[parser/ast] Reduce memory consumption for AstConsString.

The past re-factoring inadvertently increased memory consumption for
AstConsString. This implements a micro-optimization to revert and slightly
improve beyond the original state.

Example, Zone size for parsing closure.js:
  - 20,999,848 B (before refactoring)
  - 21,651,056 B (after refactoring patch; 3.1% regression)
  - 20,641,320 B (after this CL; 1.7% improvement over original)

(Reason: ZoneLinkedList requires 4 pointers to support
the std::list functionality (Zone*, head/tail ptr, payload ptr).
But since we only append and iterate in order and have the Zone*
available in the context, a super simple linked list (value + next ptr)
saves a bit of memory, especially for the common case of having 0 or 1
string segments.)

BUG=v8:6902, chromium:706935

Review-Url: https://codereview.chromium.org/2792353002
Cr-Commit-Position: refs/heads/master@{#44385}

src/ast/ast-value-factory.cc

@@ -156,12 +156,17 @@ bool AstRawString::Compare(void* a, void* b) {
 }
 
 void AstConsString::Internalize(Isolate* isolate) {
-  Handle<String> tmp(isolate->factory()->empty_string());
-  for (const AstRawString* current : strings_) {
-    // AstRawStrings are internalized before AstConsStrings, so *current is
-    // already internalized.
+  if (IsEmpty()) {
+    set_string(isolate->factory()->empty_string());
+    return;
+  }
+  // AstRawStrings are internalized before AstConsStrings, so
+  // AstRawString::string() will just work.
+  Handle<String> tmp(segment_.string->string());
+  for (AstConsString::Segment* current = segment_.next; current != nullptr;
+       current = current->next) {
     tmp = isolate->factory()
-              ->NewConsString(tmp, current->string())
+              ->NewConsString(current->string->string(), tmp)
               .ToHandleChecked();
   }
   set_string(tmp);
@@ -285,19 +290,19 @@ const AstRawString* AstValueFactory::GetString(Handle<String> literal) {
 }
 
 AstConsString* AstValueFactory::NewConsString() {
-  AstConsString* new_string = new (zone_) AstConsString(zone_);
+  AstConsString* new_string = new (zone_) AstConsString;
   DCHECK_NOT_NULL(new_string);
   AddConsString(new_string);
   return new_string;
 }
 
 AstConsString* AstValueFactory::NewConsString(const AstRawString* str) {
-  return NewConsString()->AddString(str);
+  return NewConsString()->AddString(zone_, str);
 }
 
 AstConsString* AstValueFactory::NewConsString(const AstRawString* str1,
                                               const AstRawString* str2) {
-  return NewConsString()->AddString(str1)->AddString(str2);
+  return NewConsString()->AddString(zone_, str1)->AddString(zone_, str2);
 }
 
 void AstValueFactory::Internalize(Isolate* isolate) {

src/ast/ast-value-factory.h

@@ -126,12 +126,24 @@ class AstRawString final : public ZoneObject {
 
 class AstConsString final : public ZoneObject {
  public:
-  AstConsString* AddString(const AstRawString* s) {
-    if (s && !s->IsEmpty()) strings_.push_back(s);
+  AstConsString* AddString(Zone* zone, const AstRawString* s) {
+    if (s->IsEmpty()) return this;
+    if (!IsEmpty()) {
+      // We're putting the new string to the head of the list, meaning
+      // the string segments will be in reverse order.
+      Segment* tmp = new (zone->New(sizeof(Segment))) Segment;
+      *tmp = segment_;
+      segment_.next = tmp;
+    }
+    segment_.string = s;
     return this;
   }
 
-  bool IsEmpty() const { return strings_.empty(); }
+  bool IsEmpty() const {
+    DCHECK_IMPLIES(segment_.string == nullptr, segment_.next == nullptr);
+    DCHECK_IMPLIES(segment_.string != nullptr, !segment_.string->IsEmpty());
+    return segment_.string == nullptr;
+  }
 
   void Internalize(Isolate* isolate);
 
@@ -143,7 +155,7 @@ class AstConsString final : public ZoneObject {
  private:
   friend class AstValueFactory;
 
-  explicit AstConsString(Zone* zone) : next_(nullptr), strings_(zone) {}
+  AstConsString() : next_(nullptr), segment_({nullptr, nullptr}) {}
 
   AstConsString* next() const { return next_; }
   AstConsString** next_location() { return &next_; }
@@ -156,7 +168,11 @@ class AstConsString final : public ZoneObject {
     String** string_;
   };
 
-  ZoneLinkedList<const AstRawString*> strings_;
+  struct Segment {
+    const AstRawString* string;
+    AstConsString::Segment* next;
+  };
+  Segment segment_;
 };
 
 enum class AstSymbol : uint8_t { kHomeObjectSymbol };

src/parsing/func-name-inferrer.cc

@@ -64,9 +64,9 @@ const AstConsString* FuncNameInferrer::MakeNameFromStack() {
     }
     // Add name. Separate names with ".".
     if (!result->IsEmpty()) {
-      result->AddString(ast_value_factory_->dot_string());
+      result->AddString(zone(), ast_value_factory_->dot_string());
     }
-    result->AddString(names_stack_.at(pos).name);
+    result->AddString(zone(), names_stack_.at(pos).name);
   }
   return result;
 }