[turbofan] Preprocess context slots for immutable global context loads

Main changes:
  ContextData class to hold a map of slots to ObjectData for known
    necessary lookups.
  LdaGlobal* and StaGlobal now receive an accumulator hint of the
    constant found at the lookup slot for immutable global context
    operations.

Bug: v8:7790
Change-Id: I63dc9eb8ebbbdfa4ce3b71c6aba63b3c06a3da9b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1532074Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60386}

src/compiler/js-context-specialization.cc

@@ -144,7 +144,7 @@ Reduction JSContextSpecialization::ReduceJSLoadContext(Node* node) {
 
   // Now walk up the concrete context chain for the remaining depth.
   ContextRef concrete = maybe_concrete.value();
-  concrete.Serialize();  // TODO(neis): Remove later.
+  concrete.SerializeContextChain();  // TODO(neis): Remove later.
   for (; depth > 0; --depth) {
     concrete = concrete.previous();
   }
@@ -158,6 +158,7 @@ Reduction JSContextSpecialization::ReduceJSLoadContext(Node* node) {
   // This will hold the final value, if we can figure it out.
   base::Optional<ObjectRef> maybe_value;
 
+  concrete.SerializeSlot(static_cast<int>(access.index()));
   maybe_value = concrete.get(static_cast<int>(access.index()));
   if (maybe_value.has_value() && !maybe_value->IsSmi()) {
     // Even though the context slot is immutable, the context might have escaped
@@ -206,7 +207,7 @@ Reduction JSContextSpecialization::ReduceJSStoreContext(Node* node) {
 
   // Now walk up the concrete context chain for the remaining depth.
   ContextRef concrete = maybe_concrete.value();
-  concrete.Serialize();  // TODO(neis): Remove later.
+  concrete.SerializeContextChain();  // TODO(neis): Remove later.
   for (; depth > 0; --depth) {
     concrete = concrete.previous();
   }

src/compiler/js-heap-broker.cc

@@ -406,37 +406,55 @@ class ContextData : public HeapObjectData {
  public:
   ContextData(JSHeapBroker* broker, ObjectData** storage,
               Handle<Context> object);
-  void Serialize(JSHeapBroker* broker);
+  void SerializeContextChain(JSHeapBroker* broker);
 
   ContextData* previous() const {
-    CHECK(serialized_);
+    CHECK(serialized_context_chain_);
     return previous_;
   }
 
+  void SerializeSlot(JSHeapBroker* broker, int index);
+
+  ObjectData* GetSlot(int index) {
+    auto search = slots_.find(index);
+    CHECK(search != slots_.end());
+    return search->second;
+  }
+
  private:
-  bool serialized_ = false;
+  ZoneMap<int, ObjectData*> slots_;
+  bool serialized_context_chain_ = false;
   ContextData* previous_ = nullptr;
 };
 
 ContextData::ContextData(JSHeapBroker* broker, ObjectData** storage,
                          Handle<Context> object)
-    : HeapObjectData(broker, storage, object) {}
+    : HeapObjectData(broker, storage, object), slots_(broker->zone()) {}
 
-void ContextData::Serialize(JSHeapBroker* broker) {
-  if (serialized_) return;
-  serialized_ = true;
+void ContextData::SerializeContextChain(JSHeapBroker* broker) {
+  if (serialized_context_chain_) return;
+  serialized_context_chain_ = true;
 
-  TraceScope tracer(broker, this, "ContextData::Serialize");
+  TraceScope tracer(broker, this, "ContextData::SerializeContextChain");
   Handle<Context> context = Handle<Context>::cast(object());
 
   DCHECK_NULL(previous_);
   // Context::previous DCHECK-fails when called on the native context.
   if (!context->IsNativeContext()) {
     previous_ = broker->GetOrCreateData(context->previous())->AsContext();
-    previous_->Serialize(broker);
+    previous_->SerializeContextChain(broker);
   }
 }
 
