[turbofan] Redundant branch elimination.

src/compiler/branch-elimination.cc

+// Copyright 2015 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/compiler/branch-elimination.h"
+
+#include "src/compiler/js-graph.h"
+#include "src/compiler/node-properties.h"
+#include "src/compiler/simplified-operator.h"
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+BranchElimination::BranchElimination(Editor* editor, JSGraph* js_graph,
+                                     Zone* zone)
+    : AdvancedReducer(editor),
+      node_conditions_(zone, js_graph->graph()->NodeCount()),
+      zone_(zone),
+      dead_(js_graph->graph()->NewNode(js_graph->common()->Dead())) {}
+
+
+BranchElimination::~BranchElimination() {}
+
+
+Reduction BranchElimination::Reduce(Node* node) {
+  switch (node->opcode()) {
+    case IrOpcode::kDead:
+      return NoChange();
+    case IrOpcode::kMerge:
+      return ReduceMerge(node);
+    case IrOpcode::kLoop:
+      return ReduceLoop(node);
+    case IrOpcode::kBranch:
+      return ReduceBranch(node);
+    case IrOpcode::kIfFalse:
+      return ReduceIf(node, false);
+    case IrOpcode::kIfTrue:
+      return ReduceIf(node, true);
+    case IrOpcode::kStart:
+      return ReduceStart(node);
+    default:
+      if (node->op()->ControlOutputCount() > 0) {
+        return ReduceOtherControl(node);
+      }
+      break;
+  }
+  return NoChange();
+}
+
+
+Reduction BranchElimination::ReduceBranch(Node* node) {
+  Node* condition = node->InputAt(0);
+  Node* control_input = NodeProperties::GetControlInput(node, 0);
+  const ControlPathConditions* from_input = node_conditions_.Get(control_input);
+  if (from_input != nullptr) {
+    Maybe<bool> condition_value = from_input->LookupCondition(condition);
+    // If we know the condition we can discard the branch.
+    if (condition_value.IsJust()) {
+      bool known_value = condition_value.FromJust();
+      for (Node* const use : node->uses()) {
+        switch (use->opcode()) {
+          case IrOpcode::kIfTrue:
+            Replace(use, known_value ? control_input : dead());
+            break;
+          case IrOpcode::kIfFalse:
+            Replace(use, known_value ? dead() : control_input);
+            break;
+          default:
+            UNREACHABLE();
+        }
+      }
+      return Replace(dead());
+    }
+  }
+  return TakeConditionsFromFirstControl(node);
+}
+
+
+Reduction BranchElimination::ReduceIf(Node* node, bool is_true_branch) {
+  // Add the condition to the list arriving from the input branch.
+  Node* branch = NodeProperties::GetControlInput(node, 0);
+  const ControlPathConditions* from_branch = node_conditions_.Get(branch);
+  // If we do not know anything about the predecessor, do not propagate just
+  // yet because we will have to recompute anyway once we compute the
+  // predecessor.
+  if (from_branch == nullptr) {
+    DCHECK(node_conditions_.Get(node) == nullptr);
+    return NoChange();
+  }
+  Node* condition = branch->InputAt(0);
+  return UpdateConditions(
+      node, from_branch->AddCondition(zone_, condition, is_true_branch));
+}
+
+
+Reduction BranchElimination::ReduceLoop(Node* node) {
+  // Here we rely on having only reducible loops:
+  // The loop entry edge always dominates the header, so we can just use
+  // the information from the loop entry edge.
+  return TakeConditionsFromFirstControl(node);
+}
+
+
+Reduction BranchElimination::ReduceMerge(Node* node) {
+  // Shortcut for the case when we do not know anything about some
+  // input.
+  for (int i = 0; i < node->InputCount(); i++) {
+    if (node_conditions_.Get(node->InputAt(i)) == nullptr) {
+      DCHECK(node_conditions_.Get(node) == nullptr);
+      return NoChange();
+    }
+  }
+
+  const ControlPathConditions* first = node_conditions_.Get(node->InputAt(0));
+  // Make a copy of the first input's conditions and merge with the conditions
+  // from other inputs.
