[parser] Don't store empty inner function preparse_data

Using a single bit in the inner function description preparse data of the
parent function we can avoid storing many NULL values for empty inner function
data.

This saves roughly 40KB out of 140KB PreparseScopeData on cnn.com.

Change-Id: Ib6019a8ceb99e772b398198074e171f635c0556e
Reviewed-on: https://chromium-review.googlesource.com/c/1405038
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58708}

src/parsing/preparse-data.cc

@@ -27,6 +27,10 @@ class VariableMaybeAssignedField : public BitField8<bool, 0, 1> {};
 class VariableContextAllocatedField
     : public BitField8<bool, VariableMaybeAssignedField::kNext, 1> {};
 
+class HasDataField : public BitField<bool, 0, 1> {};
+class NumberOfParametersField
+    : public BitField<uint16_t, HasDataField::kNext, 16> {};
+
 class LanguageField : public BitField8<LanguageMode, 0, 1> {};
 class UsesSuperField : public BitField8<bool, LanguageField::kNext, 1> {};
 STATIC_ASSERT(LanguageModeSize <= LanguageField::kNumValues);
@@ -170,9 +174,8 @@ void PreparseDataBuilder::DataGatheringScope::Start(
 PreparseDataBuilder::DataGatheringScope::~DataGatheringScope() {
   if (builder_ == nullptr) return;
   PreparseDataBuilder* parent = builder_->parent_;
-  if (parent != nullptr) {
-    parent->data_for_inner_functions_.push_back(builder_->HasData() ? builder_
-                                                                    : nullptr);
+  if (parent != nullptr && builder_->HasData()) {
+    parent->data_for_inner_functions_.push_back(builder_);
   }
   preparser_->set_preparse_data_builder(parent);
 }
@@ -183,14 +186,13 @@ void PreparseDataBuilder::DataGatheringScope::AddSkippableFunction(
   builder_->parent_->AddSkippableFunction(
       function_scope->start_position(), end_position,
       function_scope->num_parameters(), num_inner_functions,
-      function_scope->language_mode(), function_scope->NeedsHomeObject());
+      function_scope->language_mode(), builder_->HasData(),
+      function_scope->NeedsHomeObject());
 }
 
-void PreparseDataBuilder::AddSkippableFunction(int start_position,
-                                               int end_position,
-                                               int num_parameters,
-                                               int num_inner_functions,
-                                               LanguageMode language_mode,
+void PreparseDataBuilder::AddSkippableFunction(
+    int start_position, int end_position, int num_parameters,
+    int num_inner_functions, LanguageMode language_mode, bool has_data,
     bool uses_super_property) {
   if (bailed_out_) return;
 
@@ -199,7 +201,10 @@ void PreparseDataBuilder::AddSkippableFunction(int start_position,
   // at catching bugs in the wild so far.
   byte_data_->WriteUint32(start_position);
   byte_data_->WriteUint32(end_position);
-  byte_data_->WriteUint32(num_parameters);
+  uint32_t has_data_and_num_parameters =
+      HasDataField::encode(has_data) |
+      NumberOfParametersField::encode(num_parameters);
+  byte_data_->WriteUint32(has_data_and_num_parameters);
   byte_data_->WriteUint32(num_inner_functions);
 
   uint8_t language_and_super = LanguageField::encode(language_mode) |
@@ -255,13 +260,9 @@ Handle<PreparseData> PreparseDataBuilder::Serialize(Isolate* isolate) {
 
   int i = 0;
   for (const auto& item : data_for_inner_functions_) {
-    if (item != nullptr) {
+    DCHECK_NOT_NULL(item);
     Handle<PreparseData> child_data = item->Serialize(isolate);
-      data->set_child_data(i, *child_data);
-    } else {
-      DCHECK(data->child_data(i)->IsNull());
-    }
-    i++;
+    data->set_child_data(i++, *child_data);
   }
 
   return data;
@@ -277,11 +278,9 @@ ZonePreparseData* PreparseDataBuilder::Serialize(Zone* zone) {
 
   int i = 0;
   for (const auto& item : data_for_inner_functions_) {
-    if (item != nullptr) {
+    DCHECK_NOT_NULL(item);
     ZonePreparseData* child = item->Serialize(zone);
-      result->set_child(i, child);
-    }
-    i++;
+    result->set_child(i++, child);
   }
 
   return result;
@@ -458,16 +457,21 @@ BaseConsumedPreparseData<Data>::GetDataForSkippableFunction(
   CHECK(scope_data_->HasRemainingBytes(ByteData::kSkippableFunctionDataSize));
   int start_position_from_data = scope_data_->ReadUint32();
   CHECK_EQ(start_position, start_position_from_data);
-
   *end_position = scope_data_->ReadUint32();
   DCHECK_GT(*end_position, start_position);
-  *num_parameters = scope_data_->ReadUint32();
+
+  uint32_t has_data_and_num_parameters = scope_data_->ReadUint32();
+  bool has_data = HasDataField::decode(has_data_and_num_parameters);
+  *num_parameters =
+      NumberOfParametersField::decode(has_data_and_num_parameters);
   *num_inner_functions = scope_data_->ReadUint32();
 
   uint8_t language_and_super = scope_data_->ReadQuarter();
   *language_mode = LanguageMode(LanguageField::decode(language_and_super));
   *uses_super_property = UsesSuperField::decode(language_and_super);
 
+  if (!has_data) return nullptr;
+
   // Retrieve the corresponding PreparseData and associate it to the
   // skipped function. If the skipped functions contains inner functions, those
   // can be skipped when the skipped function is eagerly parsed.

src/parsing/preparse-data.h

@@ -123,7 +123,7 @@ class PreparseDataBuilder : public ZoneObject {
   static bool ScopeIsSkippableFunctionScope(Scope* scope);
   void AddSkippableFunction(int start_position, int end_position,
                             int num_parameters, int num_inner_functions,
-                            LanguageMode language_mode,
+                            LanguageMode language_mode, bool has_data,
                             bool uses_super_property);
 
  private: