New array bounds check elimination pass (focused on induction variables and bitwise operations).

src/bounds-check-elimination.cc

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

[titzer, 2013/07/02 13:45:06]

Please name this file with a "hydrogen-" prefix.
"hydrogen-bounds-check-elimination.cc" might be unwieldy, so perhaps
"hydrogen-bce.cc" will be OK.

[Massi, 2013/07/08 11:55:07]

hydrogen-bounds-check-removal.cc is just as long as other names...

[titzer, 2013/07/10 16:34:18]

Ok.

+// 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.h"
+
+namespace v8 {
+namespace internal {
+
+class InductionVariableBlocksTable BASE_EMBEDDED {
+ public:
+  class Element {

[titzer, 2013/07/02 13:45:06]

I find it hard to understand why this class serves both as a table of induction
variable information and also the state needed to iterate over the blocks in the
correct order. The dominator traversal here is unlike others, e.g. the one in
GVN, and I find that it obscures the underlying algorithm here. And there is
zero documentation.

[Massi, 2013/07/08 11:55:07]

Added comments.

+   public:
+    static const int kNoBlock = -1;
+
+    HBasicBlock* block() { return block_; }

[titzer, 2013/07/02 13:45:06]

Why do you have accessors for all this state in the table? Why is any other
class using the internals?

[Massi, 2013/07/08 11:55:07]

Removed unneeded accessors.

+    void set_block(HBasicBlock* block) { block_ = block; }
+    int current_successor() { return current_successor_; }
+    int backtrack_to() { return backtrack_to_; }
+    bool is_start() { return is_start_; }
+    bool is_proper_exit() { return is_proper_exit_; }
+    bool is_in_loop() { return is_in_loop_; }
+    bool has_check() { return has_check_; }
+    void set_has_check() { has_check_ = true; }
+    InductionVariableLimitUpdate* additional_limit() {
+      return &additional_limit_;
+    }
+
+    void InitializeLoop(InductionVariableData* data) {
+      ASSERT(data->limit() != NULL);
+      HLoopInformation* loop = data->phi()->block()->current_loop();
+      current_successor_ = kNoBlock;
+      backtrack_to_ = kNoBlock;
+      is_start_ = (block() == loop->loop_header());
+      is_proper_exit_ = (block() == data->induction_exit_target());
+      is_in_loop_ = loop->IsNestedInThisLoop(block()->current_loop());
+      has_check_ = false;
+    }
+
+    void ClearIterationData() {
+      current_successor_ = kNoBlock;
+      backtrack_to_ = kNoBlock;
+    }
+
+    int CurrentSuccessorBlock() {
+      if (current_successor_ < block()->end()->SuccessorCount()) {
+        return block()->end()->SuccessorAt(current_successor_)->block_id();
+      } else {
+        return kNoBlock;
+      }
+    }
+
+    int PerformStep(int from_block, bool* failure, bool* unsafe) {
+      if (!is_in_loop()) {
+        if (!is_proper_exit()) {
+          *unsafe = true;
+        }
+        return from_block;
+      }
+
+      if (is_start() &&
+          from_block != kNoBlock &&
+          from_block != CurrentSuccessorBlock()) {
+        *failure = true;
+        return from_block;
+      }
+
+      if (has_check()) {
+        return from_block;
+      }
+
+      if (current_successor_ == kNoBlock) {
+        backtrack_to_ = from_block;
+      }
+      current_successor_++;
+
+      if (CurrentSuccessorBlock() != kNoBlock) {
+        return CurrentSuccessorBlock();
+      } else {
+        return backtrack_to();
+      }
+    }
+
+    Element()

[titzer, 2013/07/02 13:45:06]

At the very least the constructor should accept the block as a parameter.

[Massi, 2013/07/08 11:55:07]

I don't think I can do it because this is the element of an array, so I need a
parameterless constructor.

