[parser/ast] Reduce memory consumption for AstConsString.

src/ast/ast-value-factory.h

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 108 matching lines @@
 #ifdef DEBUG
   // (Debug-only:) Verify the object life-cylce: Some functions may only be
   // called after internalization (that is, after a v8::internal::String has
   // been set); some only before.
   bool has_string_ = false;
 #endif
 };
 
 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);
 
   V8_INLINE Handle<String> string() const {
     DCHECK_NOT_NULL(string_);
     return Handle<String>(string_);
   }
 
  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_; }
 
   // {string_} is stored as String** instead of a Handle<String> so it can be
   // stored in a union with {next_}.
   void set_string(Handle<String> string) { string_ = string.location(); }
   union {
     AstConsString* next_;
     String** string_;
   };
 
-  ZoneLinkedList<const AstRawString*> strings_;
+  struct Segment {
+    const AstRawString* string;
+    AstConsString::Segment* next;
+  };
+  Segment segment_;
 };
 
 enum class AstSymbol : uint8_t { kHomeObjectSymbol };
 
 // AstValue is either a string, a symbol, a number, a string array, a boolean,
 // or a special value (null, undefined, the hole).
 class AstValue : public ZoneObject {
  public:
   bool IsString() const {
     return type_ == STRING;
@@ skipping 366 matching lines @@
   OTHER_CONSTANTS(F)
 #undef F
 };
 }  // namespace internal
 }  // namespace v8
 
 #undef STRING_CONSTANTS
 #undef OTHER_CONSTANTS
 
 #endif  // V8_AST_AST_VALUE_FACTORY_H_