+void ContextData::SerializeSlot(JSHeapBroker* broker, int index) {
+  TraceScope tracer(broker, this, "ContextData::SerializeSlot");
+  TRACE(broker, "Serializing script context slot " << index << ".");
+  Handle<Context> context = Handle<Context>::cast(object());
+  CHECK(index >= 0 && index < context->length());
+  ObjectData* odata = broker->GetOrCreateData(context->get(index));
+  slots_.insert(std::make_pair(index, odata));
+}
+
 class NativeContextData : public ContextData {
  public:
 #define DECL_ACCESSOR(type, name) \
@@ -1574,10 +1592,13 @@ ContextRef ContextRef::previous() const {
 
 // Not needed for TypedLowering.
 ObjectRef ContextRef::get(int index) const {
-  AllowHandleAllocation handle_allocation;
-  AllowHandleDereference handle_dereference;
-  Handle<Object> value(object()->get(index), broker()->isolate());
-  return ObjectRef(broker(), value);
+  if (broker()->mode() == JSHeapBroker::kDisabled) {
+    AllowHandleAllocation handle_allocation;
+    AllowHandleDereference handle_dereference;
+    Handle<Object> value(object()->get(index), broker()->isolate());
+    return ObjectRef(broker(), value);
+  }
+  return ObjectRef(broker(), data()->AsContext()->GetSlot(index));
 }
 
 JSHeapBroker::JSHeapBroker(Isolate* isolate, Zone* broker_zone)
@@ -2813,10 +2834,16 @@ void ModuleRef::Serialize() {
   data()->AsModule()->Serialize(broker());
 }
 
-void ContextRef::Serialize() {
+void ContextRef::SerializeContextChain() {
   if (broker()->mode() == JSHeapBroker::kDisabled) return;
   CHECK_EQ(broker()->mode(), JSHeapBroker::kSerializing);
-  data()->AsContext()->Serialize(broker());
+  data()->AsContext()->SerializeContextChain(broker());
+}
+
+void ContextRef::SerializeSlot(int index) {
+  if (broker()->mode() == JSHeapBroker::kDisabled) return;
+  CHECK_EQ(broker()->mode(), JSHeapBroker::kSerializing);
+  data()->AsContext()->SerializeSlot(broker(), index);
 }
 
 void NativeContextRef::Serialize() {
@@ -2898,6 +2925,15 @@ bool GlobalAccessFeedback::immutable() const {
   return FeedbackNexus::ImmutabilityBit::decode(index_and_immutable_);
 }
 
+base::Optional<ObjectRef> GlobalAccessFeedback::GetConstantValue() const {
+  if (IsScriptContextSlot() && immutable()) {
+    // Return the value of this global variable if it's guaranteed to be
+    // constant.
+    return script_context().get(slot_index());
+  }
+  return {};
+}
+
 ElementAccessFeedback::ElementAccessFeedback(Zone* zone)
     : ProcessedFeedback(kElementAccess),
       receiver_maps(zone),
@@ -3053,8 +3089,12 @@ GlobalAccessFeedback const* JSHeapBroker::ProcessFeedbackForGlobalAccess(
       CHECK(!contents.equals(
           ObjectRef(this, isolate()->factory()->the_hole_value())));
     }
-    return new (zone()) GlobalAccessFeedback(ContextRef(this, context),
-                                             context_slot_index, immutable);
+    ContextRef context_ref(this, context);
+    if (immutable) {
+      context_ref.SerializeSlot(context_slot_index);
+    }
+    return new (zone())
+        GlobalAccessFeedback(context_ref, context_slot_index, immutable);
   }
 
   CHECK(feedback_value->IsPropertyCell());

src/compiler/js-heap-broker.h

@@ -286,8 +286,10 @@ class ContextRef : public HeapObjectRef {
   using HeapObjectRef::HeapObjectRef;
   Handle<Context> object() const;
 