+        : block_(NULL), current_successor_(kNoBlock), backtrack_to_(kNoBlock),
+          is_start_(false), is_proper_exit_(false), has_check_(false),
+          additional_limit_() {}
+
+   private:
+    HBasicBlock* block_;
+    int current_successor_;
+    int backtrack_to_;
+    bool is_start_;
+    bool is_proper_exit_;
+    bool is_in_loop_;
+    bool has_check_;
+    InductionVariableLimitUpdate additional_limit_;
+  };
+
+  HGraph* graph() { return graph_; }
+  HBasicBlock* loop_header() { return loop_header_; }
+  Element* at(int index) { return &(elements_.at(index)); }
+  Element* at(HBasicBlock* block) { return at(block->block_id()); }
+
+  void add_check_at(int index) {
+    at(index)->set_has_check();
+  }
+  void add_check_at(HBasicBlock* block) {
+    add_check_at(block->block_id());
+  }
+
+  void InitializeLoop(InductionVariableData* data) {
+    for (int i = 0; i < graph()->blocks()->length(); i++) {
+      at(i)->InitializeLoop(data);
+    }
+    loop_header_ = data->phi()->block()->current_loop()->loop_header();
+  }
+
+  void ClearIterationData() {
+    ASSERT(loop_header() != NULL);
+    HLoopInformation* loop = loop_header()->loop_information();
+    for (int i = 0; i < loop->blocks()->length(); i++) {
+      at(loop->blocks()->at(i)->block_id())->ClearIterationData();
+    }
+  }
+
+  bool LoopPathsAreChecked(bool* unsafe) {
+    bool failure = false;
+    *unsafe = false;
+    int previous_block = Element::kNoBlock;
+    int current_block = loop_header()->block_id();
+    while (current_block != Element::kNoBlock) {
+      int next_block = at(current_block)->PerformStep(previous_block,
+                                                      &failure, unsafe);
+      previous_block = current_block;
+      current_block = next_block;
+      if (failure) return false;
+    }
+    return true;
+  }
+
+  void AddCheck(HBoundsCheck* check) {
+    at(check->block()->block_id())->set_has_check();
+  }
+
+  explicit InductionVariableBlocksTable(HGraph* graph)
+    : graph_(graph), loop_header_(NULL),
+      elements_(graph->blocks()->length(), graph->zone()) {
+    for (int i = 0; i < graph->blocks()->length(); i++) {
+      Element element;
+      element.set_block(graph->blocks()->at(i));
+      elements_.Add(element, graph->zone());
+      ASSERT(at(i)->block()->block_id() == i);
+    }
+  }
+
+  void ProcessRelatedChecks(
+      InductionVariableData::InductionVariableCheck* check,
+      InductionVariableData* data) {
+    HValue* length = check->check()->length();
+    ClearIterationData();
+    check->set_processed();
+    HBasicBlock* header =
+        data->phi()->block()->current_loop()->loop_header();
+    HBasicBlock* pre_header = header->predecessors()->at(0);
+    if (!data->limit()->IsInteger32Constant()) {
+      HBasicBlock* limit_block = data->limit()->block();
+      if (limit_block != pre_header &&
+          !limit_block->Dominates(pre_header)) {
+        return;
+      }
+    }
+    if (!(data->limit()->representation().Equals(
+            length->representation()) ||
+        data->limit()->IsInteger32Constant())) {
+      return;
+    }
+    if (check->check()->length()->block() != pre_header &&
+        !check->check()->length()->block()->Dominates(pre_header)) {
+      return;
+    }
+
+    for (InductionVariableData::InductionVariableCheck* current_check = check;
+         current_check != NULL;
+         current_check = current_check->next()) {
+      if (current_check->check()->length() != length) continue;
+
+      add_check_at(current_check->check()->block());
+      current_check->set_processed();
+    }
+
+    bool unsafe;
+    bool failure = !LoopPathsAreChecked(&unsafe);
+
+    if (failure || (unsafe && !graph()->use_optimistic_licm())) {
+      return;
+    }
+
+    bool has_upper_constant_limit = true;
+    InductionVariableData::InductionVariableCheck* current_check = check;
+    int32_t upper_constant_limit =
+        current_check != NULL && current_check->HasUpperLimit() ?
+        current_check->upper_limit() : 0;
+    while (current_check != NULL) {
+      if (check->HasUpperLimit()) {
+        if (check->upper_limit() != upper_constant_limit) {
+          has_upper_constant_limit = false;
+        }
+      } else {
+        has_upper_constant_limit = false;
+      }
+
+      current_check->check()->set_skip_check();
+      current_check = current_check->next();
+    }
+
+    HValue* limit = data->limit();
+
+    if (has_upper_constant_limit) {
+      HConstant* new_limit = new(pre_header->graph()->zone()) HConstant(
+          upper_constant_limit, length->representation());
+      new_limit->InsertBefore(pre_header->end());
+      limit = new_limit;
+    }
+    if (limit->IsInteger32Constant() &&
+        limit->block() != pre_header &&
+        !limit->block()->Dominates(pre_header)) {
+      HConstant* new_limit = new(pre_header->graph()->zone()) HConstant(
+          limit->GetInteger32Constant(), length->representation());
+      new_limit->InsertBefore(pre_header->end());
+      limit = new_limit;
+    }
+    HBoundsCheck* hoisted_check = new(pre_header->zone()) HBoundsCheck(
+        limit, check->check()->length());
+    hoisted_check->InsertBefore(pre_header->end());
+    hoisted_check->set_allow_equality(true);
+  }
+
+ private:
+  HGraph* graph_;
+  HBasicBlock* loop_header_;
+  ZoneList<Element> elements_;

[titzer, 2013/07/10 16:34:18]

I think you could get away with a plain-old C array for these elements, since it
is only allocated once and never grows.

