Prevent unnecessary memory (de-)allocations in LiteralBuffer::CopyFrom.

src/parsing/scanner.h

 // Copyright 2011 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.
 
 // Features shared by parsing and pre-parsing scanners.
 
 #ifndef V8_PARSING_SCANNER_H_
 #define V8_PARSING_SCANNER_H_
 
 #include "src/allocation.h"
@@ skipping 207 matching lines @@
   }
 
   Handle<String> Internalize(Isolate* isolate) const;
 
   void CopyFrom(const LiteralBuffer* other) {
     if (other == nullptr) {
       Reset();
     } else {
       is_one_byte_ = other->is_one_byte_;
       position_ = other->position_;
-      backing_store_.Dispose();
-      backing_store_ = other->backing_store_.Clone();
+      if (position_ < backing_store_.length()) {
+        std::copy(other->backing_store_.begin(),
+                  other->backing_store_.begin() + position_,
+                  backing_store_.begin());
+      } else {
+        backing_store_.Dispose();
+        backing_store_ = other->backing_store_.Clone();
+      }
     }
   }
 
  private:
   static const int kInitialCapacity = 16;
   static const int kGrowthFactory = 4;
   static const int kMinConversionSlack = 256;
   static const int kMaxGrowth = 1 * MB;
   inline int NewCapacity(int min_capacity) {
     int capacity = Max(min_capacity, backing_store_.length());
@@ skipping 547 matching lines @@
   bool allow_harmony_exponentiation_operator_;
 
   MessageTemplate::Template scanner_error_;
   Location scanner_error_location_;
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_PARSING_SCANNER_H_