[compiler] Introduce a simple store-store elimination, disabled by default.

src/compiler/store-store-elimination.cc

+// Copyright 2016 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/store-store-elimination.h"
+
+#include "src/compiler/all-nodes.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 {
+
+// A simple store-store elimination. When the effect chain contains the
+// following sequence,
+//
+// - StoreField[[+off_1]](x1, y1)
+// - StoreField[[+off_2]](x2, y2)
+// - StoreField[[+off_3]](x3, y3)
+//   ...
+// - StoreField[[+off_n]](xn, yn)
+//
+// where the xes are the objects and the ys are the values to be stored, then
+// we are going to say that a store is superfluous if the same offset of the
+// same object will be stored to in the future. If off_i == off_j and xi == xj
+// and i < j, then we optimize the i'th StoreField away.
+//
+// This optimization should be initiated on the last StoreField in such a
+// sequence.
+//
+// The algorithm works by walking the effect chain from the last StoreField
+// upwards. While walking, we maintain a map {futureStore} from offsets to
+// nodes; initially it is empty. As we walk the effect chain upwards, if
+// futureStore[off] = n, then any store to node {n} with offset {off} is
+// guaranteed to be useless because we do a full-width[1] store to that offset
+// of that object in the near future anyway. For example, for this effect
+// chain
+//
+// 71: StoreField(60, 0)
+// 72: StoreField(65, 8)
+// 73: StoreField(63, 8)
+// 74: StoreField(65, 16)
+// 75: StoreField(62, 8)
+//
+// just before we get to 72, we will have futureStore = {8: 63, 16: 65}.
+//
+// Here is the complete process.
+//
+// - We are at the end of a sequence of consecutive StoreFields.
+// - We start out with futureStore = empty.
+// - We then walk the effect chain upwards to find the next StoreField [2].
+//
+//   1. If the offset is not a key of {futureStore} yet, we put it in.
+//   2. If the offset is a key of {futureStore}, but futureStore[offset] is a
+//      different node, we overwrite futureStore[offset] with the current node.
+//   3. If the offset is a key of {futureStore} and futureStore[offset] equals
+//      this node, we eliminate this StoreField.
+//
+//   As long as the current effect input points to a node with a single effect
+//   output, and as long as its opcode is StoreField, we keep traversing
+//   upwards.
+//
+// [1] This optimization is unsound if we optimize away a store to an offset
+//   because we store to the same offset in the future, even though the future
+//   store is narrower than the store we optimize away. Therefore, in case (1)
+//   and (2) we only add/overwrite to the dictionary when the field access has
+//   maximal size. For simplicity of implementation, we do not try to detect
+//   case (3).
+//
+// [2] We make sure that we only traverse the linear part, that is, the part
+//   where every node has exactly one incoming and one outgoing effect edge.
+//   Also, we only keep walking upwards as long as we keep finding consecutive
+//   StoreFields on the same node.
+
+StoreStoreElimination::StoreStoreElimination(JSGraph* js_graph, Zone* temp_zone)
+    : jsgraph_(js_graph), temp_zone_(temp_zone) {}
+
+StoreStoreElimination::~StoreStoreElimination() {}
+
+void StoreStoreElimination::Run() {
+  // The store-store elimination performs work on chains of certain types of
+  // nodes. The elimination must be invoked on the lowest node in such a
+  // chain; we have a helper function IsEligibleNode that returns true
+  // precisely on the lowest node in such a chain.
+  //
+  // Because the elimination removes nodes from the graph, even remove nodes
+  // that the elimination was not invoked on, we cannot use a normal
+  // AdvancedReducer but we manually find which nodes to invoke the
+  // elimination on. Then in a next step, we invoke the elimination for each
+  // node that was eligible.
+
+  NodeVector eligible(temp_zone());  // loops over all nodes
+  AllNodes all(temp_zone(), jsgraph()->graph());
+
+  for (Node* node : all.live) {
+    if (IsEligibleNode(node)) {
+      eligible.push_back(node);
+    }
+  }
+
+  for (Node* node : eligible) {
+    ReduceEligibleNode(node);
+  }
+}
+
+namespace {
+
+// 16 bits was chosen fairly arbitrarily; it seems enough now. 8 bits is too
+// few.
+typedef uint16_t Offset;
+
+// To safely cast an offset from a FieldAccess, which has a wider range
+// (namely int).
+Offset ToOffset(int offset) {
+  CHECK(0 <= offset && offset < (1 << 8 * sizeof(Offset)));
+  return (Offset)offset;
+}
+
+Offset ToOffset(const FieldAccess& access) { return ToOffset(access.offset); }
+
+// If node has a single effect use, return that node. If node has no or
+// multiple effect uses, return nullptr.
+Node* SingleEffectUse(Node* node) {
+  Node* last_use = nullptr;
+  for (Edge edge : node->use_edges()) {
+    if (!NodeProperties::IsEffectEdge(edge)) {
+      continue;
+    }
+    if (last_use != nullptr) {
+      // more than one
+      return nullptr;
+    }
+    last_use = edge.from();
+    DCHECK_NOT_NULL(last_use);
+  }
+  return last_use;
+}
+
+// Return true if node is the last consecutive StoreField node in a linear
+// part of the effect chain.
+bool IsEndOfStoreFieldChain(Node* node) {
+  Node* next_on_chain = SingleEffectUse(node);
+  return (next_on_chain == nullptr ||
+          next_on_chain->op()->opcode() != IrOpcode::kStoreField);
+}
+
+// The argument must be a StoreField node. If there is a node before it in the
+// effect chain, and if this part of the effect chain is linear (no other
+// effect uses of that previous node), then return that previous node.
+// Otherwise, return nullptr.
+//
+// The returned node need not be a StoreField.
+Node* PreviousEffectBeforeStoreField(Node* node) {
+  DCHECK_EQ(node->op()->opcode(), IrOpcode::kStoreField);
+  DCHECK_EQ(node->op()->EffectInputCount(), 1);
+
+  Node* previous = NodeProperties::GetEffectInput(node);
+  if (previous != nullptr && node == SingleEffectUse(previous)) {
+    return previous;
+  } else {
+    return nullptr;
+  }
+}
+
+}  // namespace
+
+bool StoreStoreElimination::IsEligibleNode(Node* node) {
+  return (node->op()->opcode() == IrOpcode::kStoreField) &&
+         IsEndOfStoreFieldChain(node);
+}
+
+void StoreStoreElimination::ReduceEligibleNode(Node* node) {
+  DCHECK(IsEligibleNode(node));
+
+  if (FLAG_trace_store_elimination) {
+    printf("** StoreStoreElimination::ReduceEligibleNode activated: #%d\n",