-  void Serialize();
+  void SerializeContextChain();
   ContextRef previous() const;
+
+  void SerializeSlot(int index);
   ObjectRef get(int index) const;
 };
 
@@ -651,6 +653,8 @@ class GlobalAccessFeedback : public ProcessedFeedback {
   int slot_index() const;
   bool immutable() const;
 
+  base::Optional<ObjectRef> GetConstantValue() const;
+
  private:
   ObjectRef const cell_or_context_;
   int const index_and_immutable_;

src/compiler/js-heap-copy-reducer.cc

@@ -31,7 +31,7 @@ Reduction JSHeapCopyReducer::Reduce(Node* node) {
       if (object.IsJSFunction()) object.AsJSFunction().Serialize();
       if (object.IsJSObject()) object.AsJSObject().SerializeObjectCreateMap();
       if (object.IsModule()) object.AsModule().Serialize();
-      if (object.IsContext()) object.AsContext().Serialize();
+      if (object.IsContext()) object.AsContext().SerializeContextChain();
       break;
     }
     case IrOpcode::kJSCreateArray: {

src/compiler/serializer-for-background-compilation.cc

@@ -683,27 +683,33 @@ void SerializerForBackgroundCompilation::VisitConstructWithSpread(
   ProcessCallOrConstruct(callee, new_target, arguments, slot, true);
 }
 
-void SerializerForBackgroundCompilation::ProcessFeedbackForGlobalAccess(
+GlobalAccessFeedback const*
+SerializerForBackgroundCompilation::ProcessFeedbackForGlobalAccess(
     FeedbackSlot slot) {
-  if (slot.IsInvalid()) return;
-  if (environment()->function().feedback_vector.is_null()) return;
+  if (slot.IsInvalid()) return nullptr;
+  if (environment()->function().feedback_vector.is_null()) return nullptr;
   FeedbackSource source(environment()->function().feedback_vector, slot);
   if (!broker()->HasFeedback(source)) {
-    broker()->SetFeedback(source,
-                          broker()->ProcessFeedbackForGlobalAccess(source));
+    const GlobalAccessFeedback* feedback =
+        broker()->ProcessFeedbackForGlobalAccess(source);
+    broker()->SetFeedback(source, feedback);
+    return feedback;
   }
-  // TODO(neis, mvstanton): In the case of an immutable script context slot, we
-  // must also serialize that slot such that ContextRef::get can retrieve the
-  // value.
+  return nullptr;
 }
 
 void SerializerForBackgroundCompilation::VisitLdaGlobal(
     BytecodeArrayIterator* iterator) {
   FeedbackSlot slot = iterator->GetSlotOperand(1);
-  ProcessFeedbackForGlobalAccess(slot);
-  // TODO(neis, mvstanton): In the case of an immutable script context slot, add
-  // the value as constant hint here and below
   environment()->accumulator_hints().Clear();
+  GlobalAccessFeedback const* feedback = ProcessFeedbackForGlobalAccess(slot);
+  if (feedback != nullptr) {
+    // We may be able to contribute to accumulator constant hints.
+    base::Optional<ObjectRef> value = feedback->GetConstantValue();
+    if (value.has_value()) {
+      environment()->accumulator_hints().AddConstant(value->object());
+    }
+  }
 }
 
 void SerializerForBackgroundCompilation::VisitLdaGlobalInsideTypeof(

src/compiler/serializer-for-background-compilation.h

@@ -248,7 +248,7 @@ class SerializerForBackgroundCompilation {
                            base::Optional<Hints> new_target,
                            const HintsVector& arguments, bool with_spread);
 
-  void ProcessFeedbackForGlobalAccess(FeedbackSlot slot);
+  GlobalAccessFeedback const* ProcessFeedbackForGlobalAccess(FeedbackSlot slot);
   void ProcessFeedbackForKeyedPropertyAccess(FeedbackSlot slot,
                                              AccessMode mode);