+  ControlPathConditions* conditions =
+      new (zone_->New(sizeof(ControlPathConditions)))
+          ControlPathConditions(*first);
+  for (int i = 1; i < node->InputCount(); i++) {
+    conditions->Merge(*(node_conditions_.Get(node->InputAt(i))));
+  }
+
+  return UpdateConditions(node, conditions);
+}
+
+
+Reduction BranchElimination::ReduceStart(Node* node) {
+  return UpdateConditions(node, ControlPathConditions::Empty(zone_));
+}
+
+
+const BranchElimination::ControlPathConditions*
+BranchElimination::PathConditionsForControlNodes::Get(Node* node) {
+  if (static_cast<size_t>(node->id()) < info_for_node_.size()) {
+    return info_for_node_[node->id()];
+  }
+  return nullptr;
+}
+
+
+void BranchElimination::PathConditionsForControlNodes::Set(
+    Node* node, const ControlPathConditions* conditions) {
+  size_t index = static_cast<size_t>(node->id());
+  if (index >= info_for_node_.size()) {
+    info_for_node_.resize(index + 1, nullptr);
+  }
+  info_for_node_[index] = conditions;
+}
+
+
+Reduction BranchElimination::ReduceOtherControl(Node* node) {
+  DCHECK_EQ(1, node->op()->ControlInputCount());
+  return TakeConditionsFromFirstControl(node);
+}
+
+
+Reduction BranchElimination::TakeConditionsFromFirstControl(Node* node) {
+  // We just propagate the information from the control input (ideally,
+  // we would only revisit control uses if there is change).
+  const ControlPathConditions* from_input =
+      node_conditions_.Get(NodeProperties::GetControlInput(node, 0));
+  return UpdateConditions(node, from_input);
+}
+
+
+Reduction BranchElimination::UpdateConditions(
+    Node* node, const ControlPathConditions* conditions) {
+  const ControlPathConditions* original = node_conditions_.Get(node);
+  // Only signal that the node has Changed if the condition information has
+  // changed.
+  if (conditions != original) {
+    if (original == nullptr || *conditions != *original) {
+      node_conditions_.Set(node, conditions);
+      return Changed(node);
+    }
+  }
+  return NoChange();
+}
+
+
+// static
+const BranchElimination::ControlPathConditions*
+BranchElimination::ControlPathConditions::Empty(Zone* zone) {
+  return new (zone->New(sizeof(ControlPathConditions)))
+      ControlPathConditions(nullptr, 0);
+}
+
+
+void BranchElimination::ControlPathConditions::Merge(
+    const ControlPathConditions& other) {
+  // Change the current condition list to a longest common tail
+  // of this condition list and the other list. (The common tail
+  // should correspond to the list from the common dominator.)
+
+  // First, we throw away the prefix of the longer list, so that
+  // we have lists of the same length.
+  size_t other_size = other.condition_count_;
+  BranchCondition* other_condition = other.head_;
+  while (other_size > condition_count_) {
+    other_condition = other_condition->next;
+    other_size--;
+  }
+  while (condition_count_ > other_size) {
+    head_ = head_->next;
+    condition_count_--;
+  }
+
+  // Then we go through both lists in lock-step until we find
+  // the common tail.
+  while (head_ != other_condition) {
+    DCHECK(condition_count_ > 0);
+    condition_count_--;
+    other_condition = other_condition->next;
+    head_ = head_->next;
+  }
+}
+
+
+const BranchElimination::ControlPathConditions*
+BranchElimination::ControlPathConditions::AddCondition(Zone* zone,
+                                                       Node* condition,
+                                                       bool is_true) const {
+  DCHECK(LookupCondition(condition).IsNothing());
+
+  BranchCondition* new_head = new (zone->New(sizeof(BranchCondition)))
+      BranchCondition(condition, is_true, head_);
+
+  ControlPathConditions* conditions =
+      new (zone->New(sizeof(ControlPathConditions)))
+          ControlPathConditions(new_head, condition_count_ + 1);
+  return conditions;
+}
+
+
+Maybe<bool> BranchElimination::ControlPathConditions::LookupCondition(
+    Node* condition) const {
+  for (BranchCondition* current = head_; current != nullptr;
+       current = current->next) {
+    if (current->condition == condition) {
+      return Just<bool>(current->is_true);
+    }
+  }
+  return Nothing<bool>();
+}
+
+
+bool BranchElimination::ControlPathConditions::operator==(
+    const ControlPathConditions& other) const {
+  if (condition_count_ != other.condition_count_) return false;
+  BranchCondition* this_condition = head_;
+  BranchCondition* other_condition = other.head_;
+  while (true) {
+    if (this_condition == other_condition) return true;
+    if (this_condition->condition != other_condition->condition ||
+        this_condition->is_true != other_condition->is_true) {
+      return false;
+    }
+    this_condition = this_condition->next;
+    other_condition = other_condition->next;
+  }
+  UNREACHABLE();
+  return false;
+}
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8