[Jarin, 2016/06/23 09:58:29]

printf -> PrintF (here and elsewhere)

[bgeron, 2016/06/23 12:03:05]

Done.

+           node->id());
+  }
+  auto log = [](NodeId id, Offset off, const char* msg) {

[Jarin, 2016/06/23 09:58:28]

This does not have to be a lambda, please put it into a separate function.

[bgeron, 2016/06/23 12:03:05]

Done.

+    if (FLAG_trace_store_elimination) {
+      printf(
+          "   StoreStoreElimination::ReduceEligibleNode: "
+          "#%d[[+%d]] -- %s\n",
+          id, off, msg);
+    }
+  };
+
+  // Initialize empty futureStore.
+  ZoneMap<Offset, Node*> futureStore(temp_zone());
+
+  Node* current_node = node;
+
+  do {
+    FieldAccess access = OpParameter<FieldAccess>(current_node->op());
+    Offset offset = ToOffset(access);
+    Node* object_input = current_node->InputAt(0);
+
+    Node* previous = PreviousEffectBeforeStoreField(current_node);
+
+    if (access.machine_type.IsWidestSize()) {

[Jarin, 2016/06/23 09:58:28]

I still think this is not a good idea. If someone introduces a type that is
wider than the tagged pointer, your code will stop working without anyone
noticing.

It would be better to CHECK that the size is at most tagged and only eliminate
if the size is the same as kPointerSize.

[bgeron, 2016/06/23 12:03:05]

Done.

+      // Try to insert. If it was present, this will preserve the original
+      // value.
+      auto insert_result =
+          futureStore.insert(std::make_pair(offset, object_input));
+      if (insert_result.second) {
+        // Key was not present. This means that there is no matching
+        // StoreField to this offset in the future, so we cannot optimize
+        // current_node away. However, we will record the current StoreField
+        // in futureStore, and continue ascending up the chain.
+        log(current_node->id(), offset, "wide, key not present");
+      } else if (insert_result.first->second != object_input) {
+        // Key was present, and the value did not equal object_input. This
+        // means that there is a StoreField to this offset in the future, but
+        // the object instance comes from a different Node. We pessimistically
+        // assume that we cannot optimize current_node away. However, we will
+        // record the current StoreField in futureStore, and continue
+        // ascending up the chain.
+        insert_result.first->second = object_input;
+        log(current_node->id(), offset, "wide, diff object");
+      } else {
+        // Key was present, and the value equalled object_input. This means that
+        // soon after in the effect chain, we will do a StoreField to the same
+        // object with the same offset, therefore current_node can be optimized
+        // away. We don't need to update futureStore.
+
+        Node* previous_effect = NodeProperties::GetEffectInput(current_node);
+
+        NodeProperties::ReplaceUses(current_node, nullptr, previous_effect,
+                                    nullptr, nullptr);
+        current_node->Kill();
+        log(current_node->id(), offset, "wide, eliminated");
+      }
+    } else {
+      log(current_node->id(), offset, "narrow, not eliminated");
+    }
+
+    // Regardless of whether we eliminated node {current}, we want to
+    // continue walking up the effect chain.
+
+    current_node = previous;
+  } while (current_node != nullptr &&
+           current_node->op()->opcode() == IrOpcode::kStoreField);
+
+  if (FLAG_trace_store_elimination) {
+    printf(
+        "   StoreStoreElimination::ReduceEligibleNode: "
+        "stop chain traversal\n");
+  }
+}
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8