lower_offset_) {
+      lower_offset_ = new_offset;
+      if (HasSingleCheck()) {
+        keep_new_check = true;
+        lower_check_ = new_check;
+      } else {
+        bool result = BuildOffsetAdd(lower_check_,
+                                     &added_lower_index_,
+                                     &added_lower_offset_,
+                                     Key()->IndexBase(),
+                                     new_check->index()->representation(),
+                                     new_offset);
+        if (!result) return false;
+        lower_check_->ReplaceAllUsesWith(lower_check_->index());
+        lower_check_->SetOperandAt(0, added_lower_index_);
+      }
+    } else {
+      ASSERT(false);
+    }
+
+    if (!keep_new_check) {
+      new_check->DeleteAndReplaceWith(new_check->ActualValue());
+    }
+
+    return true;
+  }
+
+  void RemoveZeroOperations() {
+    RemoveZeroAdd(&added_lower_index_, &added_lower_offset_);
+    RemoveZeroAdd(&added_upper_index_, &added_upper_offset_);
+  }
+
+  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),
+    added_lower_index_(NULL),
+    added_lower_offset_(NULL),
+    added_upper_index_(NULL),
+    added_upper_offset_(NULL),
+    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_;
+  HInstruction* added_lower_index_;
+  HConstant* added_lower_offset_;
+  HInstruction* added_upper_index_;
+  HConstant* added_upper_offset_;
+  BoundsCheckBbData* next_in_bb_;
+  BoundsCheckBbData* father_in_dt_;
+
+  // Given an existing add instruction and a bounds check it tries to
+  // find the current context (either of the add or of the check index).
+  HValue* IndexContext(HInstruction* add, HBoundsCheck* check) {
+    if (add != NULL && add->IsAdd()) {
+      return HAdd::cast(add)->context();
+    }
+    if (check->index()->IsBinaryOperation()) {
+      return HBinaryOperation::cast(check->index())->context();
+    }
+    return NULL;
+  }
+
+  // This function returns false if it cannot build the add because the
+  // current context cannot be determined.
+  bool BuildOffsetAdd(HBoundsCheck* check,
+                      HInstruction** add,
+                      HConstant** constant,
+                      HValue* original_value,
+                      Representation representation,
+                      int32_t new_offset) {
+    HValue* index_context = IndexContext(*add, check);
+    if (index_context == NULL) return false;
+
+    HConstant* new_constant = new(BasicBlock()->zone()) HConstant(
+        new_offset, representation);
+    if (*add == NULL) {
+      new_constant->InsertBefore(check);
+      (*add) = HAdd::New(
+          BasicBlock()->zone(), index_context, original_value, new_constant);
+      (*add)->AssumeRepresentation(representation);
+      (*add)->InsertBefore(check);
+    } else {
+      new_constant->InsertBefore(*add);
+      (*constant)->DeleteAndReplaceWith(new_constant);
+    }
+    *constant = new_constant;
+    return true;
+  }
+
+  void RemoveZeroAdd(HInstruction** add, HConstant** constant) {
+    if (*add != NULL && (*add)->IsAdd() && (*constant)->Integer32Value() == 0) {
+      (*add)->DeleteAndReplaceWith(HAdd::cast(*add)->left());
+      (*constant)->DeleteAndReplaceWith(NULL);
+    }
+  }
+
+  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());
+}
+
+
+// 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* 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;
+    } else if (data->OffsetIsCovered(offset)) {
+      check->DeleteAndReplaceWith(check->ActualValue());
+    } else if (data->BasicBlock() != bb ||
+               !data->CoverCheck(check, offset)) {
+      // If the check is in the current BB we try to modify it by calling
+      // "CoverCheck", but if also that fails we record the current offsets
+      // in a new data instance because from now on they are covered.
+      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);
+      table_.Insert(key, bb_data_list, zone());
+    }
+  }
+
+  for (int i = 0; i < bb->dominated_blocks()->length(); ++i) {
+    EliminateRedundantBoundsChecks(bb->dominated_blocks()->at(i));
+  }
+
+  for (BoundsCheckBbData* data = bb_data_list;
+       data != NULL;
+       data = data->NextInBasicBlock()) {
+    data->RemoveZeroOperations();
+    if (data->FatherInDominatorTree()) {
+      table_.Insert(data->Key(), data->FatherInDominatorTree(), zone());
+    } else {
+      table_.Delete(data->Key());
+    }
+  }
+}
+
+} }  // namespace v8::internal