+};
+
+
+void HGraph::CollectInductionVariableData(
+    HBasicBlock* bb,
+    InductionVariableBlocksTable* table) {
+  bool additional_limit = false;
+
+  for (int i = 0; i < bb->phis()->length(); i++) {
+    HPhi* phi = bb->phis()->at(i);
+    phi->DetectInductionVariable();
+  }
+
+  additional_limit = InductionVariableData::ComputeInductionVariableLimit(
+      bb, table->at(bb)->additional_limit());
+
+  if (additional_limit) {
+    table->at(bb)->additional_limit()->updated_variable->
+        UpdateAdditionalLimit(table->at(bb)->additional_limit());
+  }
+
+  for (HInstruction* i = bb->first(); i != NULL; i = i->next()) {
+    if (!i->IsBoundsCheck()) continue;
+    HBoundsCheck* check = HBoundsCheck::cast(i);
+    InductionVariableData::BitwiseDecompositionResult decomposition;
+    InductionVariableData::DecomposeBitwise(check->index(), &decomposition);
+    if (!decomposition.base->IsPhi()) continue;
+    HPhi* phi = HPhi::cast(decomposition.base);
+
+    if (!phi->IsInductionVariable()) continue;
+    InductionVariableData* data = phi->induction_variable_data();
+
+    // For now ignore loops decrementing the index.
+    if (data->increment() <= 0) continue;
+    if (!data->lower_limit_is_non_negative_constant()) continue;
+
+    // TODO(mmassi): skip OSR values for check->length().
+    if (check->length() == data->limit() ||
+        check->length() == data->additional_upper_limit()) {
+      check->set_skip_check();
+      continue;
+    }
+
+    if (!phi->IsLimitedInductionVariable()) continue;
+
+    int32_t limit = data->ComputeUpperLimit(decomposition.and_mask,
+                                            decomposition.or_mask);
+    phi->induction_variable_data()->AddCheck(check, limit);
+  }
+
+  for (int i = 0; i < bb->dominated_blocks()->length(); i++) {
+    CollectInductionVariableData(bb->dominated_blocks()->at(i), table);
+  }
+
+  if (additional_limit) {
+    table->at(bb->block_id())->additional_limit()->updated_variable->
+        UpdateAdditionalLimit(table->at(bb->block_id())->additional_limit());
+  }
+}
+
+
+void HGraph::EliminateRedundantBoundsChecksUsingInductionVariables(
+    HBasicBlock* bb,
+    InductionVariableBlocksTable* table) {
+  for (int i = 0; i < bb->phis()->length(); i++) {
+    HPhi* phi = bb->phis()->at(i);
+    if (!phi->IsLimitedInductionVariable()) continue;
+
+    InductionVariableData* induction_data = phi->induction_variable_data();
+    InductionVariableData::ChecksRelatedToLength* current_length_group =
+        induction_data->checks();
+    while (current_length_group != NULL) {
+      current_length_group->CloseCurrentBlock();
+      InductionVariableData::InductionVariableCheck* current_base_check =
+          current_length_group->checks();
+      table->InitializeLoop(induction_data);
+
+      while (current_base_check != NULL) {
+        table->ProcessRelatedChecks(current_base_check, induction_data);
+        while (current_base_check != NULL && current_base_check->processed()) {
+          current_base_check = current_base_check->next();
+        }
+      }
+
+      current_length_group = current_length_group->next();
+    }
+  }
+}
+
+
+void HGraph::EliminateRedundantBoundsChecksUsingInductionVariables() {
+  InductionVariableBlocksTable table(this);
+  CollectInductionVariableData(entry_block(), &table);
+  for (int i = 0; i < blocks()->length(); i++) {
+    EliminateRedundantBoundsChecksUsingInductionVariables(blocks()->at(i),
+                                                          &table);
+  }
+}
+
+} }  // namespace v8::internal
+